allow_signal: kill the bogus ->mm check, add a note about CLONE_SIGHAND

allow_signal() checks ->mm == NULL.  Not sure why.  Perhaps to make sure
current is the kernel thread.  But this helper must not be used unless we
are the kernel thread, kill this check.

Also, document the fact that the CLONE_SIGHAND kthread must not use
allow_signal(), unless the caller really wants to change the parent's
->sighand->action as well.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/exit.c b/kernel/exit.c
index b6c90b5ef5094aef90b8cffa0fd3add3f2bbe80c..533e5f85669af0f1d1a8ffdd789b3706015b97b6 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -375,9 +375,8 @@ static void set_special_pids(struct pid *pid)
 }
 
 /*
- * Let kernel threads use this to say that they
- * allow a certain signal (since daemonize() will
- * have disabled all of them by default).
+ * Let kernel threads use this to say that they allow a certain signal.
+ * Must not be used if kthread was cloned with CLONE_SIGHAND.
  */
 int allow_signal(int sig)
 {
@@ -385,14 +384,14 @@ int allow_signal(int sig)
                return -EINVAL;
 
        spin_lock_irq(&current->sighand->siglock);
+       /* This is only needed for daemonize()'ed kthreads */
        sigdelset(&current->blocked, sig);
-       if (!current->mm) {
-               /* Kernel threads handle their own signals.
-                  Let the signal code know it'll be handled, so
-                  that they don't get converted to SIGKILL or
-                  just silently dropped */
-               current->sighand->action[(sig)-1].sa.sa_handler = (void __user *)2;
-       }
+       /*
+        * Kernel threads handle their own signals. Let the signal code
+        * know it'll be handled, so that they don't get converted to
+        * SIGKILL or just silently dropped.
+        */
+       current->sighand->action[(sig)-1].sa.sa_handler = (void __user *)2;
        recalc_sigpending();
        spin_unlock_irq(&current->sighand->siglock);
        return 0;