exec: rework the group exit and fix the race with kill

As Roland pointed out, we have the very old problem with exec.  de_thread()
sets SIGNAL_GROUP_EXIT, kills other threads, changes ->group_leader and then
clears signal->flags.  All signals (even fatal ones) sent in this window
(which is not too small) will be lost.

With this patch exec doesn't abuse SIGNAL_GROUP_EXIT.  signal_group_exit(),
the new helper, should be used to detect exit_group() or exec() in progress.
It can have more users, but this patch does only strictly necessary changes.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Robin Holt <holt@sgi.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 15 insertions, 12 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 966c5c5b6741..be923e4bc389 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -760,7 +760,7 @@ static int de_thread(struct task_struct *tsk)
          */
         read_lock(&tasklist_lock);
         spin_lock_irq(lock);
-        if (sig->flags & SIGNAL_GROUP_EXIT) {
+        if (signal_group_exit(sig)) {
                 /*
                  * Another group action in progress, just
                  * return so that the signal is processed.
@@ -778,6 +778,7 @@ static int de_thread(struct task_struct *tsk)
         if (unlikely(tsk->group_leader == task_child_reaper(tsk)))
                 task_active_pid_ns(tsk)->child_reaper = tsk;
 
+        sig->group_exit_task = tsk;
         zap_other_threads(tsk);
         read_unlock(&tasklist_lock);
 
@@ -802,7 +803,6 @@ static int de_thread(struct task_struct *tsk)
         }
 
         sig->notify_count = count;
-        sig->group_exit_task = tsk;
         while (atomic_read(&sig->count) > count) {
                 __set_current_state(TASK_UNINTERRUPTIBLE);
                 spin_unlock_irq(lock);
@@ -871,15 +871,10 @@ static int de_thread(struct task_struct *tsk)
                 leader->exit_state = EXIT_DEAD;
 
                 write_unlock_irq(&tasklist_lock);
-        }
+        }
 
         sig->group_exit_task = NULL;
         sig->notify_count = 0;
-        /*
-         * There may be one thread left which is just exiting,
-         * but it's safe to stop telling the group to kill themselves.
-         */
-        sig->flags = 0;
 
 no_thread_group:
         exit_itimers(sig);
@@ -1549,7 +1544,7 @@ static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
         int err = -EAGAIN;
 
         spin_lock_irq(&tsk->sighand->siglock);
-        if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT)) {
+        if (!signal_group_exit(tsk->signal)) {
                 tsk->signal->group_exit_code = exit_code;
                 zap_process(tsk);
                 err = 0;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 680bb03a4b90..483ea4e1accf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -555,6 +555,13 @@ struct signal_struct {
 #define SIGNAL_STOP_CONTINUED   0x00000004 /* SIGCONT since WCONTINUED reap */
 #define SIGNAL_GROUP_EXIT       0x00000008 /* group exit in progress */
 
+/* If true, all threads except ->group_exit_task have pending SIGKILL */
+static inline int signal_group_exit(const struct signal_struct *sig)
+{
+        return  (sig->flags & SIGNAL_GROUP_EXIT) ||
+                (sig->group_exit_task != NULL);
+}
+
 /*
  * Some day this will be a full-fledged user tracking system..
  */
diff --git a/kernel/exit.c b/kernel/exit.c
index 9e459fefda77..9d3d0f0b27d9 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1083,11 +1083,12 @@ do_group_exit(int exit_code)
                 struct signal_struct *const sig = current->signal;
                 struct sighand_struct *const sighand = current->sighand;
                 spin_lock_irq(&sighand->siglock);
-                if (sig->flags & SIGNAL_GROUP_EXIT)
+                if (signal_group_exit(sig))
                         /* Another thread got here before we took the lock.  */
                         exit_code = sig->group_exit_code;
                 else {
                         sig->group_exit_code = exit_code;
+                        sig->flags = SIGNAL_GROUP_EXIT;
                         zap_other_threads(current);
                 }
                 spin_unlock_irq(&sighand->siglock);
diff --git a/kernel/signal.c b/kernel/signal.c
index 1117b28488c2..6a5f97cd337a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -957,7 +957,6 @@ void zap_other_threads(struct task_struct *p)
 {
         struct task_struct *t;
 
-        p->signal->flags = SIGNAL_GROUP_EXIT;
         p->signal->group_stop_count = 0;
 
         for (t = next_thread(p); t != p; t = next_thread(t)) {
@@ -1697,7 +1696,8 @@ static int do_signal_stop(int signr)
         } else {
                 struct task_struct *t;
 
-                if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED))
+                if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||
+                    unlikely(sig->group_exit_task))
                         return 0;
                 /*
                  * There is no group stop already in progress.