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Date:   Thu, 29 Oct 2020 08:53:13 +0000
From:   Sargun Dhillon <sargun@sargun.me>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc:     Tycho Andersen <tycho@tycho.pizza>,
        Christian Brauner <christian@brauner.io>,
        Kees Cook <keescook@chromium.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Robert Sesek <rsesek@google.com>,
        Containers <containers@lists.linux-foundation.org>,
        linux-man <linux-man@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>, Jann Horn <jannh@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Will Drewry <wad@chromium.org>, bpf <bpf@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>
Subject: Re: For review: seccomp_user_notif(2) manual page [v2]
Message-ID: <20201029085312.GC29881@ircssh-2.c.rugged-nimbus-611.internal>
References: <63598b4f-6ce3-5a11-4552-cdfe308f68e4@gmail.com>
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On Mon, Oct 26, 2020 at 10:55:04AM +0100, Michael Kerrisk (man-pages) wrote:
> Hi all (and especially Tycho and Sargun),
> 
> Following review comments on the first draft (thanks to Jann, Kees,
> Christian and Tycho), I've made a lot of changes to this page.
> I've also added a few FIXMEs relating to outstanding API issues.
> I'd like a second pass review of the page before I release it.
> But also, this mail serves as a way of noting the outstanding API
> issues.
> 
> Tycho: I still have an outstanding question for you at [2].
> 
> Sargun: can you please prepare something on SECCOMP_ADDFD_FLAG_SETFD
> and SECCOMP_IOCTL_NOTIF_ADDFD to be added to this page?
> 
> I've shown the rendered version of the page below. The page source
> currently sits in a branch at
> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=seccomp_user_notif
> 
> At this point, I'm mainly interested in feedback about the FIXMEs,
> some of which relate to the text of the page itself, while the
> others relate to the various outstanding API issues. The first 
> FIXME provides a small opportunity for some bikeshedding :-);
> 
> 
> Thanks,
> 
> Michael
> 
> [1] https://lore.kernel.org/linux-man/45f07f17-18b6-d187-0914-6f341fe90857@gmail.com/
> [2] https://lore.kernel.org/linux-man/8f20d586-9609-ef83-c85a-272e37e684d8@gmail.com/
> 
> =====
> 
> SECCOMP_USER_NOTIF(2)   Linux Programmer's Manual  SECCOMP_USER_NOTIF(2)
> 
> NAME
>        seccomp_user_notif - Seccomp user-space notification mechanism
> 
>        ┌─────────────────────────────────────────────────────┐
>        │FIXME                                                │
>        ├─────────────────────────────────────────────────────┤
>        │Might "seccomp_unotify(2)" be a better name for this │
>        │page?  It's slightly shorter to type, and perhaps    │
>        │reads better when spoken.                            │
>        └─────────────────────────────────────────────────────┘
> 
> SYNOPSIS
>        #include <linux/seccomp.h>
>        #include <linux/filter.h>
>        #include <linux/audit.h>
> 
>        int seccomp(unsigned int operation, unsigned int flags, void *args);
> 
>        #include <sys/ioctl.h>
> 
>        int ioctl(int fd, SECCOMP_IOCTL_NOTIF_RECV,
>                  struct seccomp_notif *req);
>        int ioctl(int fd, SECCOMP_IOCTL_NOTIF_SEND,
>                  struct seccomp_notif_resp *resp);
>        int ioctl(int fd, SECCOMP_IOCTL_NOTIF_ID_VALID, __u64 *id);
> 
> DESCRIPTION
>        This page describes the user-space notification mechanism
>        provided by the Secure Computing (seccomp) facility.  As well as
>        the use of the SECCOMP_FILTER_FLAG_NEW_LISTENER flag, the
>        SECCOMP_RET_USER_NOTIF action value, and the
>        SECCOMP_GET_NOTIF_SIZES operation described in seccomp(2), this
>        mechanism involves the use of a number of related ioctl(2)
>        operations (described below).
> 
>    Overview
>        In conventional usage of a seccomp filter, the decision about how
>        to treat a system call is made by the filter itself.  By
>        contrast, the user-space notification mechanism allows the
>        seccomp filter to delegate the handling of the system call to
>        another user-space process.  Note that this mechanism is
>        explicitly not intended as a method implementing security policy;
>        see NOTES.
> 
>        In the discussion that follows, the thread(s) on which the
>        seccomp filter is installed is (are) referred to as the target,
>        and the process that is notified by the user-space notification
>        mechanism is referred to as the supervisor.
> 
>        A suitably privileged supervisor can use the user-space
>        notification mechanism to perform actions on behalf of the
>        target.  The advantage of the user-space notification mechanism
>        is that the supervisor will usually be able to retrieve
>        information about the target and the performed system call that
>        the seccomp filter itself cannot.  (A seccomp filter is limited
>        in the information it can obtain and the actions that it can
>        perform because it is running on a virtual machine inside the
>        kernel.)
> 
>        An overview of the steps performed by the target and the
>        supervisor is as follows:
> 
>        1. The target establishes a seccomp filter in the usual manner,
>           but with two differences:
> 
>           · The seccomp(2) flags argument includes the flag
>             SECCOMP_FILTER_FLAG_NEW_LISTENER.  Consequently, the return
>             value of the (successful) seccomp(2) call is a new
>             "listening" file descriptor that can be used to receive
>             notifications.  Only one "listening" seccomp filter can be
>             installed for a thread.
> 
>             ┌─────────────────────────────────────────────────────┐
>             │FIXME                                                │
>             ├─────────────────────────────────────────────────────┤
>             │Is the last sentence above correct?                  │
>             │                                                     │
>             │Kees Cook (25 Oct 2020) notes:                       │
>             │                                                     │
>             │I like this limitation, but I expect that it'll need │
>             │to change in the future. Even with LSMs, we see the  │
>             │need for arbitrary stacking, and the idea of there   │
>             │being only 1 supervisor will eventually break down.  │
>             │Right now there is only 1 because only container     │
>             │managers are using this feature. But if some daemon  │
>             │starts using it to isolate some thread, suddenly it  │
>             │might break if a container manager is trying to      │
>             │listen to it too, etc. I expect it won't be needed   │
>             │soon, but I do think it'll change.                   │
>             │                                                     │
>             └─────────────────────────────────────────────────────┘
> 
>           · In cases where it is appropriate, the seccomp filter returns
>             the action value SECCOMP_RET_USER_NOTIF.  This return value
>             will trigger a notification event.
> 
>        2. In order that the supervisor can obtain notifications using
>           the listening file descriptor, (a duplicate of) that file
>           descriptor must be passed from the target to the supervisor.
>           One way in which this could be done is by passing the file
>           descriptor over a UNIX domain socket connection between the
>           target and the supervisor (using the SCM_RIGHTS ancillary
>           message type described in unix(7)).
> 
>        3. The supervisor will receive notification events on the
>           listening file descriptor.  These events are returned as
>           structures of type seccomp_notif.  Because this structure and
>           its size may evolve over kernel versions, the supervisor must
>           first determine the size of this structure using the
>           seccomp(2) SECCOMP_GET_NOTIF_SIZES operation, which returns a
>           structure of type seccomp_notif_sizes.  The supervisor
>           allocates a buffer of size seccomp_notif_sizes.seccomp_notif
>           bytes to receive notification events.  In addition,the
>           supervisor allocates another buffer of size
>           seccomp_notif_sizes.seccomp_notif_resp bytes for the response
>           (a struct seccomp_notif_resp structure) that it will provide
>           to the kernel (and thus the target).
> 
>        4. The target then performs its workload, which includes system
>           calls that will be controlled by the seccomp filter.  Whenever
>           one of these system calls causes the filter to return the
>           SECCOMP_RET_USER_NOTIF action value, the kernel does not (yet)
>           execute the system call; instead, execution of the target is
>           temporarily blocked inside the kernel (in a sleep state that
>           is interruptible by signals) and a notification event is
>           generated on the listening file descriptor.
> 
>        5. The supervisor can now repeatedly monitor the listening file
>           descriptor for SECCOMP_RET_USER_NOTIF-triggered events.  To do
>           this, the supervisor uses the SECCOMP_IOCTL_NOTIF_RECV
>           ioctl(2) operation to read information about a notification
>           event; this operation blocks until an event is available.  The
>           operation returns a seccomp_notif structure containing
