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Make sure "user->sigpending" count is in sync

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The previous commit (45c18b0bb5, aka "Fix
unlikely (but possible) race condition on task->user access") fixed a
potential oops due to __sigqueue_alloc() getting its "user" pointer out
of sync with switch_user(), and accessing a user pointer that had been
de-allocated on another CPU.

It still left another (much less serious) problem, where a concurrent
__sigqueue_alloc and swich_user could cause sigqueue_alloc to do signal
pending reference counting for a _different_ user than the one it then
actually ended up using.  No oops, but we'd end up with the wrong signal
accounting.

Another case of Oleg's eagle-eyes picking up the problem.

This is trivially fixed by just making sure we load whichever "user"
structure we decide to use (it doesn't matter _which_ one we pick, we
just need to pick one) just once.

Acked-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andrew Morton <akpm@osdl.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Linus Torvalds 2006-11-04 13:03:00 -08:00
parent 45c18b0bb5
commit 10b1fbdb0a
1 changed files with 11 additions and 4 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

15
kernel/signal.c
View file

@ -267,18 +267,25 @@ static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
int override_rlimit)
{
struct sigqueue *q = NULL;
struct user_struct *user;
atomic_inc(&t->user->sigpending);
/*
* In order to avoid problems with "switch_user()", we want to make
* sure that the compiler doesn't re-load "t->user"
*/
user = t->user;
barrier();
atomic_inc(&user->sigpending);
if (override_rlimit ||
atomic_read(&t->user->sigpending) <=
atomic_read(&user->sigpending) <=
t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
q = kmem_cache_alloc(sigqueue_cachep, flags);
if (unlikely(q == NULL)) {
atomic_dec(&t->user->sigpending);
atomic_dec(&user->sigpending);
} else {
INIT_LIST_HEAD(&q->list);
q->flags = 0;
q->user = get_uid(t->user);
q->user = get_uid(user);
}
return(q);
}