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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Mon, 14 Feb 2011 18:05:03 +0100
Message-ID: <AANLkTin1ufNRPx173HgxD5jbPy3m6kK2-2ppU-jqPx0F@mail.gmail.com>
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH
To: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>, Roland McGrath <roland@redhat.com>,
        jan.kratochvil@redhat.com, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org
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On Mon, Feb 14, 2011 at 4:06 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 02/13, Denys Vlasenko wrote:
>>
>> For example, PTRACE_DETACH requires tracee to be stopped to succeed.
>> If debugger tries to detach while the tracee is running, it will get
>> an error. This forces debugger to do stupid things like sending SIGSTOP,
>> then waiting for tracee to stop, then doing PTRACE_DETACH, then
>> sending SIGCONT. Of course, while this dance is performed,
>> any SIGSTOPs/SIGCONTs which may be  sent to the tracee by other processes
>> are totally disrupted by this.
>
> Yes.
>
>> The natural (for me) fix is to make PTRACE_DETACH work even on running
>> tracee. It simply makes a lot of sense. Why on earth do we need tracee
>> to be stopped? There is no reason.
>
> Agreed, but
>
>> But this is a change in ptrace behavior, and therefore is not acceptable
>> for Roland.
>
> I agree with Roland. Not only this is too visible change, it is not clear
> what detach-with-signal can do if the tracee is not stopped.
>
> This was (very briefly) discussed recently. Probably we can implement
> PTRACE_DETACH_RUNNING (the name is random) which doesn't require the
> stopped tracee but ignores the "data" argument.

IIRC data argument is already ignored by PTRACE_CONT if it is issued in
the ptrace stop which wasn't caused by signal delivery to the tracee.

Basically, *if debugger sees SIGfoo*, it can either allow it:
ptrace(PTRACE_CONT, ...,  SIGfoo);
ignore it:
ptrace(PTRACE_CONT, ...,  0);
or even inject some other signal:
ptrace(PTRACE_CONT, ...,  SIGbar);

but if it resumes tracee from, say, post-execve ptrace stop,
it can't inject a signal: last ptrace() argument will be ignored.

So, it isn't a new precedent to make
ptrace(PTRACE_DETACH, ...,  <something>);
to ignore <something> if tracee isn't in signal-delivery-induced ptrace stop.
In particular, if it isn't in any stop at all, if it's running.

-- 
vda