>           information about the system call that is being attempted by
>           the target.
> 
>        6. The seccomp_notif structure returned by the
>           SECCOMP_IOCTL_NOTIF_RECV operation includes the same
>           information (a seccomp_data structure) that was passed to the
>           seccomp filter.  This information allows the supervisor to
>           discover the system call number and the arguments for the
>           target's system call.  In addition, the notification event
>           contains the ID of the thread that triggered the notification
>           and a unique cookie value that is used in subsequent
>           SECCOMP_IOCTL_NOTIF_ID_VALID and SECCOMP_IOCTL_NOTIF_SEND
>           operations.
> 
>           The information in the notification can be used to discover
>           the values of pointer arguments for the target's system call.
>           (This is something that can't be done from within a seccomp
>           filter.)  One way in which the supervisor can do this is to
>           open the corresponding /proc/[tid]/mem file (see proc(5)) and
>           read bytes from the location that corresponds to one of the
>           pointer arguments whose value is supplied in the notification
>           event.  (The supervisor must be careful to avoid a race
>           condition that can occur when doing this; see the description
>           of the SECCOMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation below.)
>           In addition, the supervisor can access other system
>           information that is visible in user space but which is not
>           accessible from a seccomp filter.
> 
>        7. Having obtained information as per the previous step, the
>           supervisor may then choose to perform an action in response to
>           the target's system call (which, as noted above, is not
>           executed when the seccomp filter returns the
>           SECCOMP_RET_USER_NOTIF action value).
> 
>           One example use case here relates to containers.  The target
>           may be located inside a container where it does not have
>           sufficient capabilities to mount a filesystem in the
>           container's mount namespace.  However, the supervisor may be a
>           more privileged process that does have sufficient capabilities
>           to perform the mount operation.
> 
>        8. The supervisor then sends a response to the notification.  The
>           information in this response is used by the kernel to
>           construct a return value for the target's system call and
>           provide a value that will be assigned to the errno variable of
>           the target.
> 
>           The response is sent using the SECCOMP_IOCTL_NOTIF_SEND
>           ioctl(2) operation, which is used to transmit a
>           seccomp_notif_resp structure to the kernel.  This structure
>           includes a cookie value that the supervisor obtained in the
>           seccomp_notif structure returned by the
>           SECCOMP_IOCTL_NOTIF_RECV operation.  This cookie value allows
>           the kernel to associate the response with the target.  This
>           structure must include the cookie value that the supervisor
>           obtained in the seccomp_notif structure returned by the
>           SECCOMP_IOCTL_NOTIF_RECV operation; the cookie allows the
>           kernel to associate the response with the target.
> 
>        9. Once the notification has been sent, the system call in the
>           target thread unblocks, returning the information that was
>           provided by the supervisor in the notification response.
> 
>        As a variation on the last two steps, the supervisor can send a
>        response that tells the kernel that it should execute the target
>        thread's system call; see the discussion of
>        SECCOMP_USER_NOTIF_FLAG_CONTINUE, below.
> 
>    ioctl(2) operations
>        The following ioctl(2) operations are provided to support seccomp
>        user-space notification.  For each of these operations, the first
>        (file descriptor) argument of ioctl(2) is the listening file
>        descriptor returned by a call to seccomp(2) with the
>        SECCOMP_FILTER_FLAG_NEW_LISTENER flag.
> 
>        SECCOMP_IOCTL_NOTIF_RECV
>               This operation is used to obtain a user-space notification
>               event.  If no such event is currently pending, the
>               operation blocks until an event occurs.  The third
>               ioctl(2) argument is a pointer to a structure of the
>               following form which contains information about the event.
>               This structure must be zeroed out before the call.
> 
>                   struct seccomp_notif {
>                       __u64  id;              /* Cookie */
>                       __u32  pid;             /* TID of target thread */
>                       __u32  flags;           /* Currently unused (0) */
>                       struct seccomp_data data;   /* See seccomp(2) */
>                   };
> 
>               The fields in this structure are as follows:
> 
>               id     This is a cookie for the notification.  Each such
>                      cookie is guaranteed to be unique for the
>                      corresponding seccomp filter.
> 
>                      · It can be used with the
>                        SECCOMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation
>                        to verify that the target is still alive.
> 
>                      · When returning a notification response to the
>                        kernel, the supervisor must include the cookie
>                        value in the seccomp_notif_resp structure that is
>                        specified as the argument of the
>                        SECCOMP_IOCTL_NOTIF_SEND operation.
> 
>               pid    This is the thread ID of the target thread that
>                      triggered the notification event.
> 
>               flags  This is a bit mask of flags providing further
>                      information on the event.  In the current
>                      implementation, this field is always zero.
> 
>               data   This is a seccomp_data structure containing
>                      information about the system call that triggered
>                      the notification.  This is the same structure that
>                      is passed to the seccomp filter.  See seccomp(2)
>                      for details of this structure.
> 
>               On success, this operation returns 0; on failure, -1 is
>               returned, and errno is set to indicate the cause of the
>               error.  This operation can fail with the following errors:
> 
>               EINVAL (since Linux 5.5)
>                      The seccomp_notif structure that was passed to the
>                      call contained nonzero fields.
> 
>               ENOENT The target thread was killed by a signal as the
>                      notification information was being generated, or
>                      the target's (blocked) system call was interrupted
>                      by a signal handler.
> 
>        ┌─────────────────────────────────────────────────────┐
>        │FIXME                                                │
>        ├─────────────────────────────────────────────────────┤
>        │From my experiments, it appears that if a            │
>        │SECCOMP_IOCTL_NOTIF_RECV is done after the target    │
>        │thread terminates, then the ioctl() simply blocks    │
>        │(rather than returning an error to indicate that the │
>        │target no longer exists).                            │
>        │                                                     │
>        │I found that surprising, and it required some        │
>        │contortions in the example program.  It was not      │
>        │possible to code my SIGCHLD handler (which reaps the │
>        │zombie when the worker/target terminates) to simply  │
>        │set a flag checked in the main handleNotifications() │
>        │loop, since this created an unavoidable race where   │
>        │the child might terminate just after I had checked   │
>        │the flag, but before I blocked (forever!) in the     │
>        │SECCOMP_IOCTL_NOTIF_RECV operation. Instead, I had   │
>        │to code the signal handler to simply call _exit(2)   │
>        │in order to terminate the parent process (the        │
>        │supervisor).                                         │
>        │                                                     │
>        │Is this expected behavior? It seems to me rather     │
>        │desirable that SECCOMP_IOCTL_NOTIF_RECV should give  │
>        │an error if the target has terminated.               │
>        │                                                     │
>        │Jann posted a patch to rectify this, but there was   │
>        │no response (Lore link: https://bit.ly/3jvUBxk) to   │
>        │his question about fixing this issue. (I've tried    │
>        │building with the patch, but encountered an issue    │
>        │with the target process entering D state after a     │
>        │signal.)                                             │
>        │                                                     │
>        │For now, this behavior is documented in BUGS.        │
>        │                                                     │
>        │Kees Cook commented: Let's change [this] ASAP!       │
>        └─────────────────────────────────────────────────────┘
>

