bsd_acct: plain current->real_parent access is not always safe

This is minor, but dereferencing even current real_parent is not safe on debug
kernels, since the memory, this points to, can be unmapped - RCU protection is
required.

Besides, the tgid field is deprecated and is to be replaced with task_tgid_xxx
call (the 2nd patch), so RCU will be required anyway.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/kernel/acct.c b/kernel/acct.c
index 521dfa53cb99..7ff5339a3f05 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -482,7 +482,9 @@ static void do_acct_process(struct file *file)
 #endif
 #if ACCT_VERSION==3
         ac.ac_pid = current->tgid;
-        ac.ac_ppid = current->real_parent->tgid;
+        rcu_read_lock();
+        ac.ac_ppid = rcu_dereference(current->real_parent)->tgid;
+        rcu_read_unlock();
 #endif
 
         spin_lock_irq(&current->sighand->siglock);