[PATCH 2/2] wait: WSTOPPED|WCONTINUED doesn't work if a zombie leader is traced by another process

From: Oleg Nesterov
Date: Wed Feb 26 2014 - 11:56:32 EST


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@xxxxxxxxxx>
---
kernel/exit.c | 22 +++++++++-------------
1 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 56c5ac3..790b73c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1382,20 +1382,16 @@ 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
--
1.5.5.1


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