I think I commented in another thread somewhere that the supervisor is not 
notified if the syscall is preempted. Therefore if it is performing a 
preemptible, long-running syscall, you need to poll
SECCOMP_IOCTL_NOTIF_ID_VALID in the background, otherwise you can
end up in a bad situation -- like leaking resources, or holding on to
file descriptors after the program under supervision has intended to
release them.

A very specific example is if you're performing an accept on behalf
of the program generating the notification, and the program intends
to reuse the port. You can get into all sorts of awkward situations
there.

>        SECCOMP_IOCTL_NOTIF_ID_VALID
>               This operation can be used to check that a notification ID
>               returned by an earlier SECCOMP_IOCTL_NOTIF_RECV operation
>               is still valid (i.e., that the target still exists and its
>               system call is still blocked waiting for a response).
> 
>               The third ioctl(2) argument is a pointer to the cookie
>               (id) returned by the SECCOMP_IOCTL_NOTIF_RECV operation.
> 
>               This operation is necessary to avoid race conditions that
>               can occur when the pid returned by the
>               SECCOMP_IOCTL_NOTIF_RECV operation terminates, and that
>               process ID is reused by another process.  An example of
>               this kind of race is the following
> 
>               1. A notification is generated on the listening file
>                  descriptor.  The returned seccomp_notif contains the
>                  TID of the target thread (in the pid field of the
>                  structure).
> 
>               2. The target terminates.
> 
>               3. Another thread or process is created on the system that
>                  by chance reuses the TID that was freed when the target
>                  terminated.
> 
>               4. The supervisor open(2)s the /proc/[tid]/mem file for
>                  the TID obtained in step 1, with the intention of (say)
>                  inspecting the memory location(s) that containing the
>                  argument(s) of the system call that triggered the
>                  notification in step 1.
> 
>               In the above scenario, the risk is that the supervisor may
>               try to access the memory of a process other than the
>               target.  This race can be avoided by following the call to
>               open(2) with a SECCOMP_IOCTL_NOTIF_ID_VALID operation to
>               verify that the process that generated the notification is
>               still alive.  (Note that if the target terminates after
>               the latter step, a subsequent read(2) from the file
>               descriptor may return 0, indicating end of file.)
> 
>               On success (i.e., the notification ID is still valid),
>               this operation returns 0.  On failure (i.e., the
>               notification ID is no longer valid), -1 is returned, and
>               errno is set to ENOENT.
> 
>        SECCOMP_IOCTL_NOTIF_SEND
>               This operation is used to send a notification response
>               back to the kernel.  The third ioctl(2) argument of this
>               structure is a pointer to a structure of the following
>               form:
> 
>                   struct seccomp_notif_resp {
>                       __u64 id;               /* Cookie value */
>                       __s64 val;              /* Success return value */
>                       __s32 error;            /* 0 (success) or negative
>                                                  error number */
>                       __u32 flags;            /* See below */
>                   };
> 
>               The fields of this structure are as follows:
> 
>               id     This is the cookie value that was obtained using
>                      the SECCOMP_IOCTL_NOTIF_RECV operation.  This
>                      cookie value allows the kernel to correctly
>                      associate this response with the system call that
>                      triggered the user-space notification.
> 
>               val    This is the value that will be used for a spoofed
>                      success return for the target's system call; see
>                      below.
> 
>               error  This is the value that will be used as the error
>                      number (errno) for a spoofed error return for the
>                      target's system call; see below.
> 
>               flags  This is a bit mask that includes zero or more of
>                      the following flags:
> 
>                      SECCOMP_USER_NOTIF_FLAG_CONTINUE (since Linux 5.5)
>                             Tell the kernel to execute the target's
>                             system call.
> 
>               Two kinds of response are possible:
> 
>               · A response to the kernel telling it to execute the
>                 target's system call.  In this case, the flags field
>                 includes SECCOMP_USER_NOTIF_FLAG_CONTINUE and the error
>                 and val fields must be zero.
> 
>                 This kind of response can be useful in cases where the
>                 supervisor needs to do deeper analysis of the target's
>                 system call than is possible from a seccomp filter
>                 (e.g., examining the values of pointer arguments), and,
>                 having decided that the system call does not require
>                 emulation by the supervisor, the supervisor wants the
>                 system call to be executed normally in the target.
> 
>                 The SECCOMP_USER_NOTIF_FLAG_CONTINUE flag should be used
>                 with caution; see NOTES.
> 
>               · A spoofed return value for the target's system call.  In
>                 this case, the kernel does not execute the target's
>                 system call, instead causing the system call to return a
>                 spoofed value as specified by fields of the
>                 seccomp_notif_resp structure.  The supervisor should set
>                 the fields of this structure as follows:
> 
>                 +  flags does not contain
>                    SECCOMP_USER_NOTIF_FLAG_CONTINUE.
> 
>                 +  error is set either to 0 for a spoofed "success"
>                    return or to a negative error number for a spoofed
>                    "failure" return.  In the former case, the kernel
>                    causes the target's system call to return the value
>                    specified in the val field.  In the later case, the
>                    kernel causes the target's system call to return -1,
>                    and errno is assigned the negated error value.
> 
>                 +  val is set to a value that will be used as the return
>                    value for a spoofed "success" return for the target's
>                    system call.  The value in this field is ignored if
>                    the error field contains a nonzero value.
> 
>                    ┌─────────────────────────────────────────────────────┐
>                    │FIXME                                                │
>                    ├─────────────────────────────────────────────────────┤
>                    │Kees Cook suggested:                                 │
>                    │                                                     │
>                    │Strictly speaking, this is architecture specific,    │
>                    │but all architectures do it this way. Should seccomp │
>                    │enforce val == 0 when err != 0 ?                     │
>                    └─────────────────────────────────────────────────────┘
> 
>               On success, this operation returns 0; on failure, -1 is
>               returned, and errno is set to indicate the cause of the
>               error.  This operation can fail with the following errors:
> 
>               EINPROGRESS
>                      A response to this notification has already been
>                      sent.
> 
>               EINVAL An invalid value was specified in the flags field.
> 
>               EINVAL The flags field contained
>                      SECCOMP_USER_NOTIF_FLAG_CONTINUE, and the error or
>                      val field was not zero.
> 
>               ENOENT The blocked system call in the target has been
>                      interrupted by a signal handler or the target has
>                      terminated.
> 
	SECCOMP_IOCTL_NOTIF_ADDFD (Since Linux v5.9)
		This operations is used by the supervisor to add a file
		descriptor to the process that generated the notification.
		This can be used by the supervisor to enable "emulation"
		[Probably a better word] of syscalls which return file
		descriptors, such as socket(2), or open(2).

