linux-kernel - Re: [PATCH 4/7] signal/mips: Document a conflict with SI

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Message-ID: <alpine.DEB.2.00.1708071706290.17596@tp.orcam.me.uk>
Date:   Mon, 7 Aug 2017 17:18:11 +0100
From:   "Maciej W. Rozycki" <macro@...tec.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     <linux-kernel@...r.kernel.org>, Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrei Vagin <avagin@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>, Greg KH <greg@...ah.com>,
        Andrey Vagin <avagin@...nvz.org>,
        Serge Hallyn <serge@...lyn.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Willy Tarreau <w@....eu>, <linux-arch@...r.kernel.org>,
        <linux-api@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 4/7] signal/mips: Document a conflict with SI_USER with
 SIGFPE

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?

  Maciej