wait: WSTOPPED|WCONTINUED doesn't work if a zombie leader is traced by another process
authorOleg Nesterov <oleg@redhat.com>
Mon, 7 Apr 2014 22:38:49 +0000 (15:38 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Mon, 7 Apr 2014 23:36:06 +0000 (16:36 -0700)
Even if the main thread is dead the process still can stop/continue.
However, if the leader is ptraced wait_consider_task(ptrace => false)
always skips wait_task_stopped/wait_task_continued, so WSTOPPED or
WCONTINUED can never work for the natural parent in this case.

Move the "A zombie ptracee is only visible to its ptracer" check into the
"if (!delay_group_leader(p))" block.  ->notask_error is cleared by the
"fall through" code below.

This depends on the previous change, wait_task_stopped/continued must be
avoided if !delay_group_leader() and the tracer is ->real_parent.
Otherwise WSTOPPED|WEXITED could wrongly report "stopped" when the child
is already dead (single-threaded or not).  If it is traced by another task
then the "stopped" state is fine until the debugger detaches and reveals a
zombie state.

Stupid test-case:

void *tfunc(void *arg)
{
sleep(1); // wait for zombie leader
raise(SIGSTOP);
exit(0x13);
return NULL;
}

int run_child(void)
{
pthread_t thread;

if (!fork()) {
int tracee = getppid();

assert(ptrace(PTRACE_ATTACH, tracee, 0,0) == 0);
do
ptrace(PTRACE_CONT, tracee, 0,0);
while (wait(NULL) > 0);

return 0;
}

sleep(1); // wait for PTRACE_ATTACH
assert(pthread_create(&thread, NULL, tfunc, NULL) == 0);
pthread_exit(NULL);
}

int main(void)
{
int child, stat;

child = fork();
if (!child)
return run_child();

assert(child == waitpid(-1, &stat, WSTOPPED));
assert(stat == 0x137f);

kill(child, SIGCONT);

assert(child == waitpid(-1, &stat, WCONTINUED));
assert(stat == 0xffff);

assert(child == waitpid(-1, &stat, 0));
assert(stat == 0x1300);

return 0;
}

Without this patch it hangs in waitpid(WSTOPPED), wait_task_stopped() is
never called.

Note: this doesn't fix all problems with a zombie delay_group_leader(),
WCONTINUED | WEXITED check is not exactly right.  debugger can't assume it
will be notified if another thread reaps the whole thread group.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Lennart Poettering <lpoetter@redhat.com>
Cc: Michal Schmidt <mschmidt@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
kernel/exit.c

index 92d38d4..6ed6a1d 100644 (file)
@@ -1380,20 +1380,16 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
 
        /* slay zombie? */
        if (p->exit_state == EXIT_ZOMBIE) {
-               /*
-                * A zombie ptracee is only visible to its ptracer.
-                * Notification and reaping will be cascaded to the real
-                * parent when the ptracer detaches.
-                */
-               if (likely(!ptrace) && unlikely(p->ptrace)) {
-                       /* it will become visible, clear notask_error */
-                       wo->notask_error = 0;
-                       return 0;
-               }
-
                /* we don't reap group leaders with subthreads */
-               if (!delay_group_leader(p))
-                       return wait_task_zombie(wo, p);
+               if (!delay_group_leader(p)) {
+                       /*
+                        * A zombie ptracee is only visible to its ptracer.
+                        * Notification and reaping will be cascaded to the
+                        * real parent when the ptracer detaches.
+                        */
+                       if (unlikely(ptrace) || likely(!p->ptrace))
+                               return wait_task_zombie(wo, p);
+               }
 
                /*
                 * Allow access to stopped/continued state via zombie by