Messages in this thread Patch in this message | | | Date | Sun, 8 May 2011 18:07:20 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH ptrace] ptrace: use GROUP_STOP_TRAPPING for PTRACE_DETACH too |
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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.
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