Hi Oleg,
On Tue, Dec 17, 2024 at 04:09:14PM +0100, Oleg Nesterov wrote:
> On 12/17, Oleg Nesterov wrote:
> >
> > On 11/06, Nam Cao wrote:
> > >
> > > @@ -534,6 +517,23 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
> > > ppid = task_tgid_nr_ns(task->real_parent, ns);
> > > pgid = task_pgrp_nr_ns(task, ns);
> > >
> > > + /*
> > > + * esp and eip are intentionally zeroed out. There is no
> > > + * non-racy way to read them without freezing the task.
> > > + * Programs that need reliable values can use ptrace(2).
> >
> > OK,
> >
> > but then:
> >
> > > + * The only exception is if the task is core dumping because
> > > + * a program is not able to use ptrace(2) in that case. It is
> > > + * safe because the task has stopped executing permanently.
> > > + */
> > > + if (permitted && task->signal->core_state) {
> > > + if (try_get_task_stack(task)) {
> > > + eip = KSTK_EIP(task);
> > > + esp = KSTK_ESP(task);
> > > + put_task_stack(task);
> >
> > How can the task->signal->core_state check help ?
> >
> > Suppose we have a task T1 with T1-pid == 100 and you read /proc/100/stat.
> > It is possible that the T1's sub-thread T2 starts the coredumping and sets
> > signal->core_state != NULL.
> >
> > But read(/proc/100/stat) can run before T1 gets SIGKILL from T2 and enters
> > the kernel mode?
Right, I missed that race, thanks for pointing it out.
> Can't the trivial patch below fix the problem?
It can. In fact this is the original fix we had. I thought that checking a
single "core_state" is simpler than checking 3 flags, oh well..
Can you send a proper patch, or should I do it?
Best regards,
Nam
>
> Oleg.
>
>
> --- xfs/proc/array.c
> +++ x/fs/proc/array.c
> @@ -500,7 +500,7 @@
> * a program is not able to use ptrace(2) in that case. It is
> * safe because the task has stopped executing permanently.
> */
> - if (permitted && (task->flags & (PF_EXITING|PF_DUMPCORE))) {
> + if (permitted && (task->flags & (PF_EXITING|PF_DUMPCORE|PF_POSTCOREDUMP))) {
> if (try_get_task_stack(task)) {
> eip = KSTK_EIP(task);
> esp = KSTK_ESP(task);
>
