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Subject: Re: [PATCH 04/11] signal/parisc: Document a conflict with SI_USER
 with SIGFPE
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-parisc@vger.kernel.org,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        John David Anglin <dave.anglin@bell.net>,
        linux-kernel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
        Oleg Nesterov <oleg@redhat.com>, linux-arch@vger.kernel.org
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 <20180112005940.23279-4-ebiederm@xmission.com>
 <20180112222944.GA22642@ls3530.fritz.box> <87fu5s4l4b.fsf@xmission.com>
From:   Helge Deller <deller@gmx.de>
Message-ID: <68641c9e-99c6-34d3-83aa-1241bddef33c@gmx.de>
Date:   Sun, 25 Feb 2018 20:49:41 +0100
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On 23.02.2018 01:15, Eric W. Biederman wrote:
> Helge Deller <deller@gmx.de> writes:
> 
>> * Eric W. Biederman <ebiederm@xmission.com>:
>>> Setting si_code to 0 results in a userspace seeing an si_code of 0.
>>> This is the same si_code as SI_USER.  Posix and common sense requires
>>> that SI_USER not be a signal specific si_code.  As such this use of 0
>>> for the si_code is a pretty horribly broken ABI.
>>>
>>> Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a
>>> value of __SI_KILL and now sees a value of SIL_KILL with the result
>>> that uid and pid fields are copied and which might copying the si_addr
>>> field by accident but certainly not by design.  Making this a very
>>> flakey implementation.
>>>
>>> Utilizing FPE_FIXME siginfo_layout will now return SIL_FAULT and the
>>> appropriate fields will reliably be copied.
>>>
>>> This bug is 13 years old and parsic machines are no longer being built
>>> so I don't know if it possible or worth fixing it.  But it is at least
>>> worth documenting this so other architectures don't make the same
>>> mistake.
>>
>>
>> I think we should fix it, even if we now break the ABI.
>>
>> It's about a "conditional trap" which needs to be handled by userspace.
>> I doubt there is any Linux code out which is utilizing this
>> parisc-specific trap.
>>
>> I'd suggest to add a new FPE trap si_code (e.g. FPE_CONDTRAP).
>> While at it, maybe we should include the already existing FPE_MDAOVF
>> from the frv architecture, so that arch/frv/include/uapi/asm/siginfo.h
>> can go completely.
>>
>> Suggested patch is below.
>>
>> I'm willing to test the patch below on the parisc architecture for a few
>> weeks. And it will break arch/x86/kernel/signal_compat.c which needs
>> looking at then too.
> 
> Have you managed to test this change?

Sadly I haven't done any further testing yet.
 
> I am sitting looking at another new FPE si_code and if this has been tested
> I figure FPE_CONDTRAP should get the next available FPE si_code and the
> other change should get the one that follows.

I'm fine either way. Do you have a git tree I can pull which includes
all your patches? I can then start testing.

Helge