[ 164/167] [PATCH] exit_signal: simplify the "we have changed execution domain"logic

From: Ben Hutchings
Date: Wed May 09 2012 - 02:11:49 EST

3.2-stable review patch. If anyone has any objections, please let me know.


From: Oleg Nesterov <oleg@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
fs/exec.c | 3 +++
kernel/exit.c | 7 +------
2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index b0695a9..1e94d22 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -977,6 +977,9 @@ static int de_thread(struct task_struct *tsk)
sig->notify_count = 0;

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

diff --git a/kernel/exit.c b/kernel/exit.c
index 752d2c0..51ac4ce 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)) {

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