signals: move cred_guard_mutex from task_struct to signal_struct

Oleg Nesterov pointed out we have to prevent multiple-threads-inside-exec
itself and we can reuse ->cred_guard_mutex for it.  Yes, concurrent
execve() has no worth.

Let's move ->cred_guard_mutex from task_struct to signal_struct.  It
naturally prevent multiple-threads-inside-exec.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index e87aaaaf5131..3b159c5991b7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -908,6 +908,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
         sig->oom_adj = current->signal->oom_adj;
         sig->oom_score_adj = current->signal->oom_score_adj;
 
+        mutex_init(&sig->cred_guard_mutex);
+
         return 0;
 }