[ 57/75] exit_signal: simplify the "we have changed execution domain" logic

Greg KH
2012-05-04T20:47:33Z

From: Oleg Nesterov <oleg@redhat.com>

commit e636825346b36a07ccfc8e30946d52855e21f681 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/exec.c | 3 +++
kernel/exit.c | 7 +------
2 files changed, 4 insertions(+), 6 deletions(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -975,6 +975,9 @@ static int de_thread(struct task_struct
sig->notify_count = 0;

no_thread_group:
+ /* we have changed execution domain */
+ tsk->exit_signal = SIGCHLD;
+
if (current->mm)
setmax_mm_hiwater_rss(&sig->maxrss, current->mm);

--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -827,14 +827,9 @@ static void exit_notify(struct task_stru
* 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)) {