linux-kernel - Re: [PATCH 0/3] ptrace: Stop supporting SIGKILL for PTRACE_EVENT

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Date:   Thu, 23 Jun 2022 16:55:05 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        mingo@...nel.org, bigeasy@...utronix.de,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "Robert O'Callahan" <roc@...nos.co>, Kyle Huey <khuey@...nos.co>,
        Keno Fischer <keno@...iacomputing.com>
Subject: Re: [PATCH 0/3] ptrace: Stop supporting SIGKILL for PTRACE_EVENT_EXIT

Alexander Gordeev <agordeev@...ux.ibm.com> writes:

> On Wed, Jun 22, 2022 at 11:43:37AM -0500, Eric W. Biederman wrote:
>> Recently I had a conversation where it was pointed out to me that
>> SIGKILL sent to a tracee stropped in PTRACE_EVENT_EXIT is quite
>> difficult for a tracer to handle.
>> 
>> Keeping SIGKILL working for anything after the process has been killed
>> is also a real pain from an implementation point of view.
>> 
>> So I am attempting to remove this wart in the userspace API and see
>> if anyone cares.
>
> Hi Eric,
>
> With this series s390 hits the warning exactly same way. Is that expected?

Yes.  I was working on this before I got your mysterious bug report.  I
included you because I am including everyone I know who deals with the
userspace side of this since I am very deliberately changing the user
visible behavior of PTRACE_EVENT_EXIT.

I am going to start seeing if I can find any possible explanation for
your regression report.  Since I don't have much to go on I expect I
will have to revert the last change in my ptrace_stop series that
apparently triggers the WARN_ON you reported.  I really would have
expected the WARN_ON to be triggered in the patch in which it was
introduced, not the final patch in the series.

To the best of my knowledge changing PTRACE_EVENT_EXIT is both desirable
from a userspace semantics standpoint and from a kernel implementation
standpoint.  If someone knows any differently and depends upon sending
SIGKILL to processes in PTRACE_EVENT_EXIT to steal the process away from
the tracer I would love to hear about that case.

Eric