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Date:   Tue, 21 Jun 2022 16:05:12 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Derek Bruening <bruening@google.com>,
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Subject: Re: [PATCH] rseq: x86: Fix rseq_cs get cleared when returning from
 signal handler
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Thread-Topic: rseq: x86: Fix rseq_cs get cleared when returning from signal handler
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----- On Jun 21, 2022, at 3:48 PM, Eric W. Biederman ebiederm@xmission.com wrote:

> Derek Bruening <bruening@google.com> writes:
> 
>> From the viewpoint of dynamic binary translation/instrumentation and
>> memtrace (go/memtrace), removing those RSEQ_CS_FLAG_NO_RESTART_ON_* flags
>> is a good thing as it reduces complexity and makes it easier to handle rseq
>> (which is painful enough to handle already).
> 
> It sounds like there is consensus.
> 
> Does someone want to code up a simple patch that detects when
> RSEQ_CS_NO_RESTART_ON_SIGNAL and does a WARN_ON_ONCE and fails if
> someone uses so it can be set to Linus in the next merge window.
> 
> After no one screams at that patch it should be safe to remove the
> functionality, because you have empirical proof that no one uses
> that functionality.

Sure, I can whip up something.

I'll send it to Peter Zijlstra shortly.

I plan to, as you suggest, WARN_ON_ONCE() when this happens, and return
an error when the rseq flags or rseq_cs flags contain either of the
RSEQ_CS_FLAG_NO_RESTART_ON_* flags. This error is handled by forcing a
killing the process with sigsegv:

__rseq_handle_notify_resume()
[...]
error:
        sig = ksig ? ksig->sig : 0;
        force_sigsegv(sig);

Does it look acceptable ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com