linux-kernel - Re: [PATCH 04/11] signal/parisc: Document a conflict with SI

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Date:   Thu, 22 Feb 2018 18:15:16 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Helge Deller <deller@....de>
Cc:     linux-parisc@...r.kernel.org,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        John David Anglin <dave.anglin@...l.net>,
        linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>, linux-arch@...r.kernel.org
Subject: Re: [PATCH 04/11] signal/parisc: Document a conflict with SI_USER with SIGFPE

Helge Deller <deller@....de> writes:

> * Eric W. Biederman <ebiederm@...ssion.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?

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.

Eric