Re: [PATCH v5 1/2] kernel/signal: Signal-based pre-coredump notification

[Dave Martin]

On Tue, Nov 27, 2018 at 10:54:41PM +0000, Enke Chen wrote:
> [Repost as a series, as suggested by Andrew Morton]
> 
> 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.

Since this is a notification of a change of process status, would it be
more natural to send it through SIGCHLD?

As with other supplementary child status events, a flag could be added
for wait and sigaction.sa_flags to indicate whether the parent wants
this event to be reported or not.

Then a suitable CLD_XXX could be defined for this, and we could
piggyback on PR_{SET,GET}_PDEATHSIG rather than having to have something
new.

(I hadn't been watching this thread closely, so apologies if this has
been discussed already.)

> 
> Signed-off-by: Enke Chen <enkechen@xxxxxxxxx>
> Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
> 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);

Doesn't this send si_code == SI_USER.  That seems wrong: the receiving
process wouldn't not be able to distinguish a real pre-coredump
notification from a bogus one sent by kill(2) etc.

SEND_SIG_PRIV also looks wrong, because it assumes that the sender is
"the kernel" so there is no si_pid.

This may be another reason for building on top of SIGCHLD which already
has the required (but weird) "si_pid == affected process" semantics,
rather than si_pid indicating the sender.

> +	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)

glibc has

	int prctl(int option, ...);

Some prctls() police extra arguments for zeros, but this means that
the userspace caller also has to supply pointless 0 arguments.

It's debatable which is the preferred approach.  Did you have any
particular rationale for your choice here?

[...]

Cheers
---Dave