		When the file descriptor is received by the process that
		is associated with the notification / cookie, it follows
		SCM_RIGHTS like semantics, and is evaluated by MAC.
		In addition, if it is a socket, it inherits the cgroup
		v1 classid and netprioidx of the receiving process.

		The argument of this is as follows:

			struct seccomp_notif_addfd {
				__u64 id;
				__u32 flags;
				__u32 srcfd;
				__u32 newfd;
				__u32 newfd_flags;
			};

		id
			This is the cookie value that was obtained using
			SECCOMP_IOCTL_NOTIF_RECV.

		flags
			A bitmask that includes zero or more of the
			SECCOMP_ADDFD_FLAG_* bits set

			SECCOMP_ADDFD_FLAG_SETFD - Use dup2 (or dup3?)
				like semantics when copying the file
				descriptor.

		srcfd
			The file descriptor number to copy in the
			supervisor process.

		newfd
			If the SECCOMP_ADDFD_FLAG_SETFD flag is specified
			this will be the file descriptor that is used
			in the dup2 semantics. If this file descriptor
			exists in the receiving process, it is closed
			and replaced by this file descriptor in an
			atomic fashion. If the copy process fails
			due to a MAC failure, or if srcfd is invalid,
			the newfd will not be closed in the receiving
			process.

			If SECCOMP_ADDFD_FLAG_SETFD it not set, then
			this value must be 0.

		newfd_flags
			The file descriptor flags to set on
			the file descriptor after it has been received
			by the process. The only flag that can currently
			be specified is O_CLOEXEC.

		On success, this operation returns the file descriptor
		number in the receiving process. On failure, -1 is returned.

		It can fail with the following error codes:

		EINPROGRESS
			The cookie number specified hasn't been received
			by the listener

		ENOENT
			The cookie number is not valid. This can happen
			if a response has already been sent, or if the
			syscall was interrupted

		EBADF
			If the file descriptor specified in srcfd is
			invalid, or if the fd is out of range of the
			destination program.

		EINVAL
			If flags or new_flags were unrecognized, or
			if newfd is non-zero, and SECCOMP_ADDFD_FLAG_SETFD
			has not been set.

		EMFILE
			Too many files are open by the destination process.

		[there's other error codes possible, like from the LSMs
		 or if memory can't be read / written or ebusy]
		 
