Message-ID: <55AFE46F.3090800@gmail.com>
Date: Wed, 22 Jul 2015 20:43:59 +0200
From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
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To: Alexei Starovoitov <ast@plumgrid.com>,
        Daniel Borkmann <daniel@iogearbox.net>
CC: mtk.manpages@gmail.com, linux-man <linux-man@vger.kernel.org>,
        linux-kernel@vger.kernel.org, Silvan Jegen <s.jegen@gmail.com>,
        Walter Harms <wharms@bfs.de>
Subject: Draft 3 of bpf(2) man page for review
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Hello Alexei, Daniel, et al.

Please find below another draft of the bpf(2) man page.
It has gone through some fairly substantial editing since
the last draft. 

There still remain quite a number of FIXMEs. Alexei, I am mindful
that you'd like to get a version of this page released soon. With
that in mind, I've marked some of the FIXMEs as "FIXME!!". I'd
like to get most of those FIXMEs resolved before an initial release
of the page. The other FIXMEs could be resolved incrementally later.

Cheers,

Michael

.\" Copyright (C) 2015 Alexei Starovoitov <ast@kernel.org>
.\" and Copyright (C) 2015 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH BPF 2 2015-03-10 "Linux" "Linux Programmer's Manual"
.SH NAME
bpf - perform a command on an extended eBPF map or program
.SH SYNOPSIS
.nf
.B #include <linux/bpf.h>
.sp
.BI "int bpf(int cmd, union bpf_attr *attr, unsigned int size);
.SH DESCRIPTION
The
.BR bpf ()
system call performs a range of operations related to extended
Berkeley Packet Filters.
Extended BPF (or eBPF) is similar to
the original ("classic") BPF (cBPF) used to filter network packets.
For both cBPF and eBPF programs,
the kernel statically analyzes the programs before loading them,
in order to ensure that they cannot harm the running system.
.P
eBPF extends cBPF in multiple ways, including the ability to call
a fixed set of in-kernel helper functions
.\" See 'enum bpf_func_id' in include/uapi/linux/bpf.h
(via the
.B BPF_CALL
opcode extension provided by eBPF)
and access shared data structures such as eBPF maps.
.SS Extended BPF Design/Architecture
.\"
.\" FIXME In the following line, what does "different data types" mean?
.\"       Are the values in a map not just blobs?
.\" Daniel Borkman commented:
.\"     Sort of, currently, these blobs can have different sizes of keys
.\"     and values (you can even have structs as keys). For the map itself
.\"     they are treated as blob internally. However, recently, bpf tail call
.\"     got added where you can lookup another program from an array map and
.\"     call into it. Here, that particular type of map can only have entries
.\"     of type of eBPF program fd. I think, if needed, adding a paragraph to
.\"     the tail call could be done as follow-up after we have an initial man
.\"     page in the tree included.
.\"
BPF maps are a generic data structure for storage of different data types.
A user process can create multiple maps (with key/value-pairs being
opaque bytes of data) and access them via file descriptors.
eBPF programs can access maps from inside the kernel in parallel.
.\"
.\" FIXME!! What does the previous sentence mean?
.\"
.\" Isn't "from inside the kernel" redundant? (I mean: all eBPF programs
.\" are running inside the kernel, right?)
.\" And what does "in parallel" mean?
.\" Would a simpler version of this sentence be correct? As in:
.\"     "Different eBPF programs can access the same maps in parallel."
.\" ?
.\" (Actually, the page already says something like that lower down.)
.\"
It's up to the user process and eBPF program to decide what they store
inside maps.
.P
eBPF programs are similar to kernel modules.
They are loaded by the user
process and automatically unloaded when the process exits.
.\"
.\" FIXME Daniel Borkmann commented about the preceding sentence:
.\"
.\"     Generally that's true. Btw, in 4.1 kernel, tc(8) also got support for
.\"     eBPF classifier and actions, and here it's slightly different: in tc,
.\"     we load the programs, maps etc, and push down the eBPF program fd in
.\"     order to let the kernel hold reference on the program itself.
.\"     
.\"     Thus, there, the program fd that the application owns is gone when the
.\"     application terminates, but the eBPF program itself still lives on
.\"     inside the kernel.
.\"
.\" Probably something should be said about this in this man page.
.\"
Each program is a set of instructions that is safe to run until
its completion.
An in-kernel verifier statically determines that the eBPF program
terminates and is safe to execute.
During verification, the kernel increments reference counts for each of
the maps that the eBPF program uses,
so that the selected maps cannot be removed until the program is unloaded.

