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Message-ID: <20071122161500.GA5976@tv-sign.ru>
Date: Thu, 22 Nov 2007 19:15:00 +0300
From: Oleg Nesterov <oleg@...sign.ru>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@...ru>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Roland McGrath <roland@...hat.com>,
Scott James Remnant <scott@...ntu.com>,
linux-kernel@...r.kernel.org
Subject: [PATCH -mm 2/2] wait_task_stopped: simplify and fix races with SIGCONT/SIGKILL/untrace
wait_task_stopped() has multiple races with SIGCONT/SIGKILL. tasklist_lock does
not pin the child in TASK_TRACED/TASK_STOPPED stated, almost all info reported
(including exit_code) may be wrong.
In fact, the code under write_lock_irq(tasklist_lock) is not safe. The child
may be PTRACE_DETACH'ed at this time by another subthread, in that case it is
possible we are no longer its ->parent.
Change wait_task_stopped() to take ->siglock before inspecting the task. This
guarantees that the child can't resume and (for example) clear its ->exit_code,
so we don't need to use xchg(&p->exit_code) and re-check. The only exception
is ptrace_stop() which changes ->state and ->exit_code without ->siglock held
during abort. But this can only happen if both the tracer and the tracee are
dying (coredump is in progress), we don't care.
With this patch wait_task_stopped() doesn't move the child to the end of the
->parent list on success. This optimization could be restored, but in that case
we have to take write_lock(tasklist) and do some nasty checks.
Also change the do_wait() since we don't return EAGAIN any longer.
Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
--- PT/kernel/exit.c~2_wait_task_stopped 2007-11-21 17:09:41.000000000 +0300
+++ PT/kernel/exit.c 2007-11-22 18:06:01.000000000 +0300
@@ -1352,17 +1352,35 @@ static int wait_task_stopped(struct task
int noreap, struct siginfo __user *infop,
int __user *stat_addr, struct rusage __user *ru)
{
- int retval, exit_code;
+ int retval, exit_code, why;
+ uid_t uid = 0; /* unneeded, required by compiler */
pid_t pid;
- if (!p->exit_code)
- return 0;
+ exit_code = 0;
+ spin_lock_irq(&p->sighand->siglock);
+
+ if (unlikely(!is_task_stopped_or_traced(p)))
+ goto unlock_sig;
+
if (delayed_group_leader && !(p->ptrace & PT_PTRACED) &&
p->signal->group_stop_count > 0)
/*
* A group stop is in progress and this is the group leader.
* We won't report until all threads have stopped.
*/
+ goto unlock_sig;
+
+ exit_code = p->exit_code;
+ if (!exit_code)
+ goto unlock_sig;
+
+ if (!noreap)
+ p->exit_code = 0;
+
+ uid = p->uid;
+unlock_sig:
+ spin_unlock_irq(&p->sighand->siglock);
+ if (!exit_code)
return 0;
/*
@@ -1372,65 +1390,15 @@ static int wait_task_stopped(struct task
* keep holding onto the tasklist_lock while we call getrusage and
* possibly take page faults for user memory.
*/
- pid = task_pid_nr_ns(p, current->nsproxy->pid_ns);
get_task_struct(p);
+ pid = task_pid_nr_ns(p, current->nsproxy->pid_ns);
+ why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
read_unlock(&tasklist_lock);
- if (unlikely(noreap)) {
- uid_t uid = p->uid;
- int why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
-
- exit_code = p->exit_code;
- if (unlikely(!exit_code) || unlikely(p->exit_state))
- goto bail_ref;
+ if (unlikely(noreap))
return wait_noreap_copyout(p, pid, uid,
why, exit_code,
infop, ru);
- }
-
- write_lock_irq(&tasklist_lock);
-
- /*
- * This uses xchg to be atomic with the thread resuming and setting
- * it. It must also be done with the write lock held to prevent a
- * race with the EXIT_ZOMBIE case.
- */
- exit_code = xchg(&p->exit_code, 0);
- if (unlikely(p->exit_state)) {
- /*
- * The task resumed and then died. Let the next iteration
- * catch it in EXIT_ZOMBIE. Note that exit_code might
- * already be zero here if it resumed and did _exit(0).
- * The task itself is dead and won't touch exit_code again;
- * other processors in this function are locked out.
- */
- p->exit_code = exit_code;
- exit_code = 0;
- }
- if (unlikely(exit_code == 0)) {
- /*
- * Another thread in this function got to it first, or it
- * resumed, or it resumed and then died.
- */
- write_unlock_irq(&tasklist_lock);
-bail_ref:
- put_task_struct(p);
- /*
- * We are returning to the wait loop without having successfully
- * removed the process and having released the lock. We cannot
- * continue, since the "p" task pointer is potentially stale.
- *
- * Return -EAGAIN, and do_wait() will restart the loop from the
- * beginning. Do _not_ re-acquire the lock.
- */
- return -EAGAIN;
- }
-
- /* move to end of parent's list to avoid starvation */
- remove_parent(p);
- add_parent(p);
-
- write_unlock_irq(&tasklist_lock);
retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
if (!retval && stat_addr)
@@ -1440,15 +1408,13 @@ bail_ref:
if (!retval && infop)
retval = put_user(0, &infop->si_errno);
if (!retval && infop)
- retval = put_user((short)((p->ptrace & PT_PTRACED)
- ? CLD_TRAPPED : CLD_STOPPED),
- &infop->si_code);
+ retval = put_user(why, &infop->si_code);
if (!retval && infop)
retval = put_user(exit_code, &infop->si_status);
if (!retval && infop)
retval = put_user(pid, &infop->si_pid);
if (!retval && infop)
- retval = put_user(p->uid, &infop->si_uid);
+ retval = put_user(uid, &infop->si_uid);
if (!retval)
retval = pid;
put_task_struct(p);
@@ -1555,8 +1521,6 @@ repeat:
retval = wait_task_stopped(p, ret == 2,
(options & WNOWAIT), infop,
stat_addr, ru);
- if (retval == -EAGAIN)
- goto repeat;
if (retval != 0) /* He released the lock. */
goto end;
} else if (p->exit_state == EXIT_ZOMBIE) {
-
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