do_wait: fix waiting for the group stop with the dead leader

do_wait(WSTOPPED) assumes that p->state must be == TASK_STOPPED, this is
not true if the leader is already dead.  Check SIGNAL_STOP_STOPPED instead
and use signal->group_exit_code.

Trivial test-case:

	void *tfunc(void *arg)
	{
		pause();
		return NULL;
	}

	int main(void)
	{
		pthread_t thr;
		pthread_create(&thr, NULL, tfunc, NULL);
		pthread_exit(NULL);
		return 0;
	}

It doesn't react to ^Z (and then to ^C or ^\). The task is stopped, but
bash can't see this.

The bug is very old, and it was reported multiple times. This patch was sent
more than a year ago (http://marc.info/?t=119713920000003) but it was ignored.

This change also fixes other oddities (but not all) in this area.  For
example, before this patch:

	$ sleep 100
	^Z
	[1]+  Stopped                 sleep 100
	$ strace -p `pidof sleep`
	Process 11442 attached - interrupt to quit

strace hangs in do_wait(), because ->exit_code was already consumed by
bash.  After this patch, strace happily proceeds:

	--- SIGTSTP (Stopped) @ 0 (0) ---
	restart_syscall(<... resuming interrupted call ...>

To me, this looks much more "natural" and correct.

Another example.  Let's suppose we have the main thread M and sub-thread
T, the process is stopped, and its parent did wait(WSTOPPED).  Now we can
ptrace T but not M.  This looks at least strange to me.

Imho, do_wait() should not confuse the per-thread ptrace stops with the
per-process job control stops.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Kaz Kylheku <kkylheku@gmail.com>
Cc: Michael Kerrisk <mtk.manpages@googlemail.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 12 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 167e1e3ad7c6..0c06b9efae3b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1417,6 +1417,18 @@ static int wait_task_zombie(struct task_struct *p, int options,
         return retval;
 }
 
+static int *task_stopped_code(struct task_struct *p, bool ptrace)
+{
+        if (ptrace) {
+                if (task_is_stopped_or_traced(p))
+                        return &p->exit_code;
+        } else {
+                if (p->signal->flags & SIGNAL_STOP_STOPPED)
+                        return &p->signal->group_exit_code;
+        }
+        return NULL;
+}
+
 /*
  * Handle sys_wait4 work for one task in state TASK_STOPPED.  We hold
  * read_lock(&tasklist_lock) on entry.  If we return zero, we still hold
@@ -1427,7 +1439,7 @@ static int wait_task_stopped(int ptrace, struct task_struct *p,
                              int options, struct siginfo __user *infop,
                              int __user *stat_addr, struct rusage __user *ru)
 {
-        int retval, exit_code, why;
+        int retval, exit_code, *p_code, why;
         uid_t uid = 0; /* unneeded, required by compiler */
         pid_t pid;
 
@@ -1437,22 +1449,16 @@ static int wait_task_stopped(int ptrace, struct task_struct *p,
         exit_code = 0;
         spin_lock_irq(&p->sighand->siglock);
 
-        if (unlikely(!task_is_stopped_or_traced(p)))
-                goto unlock_sig;
-
-        if (!ptrace && p->signal->group_stop_count > 0)
-                /*
-                 * A group stop is in progress and this is the group leader.
-                 * We won't report until all threads have stopped.
-                 */
+        p_code = task_stopped_code(p, ptrace);
+        if (unlikely(!p_code))
                 goto unlock_sig;
 
-        exit_code = p->exit_code;
+        exit_code = *p_code;
         if (!exit_code)
                 goto unlock_sig;
 
         if (!unlikely(options & WNOWAIT))
-                p->exit_code = 0;
+                *p_code = 0;
 
         /* don't need the RCU readlock here as we're holding a spinlock */
         uid = __task_cred(p)->uid;
@@ -1608,7 +1614,7 @@ static int wait_consider_task(struct task_struct *parent, int ptrace,
          */
         *notask_error = 0;
 
-        if (task_is_stopped_or_traced(p))
+        if (task_stopped_code(p, ptrace))
                 return wait_task_stopped(ptrace, p, options,
                                          infop, stat_addr, ru);