eBPF programs can be attached to different events.
These events can be the arrival of network packets, tracing
events, classification event by qdisc (for eBPF programs attached to a
.BR tc (8)
classifier), and other types that may be added in the future.
A new event triggers execution of the eBPF program, which
may store information about the event in eBPF maps.
Beyond storing data, eBPF programs may call a fixed set of
in-kernel helper functions.
The same eBPF program can be attached to multiple events and different
eBPF programs can access the same map:

.in +4n
.nf
tracing     tracing     tracing     packet     packet
event A     event B     event C     on eth0    on eth1
 |             |          |           |          |
 |             |          |           |          |
 --> tracing <--      tracing       socket     socket
      prog_1           prog_2       prog_3     prog_4
      |  |               |            |
   |---  -----|  |-------|           map_3
 map_1       map_2
.fi
.in
.SS Arguments
The operation to be performed by the
.BR bpf ()
system call is determined by the
.IR cmd
argument.
Each operation takes an accompanying argument,
provided via
.IR attr ,
which is a pointer to a union of type
.IR bpf_attr
(see below).
The
.I size
argument is the size of the union pointed to by
.IR attr .

The value provided in
.IR cmd
is one of the following:
.TP
.B BPF_MAP_CREATE
Create a map with and return a file descriptor that refers to the map.
.TP
.B BPF_MAP_LOOKUP_ELEM
Look up an element by key in a specified map and return its value.
.TP
.B BPF_MAP_UPDATE_ELEM
Create or update an element (key/value pair) in a specified map.
.TP
.B BPF_MAP_DELETE_ELEM
Look up and delete an element by key in a specified map.
.TP
.B BPF_MAP_GET_NEXT_KEY
Look up an element by key in a specified map and return the key
of the next element.
.TP
.B BPF_PROG_LOAD
Verify and load an eBPF program,
returning a new file descriptor associated with the program.
.P
The
.I bpf_attr
union consists of various anonymous structures that are used by different
.BR bpf ()
commands:

.in +4n
.nf
union bpf_attr {
    struct {    /* Used by BPF_MAP_CREATE */
        __u32         map_type;
        __u32         key_size;    /* size of key in bytes */
        __u32         value_size;  /* size of value in bytes */
        __u32         max_entries; /* maximum number of entries
                                      in a map */
    };

    struct {    /* Used by BPF_MAP_*_ELEM and BPF_MAP_GET_NEXT_KEY
                   commands */
        __u32         map_fd;
        __aligned_u64 key;
        union {
            __aligned_u64 value;
            __aligned_u64 next_key;
        };
        __u64         flags;
    };

    struct {    /* Used by BPF_PROG_LOAD */
        __u32         prog_type;
        __u32         insn_cnt;
        __aligned_u64 insns;      /* 'const struct bpf_insn *' */
        __aligned_u64 license;    /* 'const char *' */
        __u32         log_level;  /* verbosity level of verifier */
        __u32         log_size;   /* size of user buffer */
        __aligned_u64 log_buf;    /* user supplied 'char *'
                                     buffer */
        __u32         kern_version;
                                  /* checked when prog_type=kprobe
                                     (since Linux 4.1) */
.\"                 commit 2541517c32be2531e0da59dfd7efc1ce844644f5
    };
} __attribute__((aligned(8)));
.fi
.in
.SS eBPF maps
Maps are a generic data structure for storage of different types of data.
They allow sharing of data between eBPF kernel programs,
and also between kernel and user-space applications.

Each map type has the following attributes:

.PD 0
.IP * 3
type
.IP *
maximum number of elements
.IP *
key size in bytes
.IP *
value size in bytes
.PD
.PP
The following wrapper functions demonstrate how various
.BR bpf ()
commands can be used to access the maps.
The functions use the
.IR cmd
argument to invoke different operations.
.TP
.B BPF_MAP_CREATE
The
.B BPF_MAP_CREATE
command creates a new map,
returning a new file descriptor that refers to the map.

