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Subject[72/73] ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE race
3.0-stable review patch.  If anyone has any objections, please let me know.

------------------
Content-Length: 2354
Lines: 86
From: Oleg Nesterov <oleg@redhat.com>

commit 50b8d257486a45cba7b65ca978986ed216bbcc10 upstream.

Test-case:

int main(void)
{
int pid, status;
pid = fork();
if (!pid) {
for (;;) {
if (!fork())
return 0;
if (waitpid(-1, &status, 0) < 0) {
printf("ERR!! wait: %m\n");
return 0;
}
}
}
assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0);
assert(waitpid(-1, NULL, 0) == pid);

assert(ptrace(PTRACE_SETOPTIONS, pid, 0,
PTRACE_O_TRACEFORK) == 0);

do {
ptrace(PTRACE_CONT, pid, 0, 0);
pid = waitpid(-1, NULL, 0);
} while (pid > 0);
return 1;
}
It fails because ->real_parent sees its child in EXIT_DEAD state
while the tracer is going to change the state back to EXIT_ZOMBIE
in wait_task_zombie().

The offending commit is 823b018e which moved the EXIT_DEAD check,
but in fact we should not blame it. The original code was not
correct as well because it didn't take ptrace_reparented() into
account and because we can't really trust ->ptrace.

This patch adds the additional check to close this particular
race but it doesn't solve the whole problem. We simply can't
rely on ->ptrace in this case, it can be cleared if the tracer
is multithreaded by the exiting ->parent.

I think we should kill EXIT_DEAD altogether, we should always
remove the soon-to-be-reaped child from ->children or at least
we should never do the DEAD->ZOMBIE transition. But this is too
complex for 3.2.

Reported-and-tested-by: Denys Vlasenko <vda.linux@googlemail.com>
Tested-by: Lukasz Michalik <lmi@ift.uni.wroc.pl>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/exit.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1553,8 +1553,15 @@ static int wait_consider_task(struct wai
}

/* dead body doesn't have much to contribute */
- if (p->exit_state == EXIT_DEAD)
+ if (unlikely(p->exit_state == EXIT_DEAD)) {
+ /*
+ * But do not ignore this task until the tracer does
+ * wait_task_zombie()->do_notify_parent().
+ */
+ if (likely(!ptrace) && unlikely(ptrace_reparented(p)))
+ wo->notask_error = 0;
return 0;
+ }

/* slay zombie? */
if (p->exit_state == EXIT_ZOMBIE) {



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