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Subject: Re: [PATCH v5] kernel/signal: Signal-based pre-coredump notification
To:     Oleg Nesterov <oleg@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Khalid Aziz <khalid.aziz@oracle.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Helge Deller <deller@gmx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian@brauner.io>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Roman Gushchin <guro@fb.com>,
        Marcos Paulo de Souza <marcos.souza.org@gmail.com>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Jann Horn <jannh@google.com>, Kees Cook <keescook@chromium.org>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org,
        "Victor Kamensky (kamensky)" <kamensky@cisco.com>,
        xe-linux-external@cisco.com, Stefan Strogin <sstrogin@cisco.com>,
        Enke Chen <enkechen@cisco.com>
References: <e7ed306f-8992-9d00-bcab-5131159e8d89@cisco.com>
 <458c04d8-d189-4a26-729a-bb1d1d751534@cisco.com>
 <7741efa7-a3f8-62a1-ba52-613883164643@cisco.com>
 <84460a77-a111-404e-4bad-88104a6e246e@cisco.com>
 <20181026082812.GA10581@redhat.com>
 <21f678a8-4001-df36-c26e-e96cf203b1b1@cisco.com>
 <20181029111804.GA24820@redhat.com>
 <0c197608-3b7e-ffd1-8943-801a60beb917@cisco.com>
From:   Enke Chen <enkechen@cisco.com>
Message-ID: <ed8e0231-2b28-7cc0-4ae4-7cfcae3e0865@cisco.com>
Date:   Mon, 12 Nov 2018 15:22:07 -0800
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Hi, Folks:

Could you please take care of this patch [PATCH 5]?

