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

[Oleg Nesterov]

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 <o...@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

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