ptrace: selinux_bprm_committed_creds: simplify __wake_up_parent() code and s/parent/real_parent/

[Oleg Nesterov <oleg@redhat.com>]

(on top of David's "[PATCH] SELinux: Don't flush inherited SIGKILL during execve()",
 but can be applied independently).

selinux_bprm_committed_creds() should wake up ->real_parent, not ->parent.

We shouldn't worry about the tracer if current is ptraced, exec() must not
succeed if the tracer has no rights to trace this task after cred changing.
But we should notify ->real_parent which is, well, real parent.

Also, we don't need _irq to take tasklist, and we don't need parent's
->siglock to wake_up_interruptible(real_parent->signal->wait_chldexit).
Since we hold tasklist, real_parent->signal must be stable. Otherwise
spin_lock(siglock) is not safe too and can't help anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

--- PTRACE/security/selinux/hooks.c~SELINUX_PARENT	2009-04-29 15:45:28.000000000 +0200
+++ PTRACE/security/selinux/hooks.c	2009-04-29 15:48:41.000000000 +0200
@@ -2375,10 +2375,8 @@ static void selinux_bprm_committed_creds
 {
 	const struct task_security_struct *tsec = current_security();
 	struct itimerval itimer;
-	struct sighand_struct *psig;
 	u32 osid, sid;
 	int rc, i;
-	unsigned long flags;
 
 	osid = tsec->osid;
 	sid = tsec->sid;
@@ -2409,12 +2407,9 @@ static void selinux_bprm_committed_creds
 
 	/* Wake up the parent if it is waiting so that it can recheck
 	 * wait permission to the new task SID. */
-	read_lock_irq(&tasklist_lock);
-	psig = current->parent->sighand;
-	spin_lock_irqsave(&psig->siglock, flags);
-	wake_up_interruptible(&current->parent->signal->wait_chldexit);
-	spin_unlock_irqrestore(&psig->siglock, flags);
-	read_unlock_irq(&tasklist_lock);
+	read_lock(&tasklist_lock);
+	wake_up_interruptible(&current->real_parent->signal->wait_chldexit);
+	read_unlock(&tasklist_lock);
 }
 
 /* superblock security operations */