Thanks.  -- Enke

On 10/29/18 3:31 PM, Enke Chen wrote:
> For simplicity and consistency, this patch provides an implementation
> for signal-based fault notification prior to the coredump of a child
> process. A new prctl command, PR_SET_PREDUMP_SIG, is defined that can
> be used by an application to express its interest and to specify the
> signal for such a notification.
> 
> Changes to prctl(2):
> 
>    PR_SET_PREDUMP_SIG (since Linux 4.20.x)
>           Set the child pre-coredump signal of the calling process to
>           arg2 (either a signal value in the range 1..maxsig, or 0 to
>           clear). This is the signal that the calling process will get
>           prior to the coredump of a child process. This value is
>           cleared across execve(2), or for the child of a fork(2).
> 
>    PR_GET_PREDUMP_SIG (since Linux 4.20.x)
>           Return the current value of the child pre-coredump signal,
>           in the location pointed to by (int *) arg2.
> 
> Background:
> 
> As the coredump of a process may take time, in certain time-sensitive
> applications it is necessary for a parent process (e.g., a process
> manager) to be notified of a child's imminent death before the coredump
> so that the parent process can act sooner, such as re-spawning an
> application process, or initiating a control-plane fail-over.
> 
> One application is BFD. The early fault notification is a critical
> component for maintaining BFD sessions (with a timeout value of
> 50 msec or 100 msec) across a control-plane failure.
> 
> Currently there are two ways for a parent process to be notified of a
> child process's state change. One is to use the POSIX signal, and
> another is to use the kernel connector module. The specific events and
> actions are summarized as follows:
> 
> Process Event    POSIX Signal                Connector-based
> ----------------------------------------------------------------------
> ptrace_attach()  do_notify_parent_cldstop()  proc_ptrace_connector()
>                  SIGCHLD / CLD_STOPPED
> 
> ptrace_detach()  do_notify_parent_cldstop()  proc_ptrace_connector()
>                  SIGCHLD / CLD_CONTINUED
> 
> pre_coredump/    N/A                         proc_coredump_connector()
> get_signal()
> 
> post_coredump/   do_notify_parent()          proc_exit_connector()
> do_exit()        SIGCHLD / exit_signal
> ----------------------------------------------------------------------
> 
> As shown in the table, the signal-based pre-coredump notification is not
> currently available. In some cases using a connector-based notification
> can be quite complicated (e.g., when a process manager is written in shell
> scripts and thus is subject to certain inherent limitations), and a
> signal-based notification would be simpler and better suited.
> 
> Signed-off-by: Enke Chen <enkechen@cisco.com>
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> ---
> v4 -> v5:
> Addressed review comments from Oleg Nesterov:
> o use rcu_read_lock instead.
> o revert back to notify the real_parent.
> 
>  fs/coredump.c                | 23 +++++++++++++++++++++++
>  fs/exec.c                    |  3 +++
>  include/linux/sched/signal.h |  3 +++
>  include/uapi/linux/prctl.h   |  4 ++++
>  kernel/sys.c                 | 13 +++++++++++++
>  5 files changed, 46 insertions(+)
> 
> diff --git a/fs/coredump.c b/fs/coredump.c
> index e42e17e..740b1bb 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -536,6 +536,24 @@ static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
>  	return err;
>  }
>  
> +/*
> + * While do_notify_parent() notifies the parent of a child's death post
> + * its coredump, this function lets the parent (if so desired) know about
> + * the imminent death of a child just prior to its coredump.
> + */
> +static void do_notify_parent_predump(void)
> +{
> +	struct task_struct *parent;
> +	int sig;
> +
> +	rcu_read_lock();
> +	parent = rcu_dereference(current->real_parent);
> +	sig = parent->signal->predump_signal;
> +	if (sig != 0)
> +		do_send_sig_info(sig, SEND_SIG_NOINFO, parent, PIDTYPE_TGID);
> +	rcu_read_unlock();
> +}
> +
>  void do_coredump(const kernel_siginfo_t *siginfo)
>  {
>  	struct core_state core_state;
> @@ -590,6 +608,11 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>  	if (retval < 0)
>  		goto fail_creds;
>  
> +	/*
> +	 * Send the pre-coredump signal to the parent if requested.
> +	 */
> +	do_notify_parent_predump();
> +
>  	old_cred = override_creds(cred);
>  
>  	ispipe = format_corename(&cn, &cprm);
> diff --git a/fs/exec.c b/fs/exec.c
> index fc281b7..7714da7 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1181,6 +1181,9 @@ static int de_thread(struct task_struct *tsk)
>  	/* we have changed execution domain */
>  	tsk->exit_signal = SIGCHLD;
>  
> +	/* Clear the pre-coredump signal before loading a new binary */
> +	sig->predump_signal = 0;
> +
>  #ifdef CONFIG_POSIX_TIMERS
>  	exit_itimers(sig);
>  	flush_itimer_signals();
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 13789d1..728ef68 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -112,6 +112,9 @@ struct signal_struct {
>  	int			group_stop_count;
>  	unsigned int		flags; /* see SIGNAL_* flags below */
>  
> +	/* The signal sent prior to a child's coredump */
> +	int			predump_signal;
> +
>  	/*
>  	 * PR_SET_CHILD_SUBREAPER marks a process, like a service
>  	 * manager, to re-parent orphan (double-forking) child processes
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index c0d7ea0..79f0a8a 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -219,4 +219,8 @@ struct prctl_mm_map {
>  # define PR_SPEC_DISABLE		(1UL << 2)
>  # define PR_SPEC_FORCE_DISABLE		(1UL << 3)
>  
> +/* Whether to receive signal prior to child's coredump */
> +#define PR_SET_PREDUMP_SIG	54
> +#define PR_GET_PREDUMP_SIG	55
> +
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 123bd73..39aa3b8 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2476,6 +2476,19 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
>  			return -EINVAL;
>  		error = arch_prctl_spec_ctrl_set(me, arg2, arg3);
>  		break;
> +	case PR_SET_PREDUMP_SIG:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		if (!valid_signal((int)arg2))
> +			return -EINVAL;
> +		me->signal->predump_signal = (int)arg2;
> +		break;
> +	case PR_GET_PREDUMP_SIG:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		error = put_user(me->signal->predump_signal,
> +				 (int __user *)arg2);
> +		break;
>  	default:
>  		error = -EINVAL;
>  		break;
>