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

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Date:   Tue, 27 Apr 2021 08:26:20 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jan Kratochvil <jan.kratochvil@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Pedro Alves <palves@...hat.com>,
        Simon Marchi <simon.marchi@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] ptrace: make ptrace() fail if the tracee changed
 its pid unexpectedly

Hi Mathieu,

On 04/26, Mathieu Desnoyers wrote:
>
> > The patch doesn't add the new PTRACE_ option to not complicate the API,
> > and I _hope_ this won't cause any noticeable regression:
> >
> >	- If debugger uses PTRACE_O_TRACEEXEC and the thread did an exec
> >	  and the tracer does a ptrace request without having consumed
> >	  the exec event, it's 100% sure that the thread the ptracer
> >	  thinks it is targeting does not exist anymore, or isn't the
> >	  same as the one it thinks it is targeting.
> >
> >	- To some degree this patch adds nothing new. In the scenario
> >	  above ptrace(L) can fail with -ESRCH if it is called after the
> >	  execing sub-thread wakes the leader up and before it "steals"
> >	  the leader's pid.
>
> Hi Oleg,
>
> Is this something that should also target stable kernels ? AFAIU this change
> won't break debuggers more that they are already in this scenario. Or maybe
> it makes them fail in more obvious ways ?

Well, I am not sure this is stable material...

To me the problem is minor, and the patch adds the user-visible change.
I think it would be safer to not add stable tag.

Oleg.