Re: [PATCH ptrace] ptrace: use GROUP_STOP_TRAPPING for PTRACE_DETACH too

Oleg Nesterov
2011-05-08T16:08:55Z

On 05/08, Tejun Heo wrote:
>
> Currently GROUP_STOP_TRAPPING is used only for PTRACE_ATTACH to hide
> STOPPED -> RUNNING -> TRACED transition; however, DETACH involves
> similar transition in the reverse direction, which can be visible to
> the next ptracer if it attaches before the transition is complete.

Yes...

> This patch makes DETACH also use TRAPPING and ptrace_attach() always
> wait if TRAPPING is set to hide the transition.

I am not sure, please see below.

> Test program follows.
>
> int main(int argc, char **argv)
> {
> pid_t tracee;
> siginfo_t si = {};
> int i, nr_wait_fails = 0, nr_ptrace_fails = 0;
>
> tracee = fork();
> if (!tracee)
> while (1)
> pause();
>
> kill(tracee, SIGSTOP);
> waitid(P_PID, tracee, NULL, WSTOPPED | WNOWAIT);
>
> for (i = 0; i < 100000; i++) {
> ptrace(PTRACE_ATTACH, tracee, NULL, NULL);
> waitid(P_PID, tracee, &si, WSTOPPED | WNOHANG);
> if (!si.si_pid)
> nr_wait_fails++;

OK, this is clear, waitid(WSTOPPED | WNOHANG) can fail if it sees the
tracee inside the transition.

But,

> if (ptrace(PTRACE_DETACH, tracee, NULL, NULL)) {
> nr_ptrace_fails++;

I assume this can only fail for the same reason if waitid() fails?
Or there is something else?

> --- work.orig/kernel/ptrace.c
> +++ work/kernel/ptrace.c
> @@ -77,12 +77,15 @@ void __ptrace_unlink(struct task_struct
>
> /*
> * Reinstate GROUP_STOP_PENDING if group stop is in effect and
> - * @child isn't dead.
> + * @child isn't dead. This will trigger TRACED -> RUNNING ->
> + * STOPPED transition. As this transition can affect the next
> + * ptracer if it attaches before the transition completes, set
> + * TRAPPING too. Read comment in ptrace_attach() for more details.
> */
> if (!(child->flags & PF_EXITING) &&
> (child->signal->flags & SIGNAL_STOP_STOPPED ||
> child->signal->group_stop_count))
> - child->group_stop |= GROUP_STOP_PENDING;
> + child->group_stop |= GROUP_STOP_PENDING | GROUP_STOP_TRAPPING;

This doesn't look safe, see below. We do not know what the tracee does,
it can be even running.

> static int ptrace_attach(struct task_struct *task)
> {
> - bool wait_trap = false;
> int retval;
>
> audit_ptrace(task);
> @@ -245,7 +247,6 @@ static int ptrace_attach(struct task_str
> if (task_is_stopped(task)) {
> task->group_stop |= GROUP_STOP_PENDING | GROUP_STOP_TRAPPING;
> signal_wake_up(task, 1);
> - wait_trap = true;
> }
>
> spin_unlock(&task->sighand->siglock);
> @@ -256,9 +257,8 @@ unlock_tasklist:
> unlock_creds:
> mutex_unlock(&task->signal->cred_guard_mutex);
> out:
> - if (wait_trap)
> - wait_event(current->signal->wait_chldexit,
> - !(task->group_stop & GROUP_STOP_TRAPPING));
> + wait_event(current->signal->wait_chldexit,
> + !(task->group_stop & GROUP_STOP_TRAPPING));

Suppose that SIGCONT or, worse, SIGKILL comes in between.

Oleg.