.in +4n
.nf
int
bpf_create_map(enum bpf_map_type map_type, int key_size,
               int value_size, int max_entries)
{
    union bpf_attr attr = {
        .map_type = map_type,
        .key_size = key_size,
        .value_size = value_size,
        .max_entries = max_entries
    };

    return bpf(BPF_MAP_CREATE, &attr, sizeof(attr));
}
.fi
.in

The new map has the type specified by
.IR map_type ,
and attributes as specified in
.IR key_size ,
.IR value_size ,
and
.IR max_entries .
.\" FIXME!! In the next sentence, what does "process-local" mean?
On success, this operation returns a process-local file descriptor.
On error, \-1 is returned and
.I errno
is set to
.BR EINVAL ,
.BR EPERM ,
or
.BR ENOMEM .

The attributes
.I key_size
and
.I value_size
will be used by the verifier during program loading to check that the program
is calling
.BR bpf_map_*_elem ()
helper functions with a correctly initialized
.I key
and to check that the program doesn't access the map element
.I value
beyond the specified
.IR value_size .
For example, when a map is created with a
.IR key_size
of 8 and the eBPF program calls

.in +4n
.nf
bpf_map_lookup_elem(map_fd, fp - 4)
.fi
.in

the program will be rejected,
since the in-kernel helper function

    bpf_map_lookup_elem(map_fd, void *key)

expects to read 8 bytes from
.I key
pointer, but
.IR "fp\ -\ 4"
.\" FIXME!! I'm lost! What is 'fp' in this context?
starting address will cause out-of-bounds stack access.

Similarly, when a map is created with a
.I value_size
of 1 and the eBPF program contains

.in +4n
.nf
value = bpf_map_lookup_elem(...);
*(u32 *) value = 1;
.fi
.in

the program will be rejected, since it accesses the
.I value
pointer beyond the specified 1 byte
.I value_size
limit.

Currently, the following values are supported for
.IR map_type :

.in +4n
.nf
enum bpf_map_type {
    BPF_MAP_TYPE_UNSPEC,  /* Reserve 0 as invalid map type */
    BPF_MAP_TYPE_HASH,
    BPF_MAP_TYPE_ARRAY,
    BPF_MAP_TYPE_PROG_ARRAY,
};
.fi
.in

.I map_type
selects one of the available map implementations in the kernel.
.\" FIXME We need an explanation of why one might choose each of
.\"       these map implementations
For all map types,
eBPF programs access maps with the same
.BR bpf_map_lookup_elem ()
and
.BR bpf_map_update_elem ()
helper functions.
Further details of the various map types are given below.
.TP
.B BPF_MAP_LOOKUP_ELEM
The
.B BPF_MAP_LOOKUP_ELEM
command looks up an element with a given
.I key
in the map referred to by the file descriptor
.IR fd .

.in +4n
.nf
int
bpf_lookup_elem(int fd, void *key, void *value)
{
    union bpf_attr attr = {
        .map_fd = fd,
        .key = ptr_to_u64(key),
        .value = ptr_to_u64(value),
    };

    return bpf(BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
}
.fi
.in

If an element is found,
the operation returns zero and stores the element's value into
.IR value ,
which must point to a buffer of
.I value_size
bytes.

If no element is found, the operation returns \-1 and sets
.I errno
to
.BR ENOENT .
.TP
.B BPF_MAP_UPDATE_ELEM
The
.B BPF_MAP_UPDATE_ELEM
command
creates or updates an element with a given
.I key/value
in the map referred to by the file descriptor
.IR fd .

.in +4n
.nf
int
bpf_update_elem(int fd, void *key, void *value, __u64 flags)
{
    union bpf_attr attr = {
        .map_fd = fd,
        .key = ptr_to_u64(key),
        .value = ptr_to_u64(value),
        .flags = flags,
    };

    return bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}
.fi
.in

The
.I flags
argument should be specified as one of the following:
.RS
.TP
.B BPF_ANY
Create a new element or update an existing element.
.TP
.B BPF_NOEXIST
Create a new element only if it did not exist.
.TP
.B BPF_EXIST
Update an existing element.
.RE
.IP
On success, the operation returns zero.
On error, \-1 is returned and
.I errno
is set to
.BR EINVAL ,
.BR EPERM ,
.BR ENOMEM ,
or
.BR E2BIG .
.B E2BIG
indicates that the number of elements in the map reached the
.I max_entries
limit specified at map creation time.
.B EEXIST
will be returned if
.I flags
specifies
.B BPF_NOEXIST
and the element with
.I key
already exists in the map.
.B ENOENT
will be returned if 
.I flags
specifies
.B BPF_EXIST
and the element with
.I key
doesn't exist in the map.
.TP
.B BPF_MAP_DELETE_ELEM
The
.B BPF_MAP_DELETE_ELEM
command
deleted the element whose key is
.I key
from the map referred to by the file descriptor
.IR fd .