Does this help?
> NOTES
>    select()/poll()/epoll semantics
>        The file descriptor returned when seccomp(2) is employed with the
>        SECCOMP_FILTER_FLAG_NEW_LISTENER flag can be monitored using
>        poll(2), epoll(7), and select(2).  These interfaces indicate that
>        the file descriptor is ready as follows:
> 
>        · When a notification is pending, these interfaces indicate that
>          the file descriptor is readable.  Following such an indication,
>          a subsequent SECCOMP_IOCTL_NOTIF_RECV ioctl(2) will not block,
>          returning either information about a notification or else
>          failing with the error EINTR if the target has been killed by a
>          signal or its system call has been interrupted by a signal
>          handler.
> 
>        · After the notification has been received (i.e., by the
>          SECCOMP_IOCTL_NOTIF_RECV ioctl(2) operation), these interfaces
>          indicate that the file descriptor is writable, meaning that a
>          notification response can be sent using the
>          SECCOMP_IOCTL_NOTIF_SEND ioctl(2) operation.
> 
>          ┌─────────────────────────────────────────────────────┐
>          │FIXME                                                │
>          ├─────────────────────────────────────────────────────┤
>          │But (how) is the writable/(E)POLLOUT useful?         │
>          └─────────────────────────────────────────────────────┘
> 
>        · After the last thread using the filter has terminated and been
>          reaped using waitpid(2) (or similar), the file descriptor
>          indicates an end-of-file condition (readable in select(2);
>          POLLHUP/EPOLLHUP in poll(2)/ epoll_wait(2)).
> 
>    Design goals; use of SECCOMP_USER_NOTIF_FLAG_CONTINUE
>        The intent of the user-space notification feature is to allow
>        system calls to be performed on behalf of the target.  The
>        target's system call should either be handled by the supervisor
>        or allowed to continue normally in the kernel (where standard
>        security policies will be applied).
> 
>        Note well: this mechanism must not be used to make security
>        policy decisions about the system call, which would be inherently
>        race-prone for reasons described next.
> 
>        The SECCOMP_USER_NOTIF_FLAG_CONTINUE flag must be used with
>        caution.  If set by the supervisor, the target's system call will
>        continue.  However, there is a time-of-check, time-of-use race
>        here, since an attacker could exploit the interval of time where
>        the target is blocked waiting on the "continue" response to do
>        things such as rewriting the system call arguments.
> 
>        Note furthermore that a user-space notifier can be bypassed if
>        the existing filters allow the use of seccomp(2) or prctl(2) to
>        install a filter that returns an action value with a higher
>        precedence than SECCOMP_RET_USER_NOTIF (see seccomp(2)).
> 
>        It should thus be absolutely clear that the seccomp user-space
>        notification mechanism can not be used to implement a security
>        policy!  It should only ever be used in scenarios where a more
>        privileged process supervises the system calls of a lesser
>        privileged target to get around kernel-enforced security
>        restrictions when the supervisor deems this safe.  In other
>        words, in order to continue a system call, the supervisor should
>        be sure that another security mechanism or the kernel itself will
>        sufficiently block the system call if its arguments are rewritten
>        to something unsafe.
> 
>    Interaction with SA_RESTART signal handlers
>        Consider the following scenario:
> 
>        · The target process has used sigaction(2) to install a signal
>          handler with the SA_RESTART flag.
> 
>        · The target has made a system call that triggered a seccomp
>          user-space notification and the target is currently blocked
>          until the supervisor sends a notification response.
> 
>        · A signal is delivered to the target and the signal handler is
>          executed.
> 
>        · When (if) the supervisor attempts to send a notification
>          response, the SECCOMP_IOCTL_NOTIF_SEND ioctl(2)) operation will
>          fail with the ENOENT error.
> 
>        In this scenario, the kernel will restart the target's system
>        call.  Consequently, the supervisor will receive another user-
>        space notification.  Thus, depending on how many times the
>        blocked system call is interrupted by a signal handler, the
>        supervisor may receive multiple notifications for the same
>        instance of a system call in the target.
> 
>        One oddity is that system call restarting as described in this
>        scenario will occur even for the blocking system calls listed in
>        signal(7) that would never normally be restarted by the
>        SA_RESTART flag.
> 
>        ┌─────────────────────────────────────────────────────┐
>        │FIXME                                                │
>        ├─────────────────────────────────────────────────────┤
>        │About the above, Kees Cook commented:                │
>        │                                                     │
>        │Does this need fixing? I imagine the correct         │
>        │behavior for this case would be a response to _SEND  │
>        │of EINPROGRESS and the target would see EINTR        │
>        │normally?                                            │
>        │                                                     │
>        │I mean, it's not like seccomp doesn't already expose │
>        │weirdness with syscall restarts. Not even arm64      │
>        │compat agrees[3] with arm32 in this regard. :(       │
>        └─────────────────────────────────────────────────────┘
> 
>        ┌─────────────────────────────────────────────────────┐
>        │FIXME                                                │
>        ├─────────────────────────────────────────────────────┤
>        │Michael Kerrisk: I wonder about the effect of this   │
>        │oddity for system calls that are normally            │
>        │nonrestartable because they have timeouts. My        │
>        │understanding is that the kernel doesn't restart     │
>        │those system calls because it's impossible for the   │
>        │kernel to restart the call with the right timeout    │
>        │value. I wonder what happens when those system calls │
>        │are restarted in the scenario we're discussing.)     │
>        └─────────────────────────────────────────────────────┘
> 
> BUGS
>        If a SECCOMP_IOCTL_NOTIF_RECV ioctl(2) operation is performed
>        after the target terminates, then the ioctl(2) call simply blocks
>        (rather than returning an error to indicate that the target no
>        longer exists).
> 
>        ┌─────────────────────────────────────────────────────┐
>        │FIXME                                                │
>        ├─────────────────────────────────────────────────────┤
>        │Comment from Kees Cook:                              │
>        │                                                     │
>        │I want this fixed. It caused me no end of pain when  │
>        │building the selftests, and ended up spawning my     │
>        │implementing a global test timeout in kselftest. :P  │
>        │Before the usage counter refactor, there was no sane │
>        │way to deal with this, but now I think we're close.  │
>        │                                                     │
>        └─────────────────────────────────────────────────────┘
> 
> EXAMPLES
>        The (somewhat contrived) program shown below demonstrates the use
>        of the interfaces described in this page.  The program creates a
>        child process that serves as the "target" process.  The child
>        process installs a seccomp filter that returns the
>        SECCOMP_RET_USER_NOTIF action value if a call is made to
>        mkdir(2).  The child process then calls mkdir(2) once for each of
>        the supplied command-line arguments, and reports the result
>        returned by the call.  After processing all arguments, the child
>        process terminates.
> 
>        The parent process acts as the supervisor, listening for the
>        notifications that are generated when the target process calls
>        mkdir(2).  When such a notification occurs, the supervisor
>        examines the memory of the target process (using /proc/[pid]/mem)
>        to discover the pathname argument that was supplied to the
>        mkdir(2) call, and performs one of the following actions:
> 
>        · If the pathname begins with the prefix "/tmp/", then the
>          supervisor attempts to create the specified directory, and then
>          spoofs a return for the target process based on the return
>          value of the supervisor's mkdir(2) call.  In the event that
>          that call succeeds, the spoofed success return value is the
>          length of the pathname.
> 
>        · If the pathname begins with "./" (i.e., it is a relative
