[PATCH 2/3] coredump: ensure that SIGKILL always kills the dumping thread

[Oleg Nesterov <oleg@redhat.com>]

prepare_signal() blesses SIGKILL sent to the dumping process but
this signal can be "lost" anyway. The problems is, complete_signal()
sees SIGNAL_GROUP_EXIT and skips the "kill them all" logic. And even
if the dumping process is single-threaded (so the target is always
"correct"), the group-wide SIGKILL is not recorded in task->pending
and thus __fatal_signal_pending() won't be true. A multi-threaded
case has even more problems.

And even ignoring all technical details, SIGNAL_GROUP_EXIT doesn't
look right to me. This coredumping process is not exiting yet, it can
do a lot of work dumping the core.

With this patch the dumping process doesn't have SIGNAL_GROUP_EXIT,
we set signal->group_exit_task instead. This makes signal_group_exit()
true and thus this should equally close the races with exit/exec/stop
but allows to kill the dumping thread reliably.

Notes:
	- It is not clear what should we do with ->group_exit_code
	  if the dumper was killed, see the next change.

	- we need more (hopefully straightforward) changes to ensure
	  that SIGKILL actually interrupts the coredump. Basically we
	  need to check __fatal_signal_pending() in dump_write() and
	  dump_seek().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/coredump.c |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 2c1ef6a..835d731 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -263,7 +263,6 @@ static int zap_process(struct task_struct *start, int exit_code)
 	struct task_struct *t;
 	int nr = 0;
 
-	start->signal->flags = SIGNAL_GROUP_EXIT;
 	start->signal->group_exit_code = exit_code;
 	start->signal->group_stop_count = 0;
 
@@ -291,8 +290,9 @@ static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 	if (!signal_group_exit(tsk->signal)) {
 		mm->core_state = core_state;
 		nr = zap_process(tsk, exit_code);
+		tsk->signal->group_exit_task = tsk;
 		/* ignore all signals except SIGKILL, see prepare_signal() */
-		tsk->signal->flags |= SIGNAL_GROUP_COREDUMP;
+		tsk->signal->flags = SIGNAL_GROUP_COREDUMP;
 		clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
 	}
 	spin_unlock_irq(&tsk->sighand->siglock);
@@ -343,6 +343,7 @@ static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 				if (unlikely(p->mm == mm)) {
 					lock_task_sighand(p, &flags);
 					nr += zap_process(p, exit_code);
+					p->signal->flags = SIGNAL_GROUP_EXIT;
 					unlock_task_sighand(p, &flags);
 				}
 				break;
@@ -394,6 +395,11 @@ static void coredump_finish(struct mm_struct *mm)
 	struct core_thread *curr, *next;
 	struct task_struct *task;
 
+	spin_lock_irq(&current->sighand->siglock);
+	current->signal->group_exit_task = NULL;
+	current->signal->flags = SIGNAL_GROUP_EXIT;
+	spin_unlock_irq(&current->sighand->siglock);
+
 	next = mm->core_state->dumper.next;
 	while ((curr = next) != NULL) {
 		next = curr->next;
-- 
1.5.5.1