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[PATCH] introduce lock_task_sighand() helper

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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>
This commit is contained in:
Oleg Nesterov 2006-03-28 16:11:13 -08:00 committed by Linus Torvalds
parent aa1757f90b
commit f63ee72e0f
2 changed files with 33 additions and 14 deletions
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9
include/linux/sched.h
View file

@ -1225,6 +1225,15 @@ static inline void task_unlock(struct task_struct *p)
spin_unlock(&p->alloc_lock);
}
extern struct sighand_struct *lock_task_sighand(struct task_struct *tsk,
unsigned long *flags);
static inline void unlock_task_sighand(struct task_struct *tsk,
unsigned long *flags)
{
spin_unlock_irqrestore(&tsk->sighand->siglock, *flags);
}
#ifndef __HAVE_THREAD_FUNCTIONS
#define task_thread_info(task) (task)->thread_info

38
kernel/signal.c
View file

@ -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);
spin_unlock_irqrestore(&sp->siglock, flags);
unlock_task_sighand(p, &flags);
}
}
return ret;