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Subject[PATCH v4] kernel/signal: Signal-based pre-coredump notification
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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>
---
v3 -> v4:

Addressed review comments from Oleg Nesterov, and Eric W. Biederman,
including:
o remove the definition CLD_PREDUMP.
o code simplification.
o split out the selftest code.

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..22c40dc 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;
+
+ read_lock(&tasklist_lock);
+ parent = current->parent;
+ sig = parent->signal->predump_signal;
+ if (sig != 0)
+ do_send_sig_info(sig, SEND_SIG_NOINFO, parent, PIDTYPE_TGID);
+ read_unlock(&tasklist_lock);
+}
+
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;
--
1.8.3.1
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