[PATCH 2/8] signal: Drop signals received after a fatal signal has been processed

Eric W. Biederman
2021-12-13T22:54:44Z

In 403bad72b67d ("coredump: only SIGKILL should interrupt the
coredumping task") Oleg modified the kernel to drop all signals that
come in during a coredump except SIGKILL, and suggested that it might
be a good idea to generalize that to other cases after the process has
received a fatal signal.

Semantically it does not make sense to perform any signal delivery
after the process has already been killed.

When a signal is sent while a process is dying today the signal is
placed in the signal queue by __send_signal and a single task of the
process is woken up with signal_wake_up, if there are any tasks that
have not set PF_EXITING.

Take things one step farther and have prepare_signal report that all
signals that come after a process has been killed should be ignored.
While retaining the historical exception of allowing SIGKILL to
interrupt coredumps.

Remove the SIGNAL_GROUP_EXIT test from complete_signal, as it is no
longer possible for signal processing to reach complete_signal when
SIGNAL_GROUP_EXIT is true.

Update the comment in fs/coredump.c to make it clear coredumps are
special in being able to receive SIGKILL.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/coredump.c | 2 +-
kernel/signal.c | 5 ++---
2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 7b91fb32dbb8..a9c25f20118f 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -352,7 +352,7 @@ static int zap_process(struct task_struct *start, int exit_code, int flags)
struct task_struct *t;
int nr = 0;

- /* ignore all signals except SIGKILL, see prepare_signal() */
+ /* Allow SIGKILL, see prepare_signal() */
start->signal->flags = SIGNAL_GROUP_COREDUMP | flags;
start->signal->group_exit_code = exit_code;
start->signal->group_stop_count = 0;
diff --git a/kernel/signal.c b/kernel/signal.c
index 7e305a8ec7c2..cdccbacac685 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -914,11 +914,11 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force)
sigaddset(&dumper->pending.signal, SIGKILL);
signal_wake_up(dumper, 1);
}
- return false;
}
/*
- * The process is in the middle of dying, nothing to do.
+ * The process is in the middle of dying, drop the signal.
*/
+ return false;
} else if (sig_kernel_stop(sig)) {
/*
* This is a stop signal. Remove SIGCONT from all queues.
@@ -1039,7 +1039,6 @@ static void complete_signal(int sig, struct task_struct *p, enum pid_type type)
* then start taking the whole group down immediately.
*/
if (sig_fatal(p, sig) &&
- !(signal->flags & SIGNAL_GROUP_EXIT) &&
!sigismember(&t->real_blocked, sig) &&
(sig == SIGKILL || !p->ptrace)) {
/*
--
2.29.2