From: ebiederm@xmission.com (Eric W. Biederman)
To: "Maciej W. Rozycki" <macro@imgtec.com>
Cc: <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Oleg Nesterov <oleg@redhat.com>,
        Andrei Vagin <avagin@virtuozzo.com>,
        Thomas Gleixner <tglx@linutronix.de>, Greg KH <greg@kroah.com>,
        Andrey Vagin <avagin@openvz.org>, Serge Hallyn <serge@hallyn.com>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Peter Zijlstra <peterz@infradead.org>, Willy Tarreau <w@1wt.eu>,
        <linux-arch@vger.kernel.org>, <linux-api@vger.kernel.org>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Ralf Baechle <ralf@linux-mips.org>
References: <87o9shg7t7.fsf_-_@xmission.com>
        <20170718140651.15973-4-ebiederm@xmission.com>
        <alpine.DEB.2.00.1708071706290.17596@tp.orcam.me.uk>
Date: Tue, 08 Aug 2017 10:29:18 -0500
In-Reply-To: <alpine.DEB.2.00.1708071706290.17596@tp.orcam.me.uk> (Maciej
        W. Rozycki's message of "Mon, 7 Aug 2017 17:18:11 +0100")
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Subject: Re: [PATCH 4/7] signal/mips: Document a conflict with SI_USER with SIGFPE
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"Maciej W. Rozycki" <macro@imgtec.com> writes:

> On Tue, 18 Jul 2017, Eric W. Biederman wrote:
>
>> diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
>> index b68b4d0726d3..6c9cca9c5341 100644
>> --- a/arch/mips/kernel/traps.c
>> +++ b/arch/mips/kernel/traps.c
>> @@ -735,7 +735,7 @@ void force_fcr31_sig(unsigned long fcr31, void __user *fault_addr,
>>  	else if (fcr31 & FPU_CSR_INE_X)
>>  		si.si_code = FPE_FLTRES;
>>  	else
>> -		si.si_code = __SI_FAULT;
>> +		si.si_code = FPE_FIXME;
>
>  This is an "impossible" state to reach unless your hardware is on fire.  
> One or more of the FCSR Cause bits will have been set (in `fcr31') or the 
> FPE exception would not have happened.
>
>  Of course there could be a simulator bug, or we could have breakage 
> somewhere causing `process_fpemu_return' to be called with SIGFPE and 
> inconsistent `fcr31'.  So we need to handle it somehow.
>
>  So what would be the right value of `si_code' to use here for such an 
> unexpected exception condition?  I think `BUG()' would be too big a 
> hammer here.  Or wouldn't it?

The possible solutions I can think of are:

WARN_ON_ONCE with a comment.

Add a new si_code to uapi/asm-generic/siginfo.h perhaps FPE_IMPOSSIBLE.
Like syscall numbers si_codes are cheap.

Call force_sig() instead of force_sig_info, using just a generic
si_code.

If this is truly impossible and the compiler doesn't complain just drop
the code.

Eric