>          pathname), the supervisor sends a
>          SECCOMP_USER_NOTIF_FLAG_CONTINUE response to the kernel to say
>          that the kernel should execute the target process's mkdir(2)
>          call.
> 
>        · If the pathname begins with some other prefix, the supervisor
>          spoofs an error return for the target process, so that the
>          target process's mkdir(2) call appears to fail with the error
>          EOPNOTSUPP ("Operation not supported").  Additionally, if the
>          specified pathname is exactly "/bye", then the supervisor
>          terminates.
> 
>        This program can be used to demonstrate various aspects of the
>        behavior of the seccomp user-space notification mechanism.  To
>        help aid such demonstrations, the program logs various messages
>        to show the operation of the target process (lines prefixed "T:")
>        and the supervisor (indented lines prefixed "S:").
> 
>        In the following example, the target attempts to create the
>        directory /tmp/x.  Upon receiving the notification, the
>        supervisor creates the directory on the target's behalf, and
>        spoofs a success return to be received by the target process's
>        mkdir(2) call.
> 
>            $ ./seccomp_unotify /tmp/x
>            T: PID = 23168
> 
>            T: about to mkdir("/tmp/x")
>                    S: got notification (ID 0x17445c4a0f4e0e3c) for PID 23168
>                    S: executing: mkdir("/tmp/x", 0700)
>                    S: success! spoofed return = 6
>                    S: sending response (flags = 0; val = 6; error = 0)
>            T: SUCCESS: mkdir(2) returned 6
> 
>            T: terminating
>                    S: target has terminated; bye
> 
>        In the above output, note that the spoofed return value seen by
>        the target process is 6 (the length of the pathname /tmp/x),
>        whereas a normal mkdir(2) call returns 0 on success.
> 
>        In the next example, the target attempts to create a directory
>        using the relative pathname ./sub.  Since this pathname starts
>        with "./", the supervisor sends a
>        SECCOMP_USER_NOTIF_FLAG_CONTINUE response to the kernel, and the
>        kernel then (successfully) executes the target process's mkdir(2)
>        call.
> 
>            $ ./seccomp_unotify ./sub
>            T: PID = 23204
> 
>            T: about to mkdir("./sub")
>                    S: got notification (ID 0xddb16abe25b4c12) for PID 23204
>                    S: target can execute system call
>                    S: sending response (flags = 0x1; val = 0; error = 0)
>            T: SUCCESS: mkdir(2) returned 0
> 
>            T: terminating
>                    S: target has terminated; bye
> 
>        If the target process attempts to create a directory with a
>        pathname that doesn't start with "." and doesn't begin with the
>        prefix "/tmp/", then the supervisor spoofs an error return
>        (EOPNOTSUPP, "Operation not  supported") for the target's
>        mkdir(2) call (which is not executed):
> 
>            $ ./seccomp_unotify /xxx
>            T: PID = 23178
> 
>            T: about to mkdir("/xxx")
>                    S: got notification (ID 0xe7dc095d1c524e80) for PID 23178
>                    S: spoofing error response (Operation not supported)
>                    S: sending response (flags = 0; val = 0; error = -95)
>            T: ERROR: mkdir(2): Operation not supported
> 
>            T: terminating
>                    S: target has terminated; bye
> 
>        In the next example, the target process attempts to create a
>        directory with the pathname /tmp/nosuchdir/b.  Upon receiving the
>        notification, the supervisor attempts to create that directory,
>        but the mkdir(2) call fails because the directory /tmp/nosuchdir
>        does not exist.  Consequently, the supervisor spoofs an error
>        return that passes the error that it received back to the target
>        process's mkdir(2) call.
> 
>            $ ./seccomp_unotify /tmp/nosuchdir/b
>            T: PID = 23199
> 
>            T: about to mkdir("/tmp/nosuchdir/b")
>                    S: got notification (ID 0x8744454293506046) for PID 23199
>                    S: executing: mkdir("/tmp/nosuchdir/b", 0700)
>                    S: failure! (errno = 2; No such file or directory)
>                    S: sending response (flags = 0; val = 0; error = -2)
>            T: ERROR: mkdir(2): No such file or directory
> 
>            T: terminating
>                    S: target has terminated; bye
> 
>        If the supervisor receives a notification and sees that the
>        argument of the target's mkdir(2) is the string "/bye", then (as
>        well as spoofing an EOPNOTSUPP error), the supervisor terminates.
>        If the target process subsequently executes another mkdir(2) that
>        triggers its seccomp filter to return the SECCOMP_RET_USER_NOTIF
>        action value, then the kernel causes the target process's system
>        call to fail with the error ENOSYS ("Function not implemented").
>        This is demonstrated by the following example:
> 
>            $ ./seccomp_unotify /bye /tmp/y
>            T: PID = 23185
> 
>            T: about to mkdir("/bye")
>                    S: got notification (ID 0xa81236b1d2f7b0f4) for PID 23185
>                    S: spoofing error response (Operation not supported)
>                    S: sending response (flags = 0; val = 0; error = -95)
>                    S: terminating **********
>            T: ERROR: mkdir(2): Operation not supported
> 
>            T: about to mkdir("/tmp/y")
>            T: ERROR: mkdir(2): Function not implemented
> 
>            T: terminating
> 
>    Program source
>        #define _GNU_SOURCE
>        #include <sys/types.h>
>        #include <sys/prctl.h>
>        #include <fcntl.h>
>        #include <limits.h>
>        #include <signal.h>
>        #include <stddef.h>
>        #include <stdint.h>
>        #include <stdbool.h>
>        #include <linux/audit.h>
>        #include <sys/syscall.h>
>        #include <sys/stat.h>
>        #include <linux/filter.h>
>        #include <linux/seccomp.h>
>        #include <sys/ioctl.h>
>        #include <stdio.h>
>        #include <stdlib.h>
>        #include <unistd.h>
>        #include <errno.h>
>        #include <sys/socket.h>
>        #include <sys/un.h>
> 
>        #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
>                                } while (0)
> 
>        /* Send the file descriptor 'fd' over the connected UNIX domain socket
>           'sockfd'. Returns 0 on success, or -1 on error. */
> 
>        static int
>        sendfd(int sockfd, int fd)
>        {
>            struct msghdr msgh;
>            struct iovec iov;
>            int data;
>            struct cmsghdr *cmsgp;
> 
>            /* Allocate a char array of suitable size to hold the ancillary data.
>               However, since this buffer is in reality a 'struct cmsghdr', use a
>               union to ensure that it is suitably aligned. */
>            union {
>                char   buf[CMSG_SPACE(sizeof(int))];
>                                /* Space large enough to hold an 'int' */
>                struct cmsghdr align;
>            } controlMsg;
> 
>            /* The 'msg_name' field can be used to specify the address of the
>               destination socket when sending a datagram. However, we do not
>               need to use this field because 'sockfd' is a connected socket. */
> 
>            msgh.msg_name = NULL;
>            msgh.msg_namelen = 0;
> 
>            /* On Linux, we must transmit at least one byte of real data in
>               order to send ancillary data. We transmit an arbitrary integer
>               whose value is ignored by recvfd(). */
> 
>            msgh.msg_iov = &iov;
>            msgh.msg_iovlen = 1;
>            iov.iov_base = &data;
>            iov.iov_len = sizeof(int);
>            data = 12345;
> 
>            /* Set 'msghdr' fields that describe ancillary data */
> 
>            msgh.msg_control = controlMsg.buf;
>            msgh.msg_controllen = sizeof(controlMsg.buf);
> 
>            /* Set up ancillary data describing file descriptor to send */
> 
>            cmsgp = CMSG_FIRSTHDR(&msgh);
>            cmsgp->cmsg_level = SOL_SOCKET;
>            cmsgp->cmsg_type = SCM_RIGHTS;
>            cmsgp->cmsg_len = CMSG_LEN(sizeof(int));
>            memcpy(CMSG_DATA(cmsgp), &fd, sizeof(int));
> 
>            /* Send real plus ancillary data */
> 
>            if (sendmsg(sockfd, &msgh, 0) == -1)
>                return -1;
> 
>            return 0;
>        }
> 
>        /* Receive a file descriptor on a connected UNIX domain socket. Returns
>           the received file descriptor on success, or -1 on error. */
> 
>        static int
>        recvfd(int sockfd)
>        {
>            struct msghdr msgh;
>            struct iovec iov;
>            int data, fd;
>            ssize_t nr;
> 
>            /* Allocate a char buffer for the ancillary data. See the comments
>               in sendfd() */
>            union {
>                char   buf[CMSG_SPACE(sizeof(int))];
>                struct cmsghdr align;
>            } controlMsg;
