linux-kernel - Re: [PATCH RESEND2] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly

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Message-ID: <20210511175341.GA14488@redhat.com>
Date:   Tue, 11 May 2021 19:53:42 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jan Kratochvil <jan.kratochvil@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Pedro Alves <palves@...hat.com>,
        Simon Marchi <simon.marchi@...icios.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND2] ptrace: make ptrace() fail if the tracee changed
 its pid unexpectedly

On 05/11, Linus Torvalds wrote:
>
> On Tue, May 11, 2021 at 9:56 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > This patch makes ptrace() fail in this case until debugger does wait()
> > and consumes PTHREAD_EVENT_EXEC which reports old_pid.
>
> I'm ok with the patch, just wondering which way it's supposed to come
> to me. Should I just apply it directly?

would be nice!

> That said, why this:
>
> > +       rcu_read_lock();
> > +       pid = task_pid_nr_ns(task, task_active_pid_ns(task->parent));
> > +       rcu_read_unlock();
>
> I don't see why the RCU read lock would be needed? task_pid_nr_ns()
> does any required locking itself, afaik.
>
> And even if it wasn't, this all happens with siglock held, can
> anything actually change.

... and with tasklist_lock held.

Hmm. Linus, I am shy to admit I can't answer immediately, I'll recheck
tomorrow after sleep. But it seems you are right.

Thanks!

Oleg.