.in +4n
.nf
int
bpf_delete_elem(int fd, void *key)
{
    union bpf_attr attr = {
        .map_fd = fd,
        .key = ptr_to_u64(key),
    };

    return bpf(BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));
}
.fi
.in

On success, zero is returned.
If the element is not found, \-1 is returned and
.I errno
is set to
.BR ENOENT .
.TP
.B BPF_MAP_GET_NEXT_KEY
The
.B BPF_MAP_GET_NEXT_KEY
command looks up an element by
.I key
in the map referred to by the file descriptor
.IR fd 
and sets the
.I next_key
pointer to the key of the next element.

.nf
.in +4n
int
bpf_get_next_key(int fd, void *key, void *next_key)
{
    union bpf_attr attr = {
        .map_fd = fd,
        .key = ptr_to_u64(key),
        .next_key = ptr_to_u64(next_key),
    };

    return bpf(BPF_MAP_GET_NEXT_KEY, &attr, sizeof(attr));
}
.fi
.in

If
.I key
is found, the operation returns zero and sets the
.I next_key
pointer to the key of the next element.
If
.I key
is not found, the operation returns zero and sets the
.I next_key
pointer to the key of the first element.
If
.I key
is the last element, \-1 is returned and
.I errno
is set to
.BR ENOENT .
Other possible
.I errno
values are
.BR ENOMEM ,
.BR EFAULT ,
.BR EPERM ,
and
.BR EINVAL .
This method can be used to iterate over all elements in the map.
.TP
.B close(map_fd)
Delete the map referred to by the file descriptor
.IR map_fd .
When the user-space program that created a map exits, all maps will
be deleted automatically (but see NOTES).
.\"
.SS eBPF map types
The following map types are supported:
.TP
.B BPF_MAP_TYPE_HASH
.\" commit 0f8e4bd8a1fc8c4185f1630061d0a1f2d197a475
.\" FIXME!! Please review the following list of points, which draws
.\" heavily from the commit message, but reworks the text significantly
.\" and so may have introduced errors.
Hash-table maps have the following characteristics:
.RS
.IP * 3
Maps are created and destroyed by user-space programs.
Both user-space and eBPF programs
can perform lookuo, update, and delete operations.
.IP *
The kernel takes care of allocating and freeing key/value pairs.
.IP *
The
.BR map_update_elem ()
helper with fail to insert new element when the
.I max_entries
limit is reached.
(This ensures that eBPF programs cannot exhaust memory.)
.IP *
.BR map_update_elem ()
replaces existing elements atomically.
.RE
.IP
Hash-table maps are 
optimized for speed of lookup.
.TP
.B BPF_MAP_TYPE_ARRAY
.\" commit 28fbcfa08d8ed7c5a50d41a0433aad222835e8e3
.\" FIXME!! Please review the following list of points, which draws
.\" heavily from the commit message, but reworks the text significantly
.\" and so may have introduced errors.
Array maps have the following characteristics:
.RS
.IP * 3
Optimized for fastest possible lookup.
In the future ithe verifier/JIT compiler
may recognize lookup() operations that employ a constant key
and optimize it into constant pointer.
It is possible to optimize a non-constant
key into direct pointer arithmetic as well, since pointers and
.I value_size
are constant for the life of the eBPF program.
In other words,
.BR array_map_lookup_elem ()
may be 'inlined' by the verifier/JIT compiler
while preserving concurrent access to this map from user space.
.IP *
All array elements pre-allocated and zero initialized at init time
.IP *
The key is an array index, and must be exactly four bytes.
.IP *
.BR map_delete_elem ()
fails with the error
.BR EINVAL ,
since elements cannot be deleted.
.IP *
.BR map_update_elem ()
replaces elements in an non-atomic fashion;
for atomic updates, a hash-table map should be used instead.
.RE
.IP
Among the uses for array maps are the following:
.RS
.IP * 3
As "global" eBPF variables: an array of 1 element whose key is (index) 0
and where the value is a collection of 'global' variables which
eBPF programs can use to keep state between events.
.IP *
Aggregation of tracing events into a fixed set of buckets.
.RE
.TP
.BR BPF_MAP_TYPE_PROG_ARRAY " (since Linux 4.2)"
.\" FIXME: we need documentation of BPF_MAP_TYPE_PROG_ARRAY
[To be completed]
.\"
.SS eBPF programs
The
.B BPF_PROG_LOAD
command is used to load an eBPF program into the kernel.
The return value for this command is a new file descriptor associated
with this eBPF program.

