linux-kernel - Re: [RFC 5/5 v2] uprobes: add global breakpoints

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Date:	Thu, 30 Aug 2012 22:42:09 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakaynahalli <ananth@...ibm.com>,
	stan_shebs@...tor.com, gdb-patches@...rceware.org
Subject: Re: [RFC 5/5 v2] uprobes: add global breakpoints

On 08/29/2012 05:49 PM, Oleg Nesterov wrote:
>> That would help but would require a change in ptrace_attach() or
>> something in gdb/strace/…
>
> Well, I still think you should not touch ptrace_attach() at all.

Okay.

>> One thing I just noticed: If I don't register a handler for SIGUSR1 and
>> send one to the application while it is in TASK_KILLABLE then the
>> signal gets delivered.
>
> Not really delivered... OK, it can be delivered (dequeued) before
> the task sees SIGKILL, but this can be changed.
>
> In short: in this case the task is correctly SIGKILL'ed. See sig_fatal()
> in complete_signal().
>
>> If I register a signal handler for it than it
>> gets blocked and delivered once I resume the task.
>
> Sure, if you have a handler, the signal is not fatal.
>
>> Shouldn't it get blocked even if I don't register a handler for it?
>
> No.

Now, that I read again it looks like a brain fart on my side.

>> ach, those signals make everything complicated. I though signals are
>> blocked until the single step is done
>
> Yes, see uprobe_deny_signal().
>
>> but my test just showed my
>> something different.
>
> I guess you missed the UTASK_SSTEP_TRAPPED logic.
>
> But this doesn't matter. Surely we must not "block" signals _after_
> the single step is done, and this is the problem.
>
>> Okay, what now?
>
> IMHO: don't do this ;)
>
>> Blocking signals isn't probably a good idea.
>
> This is bad and wrong idea, I think.
>
> And, once again. Whatever you do, you can race with uprobe_register().
> I mean, you must never expect that the task will hit the same uprobe
> again, even if you are going to re-execute the same insn.

After witting why I think you are wrong I understood what you meant :)
So let me try to get this right…

>
> Oleg.

Sebastian
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