>            struct cmsghdr *cmsgp;
> 
>            /* The 'msg_name' field can be used to obtain the address of the
>               sending socket. However, we do not need this information. */
> 
>            msgh.msg_name = NULL;
>            msgh.msg_namelen = 0;
> 
>            /* Specify buffer for receiving real data */
> 
>            msgh.msg_iov = &iov;
>            msgh.msg_iovlen = 1;
>            iov.iov_base = &data;       /* Real data is an 'int' */
>            iov.iov_len = sizeof(int);
> 
>            /* Set 'msghdr' fields that describe ancillary data */
> 
>            msgh.msg_control = controlMsg.buf;
>            msgh.msg_controllen = sizeof(controlMsg.buf);
> 
>            /* Receive real plus ancillary data; real data is ignored */
> 
>            nr = recvmsg(sockfd, &msgh, 0);
>            if (nr == -1)
>                return -1;
> 
>            cmsgp = CMSG_FIRSTHDR(&msgh);
> 
>            /* Check the validity of the 'cmsghdr' */
> 
>            if (cmsgp == NULL ||
>                    cmsgp->cmsg_len != CMSG_LEN(sizeof(int)) ||
>                    cmsgp->cmsg_level != SOL_SOCKET ||
>                    cmsgp->cmsg_type != SCM_RIGHTS) {
>                errno = EINVAL;
>                return -1;
>            }
> 
>            /* Return the received file descriptor to our caller */
> 
>            memcpy(&fd, CMSG_DATA(cmsgp), sizeof(int));
>            return fd;
>        }
> 
>        static void
>        sigchldHandler(int sig)
>        {
>            char msg[] = "\tS: target has terminated; bye\n";
> 
>            write(STDOUT_FILENO, msg, sizeof(msg) - 1);
>            _exit(EXIT_SUCCESS);
>        }
> 
>        static int
>        seccomp(unsigned int operation, unsigned int flags, void *args)
>        {
>            return syscall(__NR_seccomp, operation, flags, args);
>        }
> 
>        /* The following is the x86-64-specific BPF boilerplate code for checking
>           that the BPF program is running on the right architecture + ABI. At
>           completion of these instructions, the accumulator contains the system
>           call number. */
> 
>        /* For the x32 ABI, all system call numbers have bit 30 set */
> 
>        #define X32_SYSCALL_BIT         0x40000000
> 
>        #define X86_64_CHECK_ARCH_AND_LOAD_SYSCALL_NR \
>                BPF_STMT(BPF_LD | BPF_W | BPF_ABS, \
>                        (offsetof(struct seccomp_data, arch))), \
>                BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, AUDIT_ARCH_X86_64, 0, 2), \
>                BPF_STMT(BPF_LD | BPF_W | BPF_ABS, \
>                         (offsetof(struct seccomp_data, nr))), \
>                BPF_JUMP(BPF_JMP | BPF_JGE | BPF_K, X32_SYSCALL_BIT, 0, 1), \
>                BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS)
> 
>        /* installNotifyFilter() installs a seccomp filter that generates
>           user-space notifications (SECCOMP_RET_USER_NOTIF) when the process
>           calls mkdir(2); the filter allows all other system calls.
> 
>           The function return value is a file descriptor from which the
>           user-space notifications can be fetched. */
> 
>        static int
>        installNotifyFilter(void)
>        {
>            struct sock_filter filter[] = {
>                X86_64_CHECK_ARCH_AND_LOAD_SYSCALL_NR,
> 
>                /* mkdir() triggers notification to user-space supervisor */
> 
>                BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_mkdir, 0, 1),
>                BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_USER_NOTIF),
> 
>                /* Every other system call is allowed */
> 
>                BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
>            };
> 
>            struct sock_fprog prog = {
>                .len = sizeof(filter) / sizeof(filter[0]),
>                .filter = filter,
>            };
> 
>            /* Install the filter with the SECCOMP_FILTER_FLAG_NEW_LISTENER flag;
>               as a result, seccomp() returns a notification file descriptor. */
> 
>            int notifyFd = seccomp(SECCOMP_SET_MODE_FILTER,
>                                   SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
>            if (notifyFd == -1)
>                errExit("seccomp-install-notify-filter");
> 
>            return notifyFd;
>        }
> 
>        /* Close a pair of sockets created by socketpair() */
> 
>        static void
>        closeSocketPair(int sockPair[2])
>        {
>            if (close(sockPair[0]) == -1)
>                errExit("closeSocketPair-close-0");
>            if (close(sockPair[1]) == -1)
>                errExit("closeSocketPair-close-1");
>        }
> 
>        /* Implementation of the target process; create a child process that:
> 
>           (1) installs a seccomp filter with the
>               SECCOMP_FILTER_FLAG_NEW_LISTENER flag;
>           (2) writes the seccomp notification file descriptor returned from
>               the previous step onto the UNIX domain socket, 'sockPair[0]';
>           (3) calls mkdir(2) for each element of 'argv'.
> 
>           The function return value in the parent is the PID of the child
>           process; the child does not return from this function. */
> 
>        static pid_t
>        targetProcess(int sockPair[2], char *argv[])
>        {
>            pid_t targetPid = fork();
>            if (targetPid == -1)
>                errExit("fork");
> 
>            if (targetPid > 0)          /* In parent, return PID of child */
>                return targetPid;
> 
>            /* Child falls through to here */
> 
>            printf("T: PID = %ld\n", (long) getpid());
> 
>            /* Install seccomp filter(s) */
> 
>            if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
>                errExit("prctl");
> 
>            int notifyFd = installNotifyFilter();
> 
>            /* Pass the notification file descriptor to the tracing process over
>               a UNIX domain socket */
> 
>            if (sendfd(sockPair[0], notifyFd) == -1)
>                errExit("sendfd");
> 
>            /* Notification and socket FDs are no longer needed in target */
> 
>            if (close(notifyFd) == -1)
>                errExit("close-target-notify-fd");
> 
>            closeSocketPair(sockPair);
> 
>            /* Perform a mkdir() call for each of the command-line arguments */
> 
>            for (char **ap = argv; *ap != NULL; ap++) {
>                printf("\nT: about to mkdir(\"%s\")\n", *ap);
> 
>                int s = mkdir(*ap, 0700);
>                if (s == -1)
>                    perror("T: ERROR: mkdir(2)");
>                else
>                    printf("T: SUCCESS: mkdir(2) returned %d\n", s);
>            }
> 
>            printf("\nT: terminating\n");
>            exit(EXIT_SUCCESS);
>        }
> 
>        /* Check that the notification ID provided by a SECCOMP_IOCTL_NOTIF_RECV
>           operation is still valid. It will no longer be valid if the process
>           has terminated. This operation can be used when accessing /proc/PID
>           files in the target process in order to avoid TOCTOU race conditions
>           where the PID that is returned by SECCOMP_IOCTL_NOTIF_RECV terminates
>           and is reused by another process. */
> 
>        static void
>        checkNotificationIdIsValid(int notifyFd, uint64_t id)
>        {
>            if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_ID_VALID, &id) == -1)
>                errExit("\tS: notification ID check: "
>                        "target has terminated!!!\n");
>        }
> 
>        /* Access the memory of the target process in order to discover the
>           pathname that was given to mkdir() */
> 
>        static bool
>        getTargetPathname(struct seccomp_notif *req, int notifyFd,
>                          char *path, size_t len)
>        {
>            char procMemPath[PATH_MAX];
> 
>            snprintf(procMemPath, sizeof(procMemPath), "/proc/%d/mem", req->pid);
> 
>            int procMemFd = open(procMemPath, O_RDONLY);
>            if (procMemFd == -1)
>                errExit("\tS: open");
> 
>            /* Check that the process whose info we are accessing is still alive.
>               If the SECCOMP_IOCTL_NOTIF_ID_VALID operation (performed
>               in checkNotificationIdIsValid()) succeeds, we know that the
>               /proc/PID/mem file descriptor that we opened corresponds to the
>               process for which we received a notification. If that process
>               subsequently terminates, then read() on that file descriptor
>               will return 0 (EOF). */
> 
>            checkNotificationIdIsValid(notifyFd, req->id);
> 
>            /* Read bytes at the location containing the pathname argument
>               (i.e., the first argument) of the mkdir(2) call */
> 
>            ssize_t nread = pread(procMemFd, path, len, req->data.args[0]);
>            if (nread == -1)
>                errExit("pread");
> 
>            if (nread == 0) {
>                fprintf(stderr, "\tS: pread() of /proc/PID/mem "
>                        "returned 0 (EOF)\n");
>                exit(EXIT_FAILURE);
>            }
> 
>            if (close(procMemFd) == -1)
