[PATCH] pid: replace is_orphaned_pgrp with is_current_pgrp_orphaned

Every call to is_orphaned_pgrp passed in process_group(current) which is racy
with respect to another thread changing our process group.  It didn't bite us
because we were dealing with integers and the worse we would get would be a
stale answer.

In switching the checks to use struct pid to be a little more efficient and
prepare the way for pid namespaces this race became apparent.

So I simplified the calls to the more specialized is_current_pgrp_orphaned so
I didn't have to worry about making logic changes to avoid the race.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/n_tty.c

@@ -1188,7 +1188,7 @@ static int job_control(struct tty_struct *tty, struct file *file)
 			printk("read_chan: tty->pgrp <= 0!\n");
 		else if (process_group(current) != tty->pgrp) {
 			if (is_ignored(SIGTTIN) ||
-			    is_orphaned_pgrp(process_group(current)))
+			    is_current_pgrp_orphaned())
 				return -EIO;
 			kill_pg(process_group(current), SIGTTIN, 1);
 			return -ERESTARTSYS;

drivers/char/tty_io.c

@@ -1118,7 +1118,7 @@ int tty_check_change(struct tty_struct * tty)
 		return 0;
 	if (is_ignored(SIGTTOU))
 		return 0;
-	if (is_orphaned_pgrp(process_group(current)))
+	if (is_current_pgrp_orphaned())
 		return -EIO;
 	(void) kill_pg(process_group(current), SIGTTOU, 1);
 	return -ERESTARTSYS;

include/linux/tty.h

@@ -303,7 +303,7 @@ extern int tty_read_raw_data(struct tty_struct *tty, unsigned char *bufp,
 			     int buflen);
 extern void tty_write_message(struct tty_struct *tty, char *msg);
 
-extern int is_orphaned_pgrp(int pgrp);
+extern int is_current_pgrp_orphaned(void);
 extern int is_ignored(int sig);
 extern int tty_signal(int sig, struct tty_struct *tty);
 extern void tty_hangup(struct tty_struct * tty);

kernel/exit.c

@@ -229,12 +229,12 @@ static int will_become_orphaned_pgrp(struct pid *pgrp, struct task_struct *ignor
 	return ret;	/* (sighing) "Often!" */
 }
 
-int is_orphaned_pgrp(int pgrp)
+int is_current_pgrp_orphaned(void)
 {
 	int retval;
 
 	read_lock(&tasklist_lock);
-	retval = will_become_orphaned_pgrp(find_pid(pgrp), NULL);
+	retval = will_become_orphaned_pgrp(task_pgrp(current), NULL);
 	read_unlock(&tasklist_lock);
 
 	return retval;

kernel/signal.c

@@ -1909,7 +1909,7 @@ relock:
 
 				/* signals can be posted during this window */
 
-				if (is_orphaned_pgrp(process_group(current)))
+				if (is_current_pgrp_orphaned())
 					goto relock;
 
 				spin_lock_irq(&current->sighand->siglock);