exit_signal: simplify the "we have changed execution domain" logic

exit_notify() checks "tsk->self_exec_id != tsk->parent_exec_id"
to handle the "we have changed execution domain" case.

We can change do_thread() to always set ->exit_signal = SIGCHLD
and remove this check to simplify the code.

We could change setup_new_exec() instead, this looks more logical
because it increments ->self_exec_id. But note that de_thread()
already resets ->exit_signal if it changes the leader, let's keep
both changes close to each other.

Note that we change ->exit_signal lockless, this changes the rules.
Thereafter ->exit_signal is not stable under tasklist but this is
fine, the only possible change is OLDSIG -> SIGCHLD. This can race
with eligible_child() but the race is harmless. We can race with
reparent_leader() which changes our ->exit_signal in parallel, but
it does the same change to SIGCHLD.

The noticeable user-visible change is that the execing task is not
"visible" to do_wait()->eligible_child(__WCLONE) right after exec.
To me this looks more logical, and this is consistent with mt case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 6 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 752d2c0abd19..51ac4ced1313 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -827,14 +827,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
          * If the parent exec id doesn't match the exec id we saved
          * when we started then we know the parent has changed security
          * domain.
-         *
-         * If our self_exec id doesn't match our parent_exec_id then
-         * we have changed execution domain as these two values started
-         * the same after a fork.
          */
         if (thread_group_leader(tsk) && tsk->exit_signal != SIGCHLD &&
-            (tsk->parent_exec_id != tsk->real_parent->self_exec_id ||
-             tsk->self_exec_id != tsk->parent_exec_id))
+            tsk->parent_exec_id != tsk->real_parent->self_exec_id)
                 tsk->exit_signal = SIGCHLD;
 
         if (unlikely(tsk->ptrace)) {