(don't know how to test this, hopefully Roland can take a look and ack/nack)
wait_task_stopped() writes to p->exit_code before checking that this is still
safe to do. Suppose for example that the child resumed and entered do_exit()
at the time we re-acquired tasklist lock. We can see ->exit_code != 0 and
->exit_state == 0 (the window is not that small), in that case we report the
the wrong value, and confuse the subsequent wait_task_zombie() because we
clear ->exit_code.
Change the code to ensure that the child is still TASK_TRACED/TASK_STOPPED
when we play with its ->exit_code. Actually, now we return EAGAIN if the
->state is "valid" but was was changed in between, this looks like a right
thing to do.
Signed-off-by: Oleg Nesterov <[email protected]>
--- 24/kernel/exit.c~3_REAP 2007-11-16 18:18:24.000000000 +0300
+++ 24/kernel/exit.c 2007-11-16 19:31:49.000000000 +0300
@@ -1357,6 +1357,7 @@ static int wait_task_stopped(struct task
int __user *stat_addr, struct rusage __user *ru)
{
int retval, exit_code = p->exit_code;
+ long state = p->state;
pid_t pid;
if (!exit_code)
@@ -1392,26 +1393,14 @@ static int wait_task_stopped(struct task
write_lock_irq(&tasklist_lock);
/*
- * This uses xchg to be atomic with the thread resuming and setting
- * it. It must also be done with the write lock held to prevent a
- * race with the EXIT_ZOMBIE case.
+ * This uses xchg to be atomic with the thread resuming and setting it
*/
- exit_code = xchg(&p->exit_code, 0);
- if (unlikely(p->exit_state)) {
- /*
- * The task resumed and then died. Let the next iteration
- * catch it in EXIT_ZOMBIE. Note that exit_code might
- * already be zero here if it resumed and did _exit(0).
- * The task itself is dead and won't touch exit_code again;
- * other processors in this function are locked out.
- */
- p->exit_code = exit_code;
- exit_code = 0;
- }
- if (unlikely(exit_code == 0)) {
+ if (unlikely(p->state != state) ||
+ !likely(exit_code = xchg(&p->exit_code, 0))) {
/*
- * Another thread in this function got to it first, or it
- * resumed, or it resumed and then died.
+ * The task resumed and changed its state, or another thread
+ * stealed its ->exit_code. Let the next iteration inspect the
+ * child again.
*/
write_unlock_irq(&tasklist_lock);
put_task_struct(p);
-
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