.in +4n
.nf
char bpf_log_buf[LOG_BUF_SIZE];

int
bpf_prog_load(enum bpf_prog_type prog_type,
              const struct bpf_insn *insns, int insn_cnt,
              const char *license)
{
    union bpf_attr attr = {
        .prog_type = prog_type,
        .insns = ptr_to_u64(insns),
        .insn_cnt = insn_cnt,
        .license = ptr_to_u64(license),
        .log_buf = ptr_to_u64(bpf_log_buf),
        .log_size = LOG_BUF_SIZE,
        .log_level = 1,
    };

    return bpf(BPF_PROG_LOAD, &attr, sizeof(attr));
}
.fi
.in

.I prog_type
is one of the available program types:

.in +4n
.nf
enum bpf_prog_type {
    BPF_PROG_TYPE_UNSPEC,        /* Reserve 0 as invalid
                                    program type */
    BPF_PROG_TYPE_SOCKET_FILTER,
    BPF_PROG_TYPE_KPROBE,
    BPF_PROG_TYPE_SCHED_CLS,
    BPF_PROG_TYPE_SCHED_ACT,
};
.fi
.in

For further details of eBPF program types, see below.

The remaining fields of
.I bpf_attr
are set as follows:
.IP * 3
.I insns
is an array of
.I "struct bpf_insn"
instructions.
.IP *
.I insn_cnt
is the number of instructions in the program referred to by
.IR insns .
.IP *
.I license
is a license string, which must be GPL compatible to call helper functions
marked
.IR gpl_only .
.IP *
.I log_buf
is a pointer to a caller-allocated buffer in which the in-kernel
verifier can store the verification log.
This log is a multi-line string that can be checked by
the program author in order to understand how the verifier came to
the conclusion that the BPF program is unsafe.
The format of the output can change at any time as the verifier evolves.
.IP *
.I log_size
size of the buffer pointed to by
.IR log_bug .
If the size of the buffer is not large enough to store all
verifier messages, \-1 is returned and
.I errno
is set to
.BR ENOSPC .
.IP *
.I log_level
verbosity level of the verifier.
A value of zero means that the verifier will
not provide a log.
.P
Applying
.BR close (2)
to the file descriptor returned by
.B BPF_PROG_LOAD
will unload the eBPF program (but see NOTES).

Maps are accessible from eBPF programs and are used to exchange data between
eBPF programs and between eBPF programs and user-space programs.
For example,
eBPF programs can process various events (like kprobe, packets) and
store their data into a map,
and user-space programs can then fetch data from the map.
Conversely, user-space programs can use a map as a configuration mechanism,
populating the map with values checked by the eBPF program,
which then modifies its behavior on the fly according to those values.
.SS eBPF program types
By picking
.IR prog_type ,
the program author selects a set of helper functions that can be called from
the eBPF program and the corresponding format of
.I struct bpf_context
(which is the data blob passed into the eBPF program as the first argument).
For example, programs loaded with a
.I prog_type
of
.B BPF_PROG_TYPE_SOCKET_FILTER
may call the
.BR bpf_map_lookup_elem ()
helper,
whereas some other program types may not be able to employ this helper.
The set of functions available to eBPF programs of a given type may increase
in the future.