>                errExit("close-/proc/PID/mem");
> 
>            /* We have no guarantees about what was in the memory of the target
>               process. We therefore treat the buffer returned by pread() as
>               untrusted input. The buffer should be terminated by a null byte;
>               if not, then we will trigger an error for the target process. */
> 
>            if (strnlen(path, nread) < nread)
>                return true;
> 
>            return false;
>        }
> 
>        /* Handle notifications that arrive via the SECCOMP_RET_USER_NOTIF file
>           descriptor, 'notifyFd'. */
> 
>        static void
>        handleNotifications(int notifyFd)
>        {
>            struct seccomp_notif_sizes sizes;
>            char path[PATH_MAX];
> 
>            /* Discover the sizes of the structures that are used to receive
>               notifications and send notification responses, and allocate
>               buffers of those sizes. */
> 
>            if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) == -1)
>                errExit("\tS: seccomp-SECCOMP_GET_NOTIF_SIZES");
> 
>            struct seccomp_notif *req = malloc(sizes.seccomp_notif);
>            if (req == NULL)
>                errExit("\tS: malloc");
> 
>            /* When allocating the response buffer, we must allow for the fact
>               that the user-space binary may have been built with user-space
>               headers where 'struct seccomp_notif_resp' is bigger than the
>               response buffer expected by the (older) kernel. Therefore, we
>               allocate a buffer that is the maximum of the two sizes. This
>               ensures that if the supervisor places bytes into the response
>               structure that are past the response size that the kernel expects,
>               then the supervisor is not touching an invalid memory location. */
> 
>            size_t resp_size = sizes.seccomp_notif_resp;
>            if (sizeof(struct seccomp_notif_resp) > resp_size)
>                resp_size = sizeof(struct seccomp_notif_resp);
> 
>            struct seccomp_notif_resp *resp = malloc(resp_size);
>            if (resp == NULL)
>                errExit("\tS: malloc");
> 
>            /* Loop handling notifications */
> 
>            for (;;) {
>                /* Wait for next notification, returning info in '*req' */
> 
>                memset(req, 0, sizes.seccomp_notif);
>                if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_RECV, req) == -1) {
>                    if (errno == EINTR)
>                        continue;
>                    errExit("\tS: ioctl-SECCOMP_IOCTL_NOTIF_RECV");
>                }
> 
>                printf("\tS: got notification (ID %#llx) for PID %d\n",
>                        req->id, req->pid);
> 
>                /* The only system call that can generate a notification event
>                   is mkdir(2). Nevertheless, we check that the notified system
>                   call is indeed mkdir() as kind of future-proofing of this
>                   code in case the seccomp filter is later modified to
>                   generate notifications for other system calls. */
> 
>                if (req->data.nr != __NR_mkdir) {
>                    printf("\tS: notification contained unexpected "
>                            "system call number; bye!!!\n");
>                    exit(EXIT_FAILURE);
>                }
> 
>                bool pathOK = getTargetPathname(req, notifyFd, path,
>                                                sizeof(path));
> 
>                /* Prepopulate some fields of the response */
> 
>                resp->id = req->id;     /* Response includes notification ID */
>                resp->flags = 0;
>                resp->val = 0;
> 
>                /* If the target pathname was not valid, trigger an EINVAL error;
>                   if the directory is in /tmp, then create it on behalf of the
>                   supervisor; if the pathname starts with '.', tell the kernel
>                   to let the target process execute the mkdir(); otherwise, give
>                   an error for a directory pathname in any other location. */
> 
>                if (!pathOK) {
>                    resp->error = -EINVAL;
>                    printf("\tS: spoofing error for invalid pathname (%s)\n",
>                            strerror(-resp->error));
>                } else if (strncmp(path, "/tmp/", strlen("/tmp/")) == 0) {
>                    printf("\tS: executing: mkdir(\"%s\", %#llo)\n",
>                            path, req->data.args[1]);
> 
>                    if (mkdir(path, req->data.args[1]) == 0) {
>                        resp->error = 0;            /* "Success" */
>                        resp->val = strlen(path);   /* Used as return value of
>                                                       mkdir() in target */
>                        printf("\tS: success! spoofed return = %lld\n",
>                                resp->val);
>                    } else {
> 
>                        /* If mkdir() failed in the supervisor, pass the error
>                           back to the target */
> 
>                        resp->error = -errno;
>                        printf("\tS: failure! (errno = %d; %s)\n", errno,
>                                strerror(errno));
>                    }
>                } else if (strncmp(path, "./", strlen("./")) == 0) {
>                    resp->error = resp->val = 0;
>                    resp->flags = SECCOMP_USER_NOTIF_FLAG_CONTINUE;
>                    printf("\tS: target can execute system call\n");
>                } else {
>                    resp->error = -EOPNOTSUPP;
>                    printf("\tS: spoofing error response (%s)\n",
>                            strerror(-resp->error));
>                }
> 
>                /* Send a response to the notification */
> 
>                printf("\tS: sending response "
>                        "(flags = %#x; val = %lld; error = %d)\n",
>                        resp->flags, resp->val, resp->error);
> 
>                if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_SEND, resp) == -1) {
>                    if (errno == ENOENT)
>                        printf("\tS: response failed with ENOENT; "
>                                "perhaps target process's syscall was "
>                                "interrupted by a signal?\n");
>                    else
>                        perror("ioctl-SECCOMP_IOCTL_NOTIF_SEND");
>                }
> 
>                /* If the pathname is just "/bye", then the supervisor
>                   terminates. This allows us to see what happens if the
>                   target process makes further calls to mkdir(2). */
> 
>                if (strcmp(path, "/bye") == 0) {
>                    printf("\tS: terminating **********\n");
>                    exit(EXIT_FAILURE);
>                }
>            }
>        }
> 
>        /* Implementation of the supervisor process:
> 
>           (1) obtains the notification file descriptor from 'sockPair[1]'
>           (2) handles notifications that arrive on that file descriptor. */
> 
>        static void
>        supervisor(int sockPair[2])
>        {
>            int notifyFd = recvfd(sockPair[1]);
>            if (notifyFd == -1)
>                errExit("recvfd");
> 
>            closeSocketPair(sockPair);  /* We no longer need the socket pair */
> 
>            handleNotifications(notifyFd);
>        }
> 
>        int
>        main(int argc, char *argv[])
>        {
>            int sockPair[2];
> 
>            setbuf(stdout, NULL);
> 
>            if (argc < 2) {
>                fprintf(stderr, "At least one pathname argument is required\n");
>                exit(EXIT_FAILURE);
>            }
> 
>            /* Create a UNIX domain socket that is used to pass the seccomp
>               notification file descriptor from the target process to the
>               supervisor process. */
> 
>            if (socketpair(AF_UNIX, SOCK_STREAM, 0, sockPair) == -1)
>                errExit("socketpair");
> 
>            /* Create a child process--the "target"--that installs seccomp
>               filtering. The target process writes the seccomp notification
>               file descriptor onto 'sockPair[0]' and then calls mkdir(2) for
>               each directory in the command-line arguments. */
> 
>            (void) targetProcess(sockPair, &argv[optind]);
> 
>            /* Catch SIGCHLD when the target terminates, so that the
>               supervisor can also terminate. */
> 
>            struct sigaction sa;
>            sa.sa_handler = sigchldHandler;
>            sa.sa_flags = 0;
>            sigemptyset(&sa.sa_mask);
>            if (sigaction(SIGCHLD, &sa, NULL) == -1)
>                errExit("sigaction");
> 
>            supervisor(sockPair);
> 
>            exit(EXIT_SUCCESS);
>        }
> 
> SEE ALSO
>        ioctl(2), seccomp(2)
> 
>        A further example program can be found in the kernel source file
>        samples/seccomp/user-trap.c.
> 
> Linux                          2020-10-01          SECCOMP_USER_NOTIF(2)
> 
> 
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/

Thanks Michael!