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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.

[akpm@linux-foundation.org: fix up after Willy renamed everything]
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2008-02-08 04:19:01 -08:00 committed by Linus Torvalds
parent 6405f7f467
commit ee7c82da83
1 changed files with 26 additions and 62 deletions
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88
kernel/exit.c
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@ -1355,17 +1355,35 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
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(!task_is_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;
/*
@ -1375,65 +1393,15 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
* 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)
@ -1443,15 +1411,13 @@ 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);
@ -1558,8 +1524,6 @@ 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) {