The following program types are supported:
.TP
.BR BPF_PROG_TYPE_SOCKET_FILTER " (since Linux 3.19)"
Currently, the set of functions for
.B BPF_PROG_TYPE_SOCKET_FILTER
is:

.in +4n
.nf
bpf_map_lookup_elem(map_fd, void *key)
                    /* look up key in a map_fd */
bpf_map_update_elem(map_fd, void *key, void *value)
                    /* update key/value */
bpf_map_delete_elem(map_fd, void *key)
                    /* delete key in a map_fd */
.fi
.in

.\" FIXME The following paragraph needs amending. Alexei commented:
.\"
.\"     Actually now in case of SOCKET_FILTER, SCHED_CLS, SCHED_ACT
.\"     the program can now access skb fields.
.\"     See 'struct __sk_buff' and commit 9bac3d6d548e5
.\"
.\" Do we want some text here to explain how the program access __sk_buff?
The
.I bpf_context
argument is a pointer to a
.IR "struct sk_buff" .
Programs cannot access the fields of
.I sk_buff
directly.
.\"
.TP
.BR BPF_PROG_TYPE_KPROBE " (since Linux 4.1)
.\" commit 2541517c32be2531e0da59dfd7efc1ce844644f5
[To be documented]
.\" FIXME Document this program type
.\"	  Describe allowed helper functions for this program type
.\"	  Describe bpf_context for this program type
.\" FIXME We need text here to describe 'kern_version'
.TP
.BR BPF_PROG_TYPE_SCHED_CLS " (since Linux 4.1)
.\" commit 96be4325f443dbbfeb37d2a157675ac0736531a1
.\" commit e2e9b6541dd4b31848079da80fe2253daaafb549
[To be documented]
.\" FIXME Document this program type
.\"	  Describe allowed helper functions for this program type
.\"	  Describe bpf_context for this program type
.TP
.BR BPF_PROG_TYPE_SCHED_ACT " (since Linux 4.1)
.\" commit 94caee8c312d96522bcdae88791aaa9ebcd5f22c
.\" commit a8cb5f556b567974d75ea29c15181c445c541b1f
[To be documented]
.\" FIXME Document this program type
.\"	  Describe allowed helper functions for this program type
.\"	  Describe bpf_context for this program type
.SS Events
Once a program is loaded, it can be attached to an event.
Various kernel subsystems have different ways to do so.

Since Linux 3.19,
.\" commit 89aa075832b0da4402acebd698d0411dcc82d03e
the following call will attach the program
.I prog_fd
to the socket
.IR sockfd ,
which was created by an earlier call to
.BR socket (2):

.in +4n
.nf
setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_BPF,
           &prog_fd, sizeof(prog_fd));
.fi
.in

Since Linux 4.1,
.\" commit 2541517c32be2531e0da59dfd7efc1ce844644f5
the following call may be used to attach
the eBPF program referred to by the file descriptor
.I prog_fd
to a perf event file descriptor,
.IR event_fd ,
that was created by a previous call to
.BR perf_event_open (2):

.in +4n
.nf
ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
.fi
.in
.\"
.\"
.SH EXAMPLES
.nf
/* bpf+sockets example:
 * 1. create array map of 256 elements
 * 2. load program that counts number of packets received
 *    r0 = skb->data[ETH_HLEN + offsetof(struct iphdr, protocol)]
 *    map[r0]++
 * 3. attach prog_fd to raw socket via setsockopt()
 * 4. print number of received TCP/UDP packets every second
 */
int
main(int argc, char **argv)
{
    int sock, map_fd, prog_fd, key;
    long long value = 0, tcp_cnt, udp_cnt;

    map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(key),
                            sizeof(value), 256);
    if (map_fd < 0) {
        printf("failed to create map '%s'\\n", strerror(errno));
        /* likely not run as root */
        return 1;
    }

    struct bpf_insn prog[] = {
        BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),        /* r6 = r1 */
        BPF_LD_ABS(BPF_B, ETH_HLEN + offsetof(struct iphdr, protocol)),
                                /* r0 = ip->proto */
        BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -4),
                                /* *(u32 *)(fp - 4) = r0 */
        BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),       /* r2 = fp */
        BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4),      /* r2 = r2 - 4 */
        BPF_LD_MAP_FD(BPF_REG_1, map_fd),           /* r1 = map_fd */
        BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem),
                                /* r0 = map_lookup(r1, r2) */
        BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
                                /* if (r0 == 0) goto pc+2 */
        BPF_MOV64_IMM(BPF_REG_1, 1),                /* r1 = 1 */
        BPF_XADD(BPF_DW, BPF_REG_0, BPF_REG_1, 0, 0),
                                /* lock *(u64 *) r0 += r1 */
.\"                                == atomic64_add
        BPF_MOV64_IMM(BPF_REG_0, 0),                /* r0 = 0 */
        BPF_EXIT_INSN(),                            /* return r0 */
    };

    prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog,
                            sizeof(prog), "GPL");

    sock = open_raw_sock("lo");

    assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd,
                      sizeof(prog_fd)) == 0);

    for (;;) {
        key = IPPROTO_TCP;
        assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
        key = IPPROTO_UDP
        assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
        printf("TCP %lld UDP %lld packets\n", tcp_cnt, udp_cnt);
        sleep(1);
    }

    return 0;
}
.fi

Some complete working code can be found in the
.IR samples/bpf
directory in the kernel source tree.
.SH RETURN VALUE
For a successful call, the return value depends on the operation:
.TP
.B BPF_MAP_CREATE
The new file descriptor associated with the eBPF map.
.TP
.B BPF_PROG_LOAD
The new file descriptor associated with the eBPF program.
.TP
All other commands
Zero.
.PP
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EPERM
The call was made without sufficient privilege
(without the
.B CAP_SYS_ADMIN
capability).
.TP
.B ENOMEM
Cannot allocate sufficient memory.
.TP
.B EBADF
.I fd
is not an open file descriptor
.TP
.B EFAULT
One of the pointers
.RI ( key
or
.I value
or
.I log_buf
or
.IR insns )
is outside the accessible address space.
.TP
.B EINVAL
The value specified in
.I cmd
is not recognized by this kernel.
.TP
.B EINVAL
For
.BR BPF_MAP_CREATE ,
either
.I map_type
or attributes are invalid.
.TP
.B EINVAL
For
.BR BPF_MAP_*_ELEM
commands,
some of the fields of
.I "union bpf_attr"
that are not used by this command
are not set to zero.
.TP
.B EINVAL
For
.BR BPF_PROG_LOAD,
indicates an attempt to load an invalid program.
BPF programs can be deemed
einvalid due to unrecognized instructions, the use of reserved fields, jumps
out of range, infinite loops or calls of unknown functions.
.TP
.BR EACCES
For
.BR BPF_PROG_LOAD,
even though all program instructions are valid, the program has been
rejected because it was deemed unsafe.
This may be because it may have
accessed a disallowed memory region or an uninitialized stack/register or
because the function constraints don't match the actual types or because
there was a misaligned memory access.
In this case, it is recommended to call
.BR bpf ()
again with
.I log_level = 1
and examine
.I log_buf
for the specific reason provided by the verifier.
.TP
.BR ENOENT
For
.B BPF_MAP_LOOKUP_ELEM
or
.BR BPF_MAP_DELETE_ELEM ,
indicates that the element with the given
.I key
was not found.
.TP
.BR E2BIG
The BPF program is too large or a map reached the
.I max_entries
limit (maximum number of elements).
.SH VERSIONS
The
.BR bpf ()
system call first appeared in Linux 3.18.
.SH CONFORMING TO
The
.BR bpf ()
system call is Linux-specific.
.SH NOTES
In the current implementation, all
.BR bpf ()
commands require the caller to have the
.B CAP_SYS_ADMIN
capability.

.\" FIXME!! Alexei, is the following correct?
eBPF objects (maps and programs) can be shared between processes.
For example, after
.BR fork (2),
the child inherits file descriptors referring to the same eBPF objects.
In addition, file descriptors referring to eBPF objects can be
transferred over UNIX domain sockets.
File descriptors referring to eBPF objects can be duplicated
in the usual way, using
.BR dup (2)
and similar calls.
An eBPF object is deallocated only after all file descriptors
referring to the object have been closed.

eBPF programs can be written in a restricted C that is compiled (using the
.B clang
compiler) into eBPF bytecode and executed on the in-kernel virtual machine or
just-in-time compiled into native code.
(Various features are omitted from this restricted C, such as loops, 
global variables, variadic functions, floating-point numbers,
and passing structures as function arguments.)
Some examples can be found in the
.I samples/bpf/*_kern.c
files in the kernel source tree.
.\" There are also examples for the tc classifier, in the iproute2
.\" project, in examples/bpf
.SH SEE ALSO
.BR seccomp (2),
.BR socket (7),
.BR tc (8),
.BR tc-bpf (8)

Both classic and extended BPF are explained in the kernel source file
.IR Documentation/networking/filter.txt .

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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