[PATCH] introduce lock_task_sighand() helper

Add lock_task_sighand() helper and converts group_send_sig_info() to use
it.  Hopefully we will have more users soon.

This patch also removes '!sighand->count' and '!p->usage' checks, I think
they both are bogus, racy and unneeded (but probably it makes sense to
restore them as BUG_ON()s).

->sighand is cleared and it's ->count is decremented in release_task() with
sighand->siglock held, so it is a bug to have '!p->usage || !->count' after
we already locked and verified it is the same.  On the other hand, an
already dead task without ->sighand can have a non-zero ->usage due to
ptrace, for example.

If we read the stale value of ->sighand we must see the change after
spin_lock(), because that change was done while holding that same old
->sighand.siglock.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 24 insertions, 14 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index b0b1ca9daa33..819fa49aa70a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1120,27 +1120,37 @@ void zap_other_threads(struct task_struct *p)
 /*
  * Must be called under rcu_read_lock() or with tasklist_lock read-held.
  */
+struct sighand_struct *lock_task_sighand(struct task_struct *tsk, unsigned long *flags)
+{
+        struct sighand_struct *sighand;
+
+        for (;;) {
+                sighand = rcu_dereference(tsk->sighand);
+                if (unlikely(sighand == NULL))
+                        break;
+
+                spin_lock_irqsave(&sighand->siglock, *flags);
+                if (likely(sighand == tsk->sighand))
+                        break;
+                spin_unlock_irqrestore(&sighand->siglock, *flags);
+        }
+
+        return sighand;
+}
+
 int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 {
         unsigned long flags;
-        struct sighand_struct *sp;
         int ret;
 
-retry:
         ret = check_kill_permission(sig, info, p);
-        if (!ret && sig && (sp = rcu_dereference(p->sighand))) {
-                spin_lock_irqsave(&sp->siglock, flags);
-                if (p->sighand != sp) {
-                        spin_unlock_irqrestore(&sp->siglock, flags);
-                        goto retry;
-                }
-                if ((atomic_read(&sp->count) == 0) ||
-                                (atomic_read(&p->usage) == 0)) {
-                        spin_unlock_irqrestore(&sp->siglock, flags);
-                        return -ESRCH;
+
+        if (!ret && sig) {
+                ret = -ESRCH;
+                if (lock_task_sighand(p, &flags)) {
+                        ret = __group_send_sig_info(sig, info, p);
+                        unlock_task_sighand(p, &flags);
                 }
-                ret = __group_send_sig_info(sig, info, p);
-                spin_unlock_irqrestore(&sp->siglock, flags);
         }
 
         return ret;