linux-kernel - Re: sparc/ppc/arm compat siginfo ABI regressions: sending SIGFPE via kill() returns wrong values in si_pid and si

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Message-ID: <20180413170827.GB16308@e103592.cambridge.arm.com>
Date:   Fri, 13 Apr 2018 18:08:28 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        sparclinux <sparclinux@...r.kernel.org>,
        ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: sparc/ppc/arm compat siginfo ABI regressions: sending SIGFPE via
 kill() returns wrong values in si_pid and si_uid

On Fri, Apr 13, 2018 at 09:33:17AM -0700, Linus Torvalds wrote:
> On Fri, Apr 13, 2018 at 2:42 AM, Russell King - ARM Linux
> <linux@...linux.org.uk> wrote:
> >
> > Yes, it does solve the problem at hand with strace - the exact patch I
> > tested against 4.16 is below.
> 
> Ok, good.
> 
> > However, FPE_FLTUNK is not defined in older kernels, so while we can
> > fix it this way for the current merge window, that doesn't help 4.16.
> 
> I wonder if we should even bother with FPE_FLTUNK.
> 
> I suspect we might as well use FPE_FLTINV, I suspect, and not have
> this complexity at all. That case is not worth worrying about, since
> it's a "this shouldn't happen anyway" and the *real* reason will be in
> the kernel logs due to vfs_panic().
> 
> So it's not like this is something that the user should ever care
> about the si_code about.

Ack, my intended meaning for FPE_FLTUNK is that the fp exception is
either spurious or we can't tell easily (or possibly at all) which
FPE_XXX should be returned.  It's up to userspace to figure it out
if it really cares.  Previously we were accidentally returning SI_USER
in si_code for arm64.

This case on arm looks like a more serious error for which FPE_FLTINV
may be more appropriate anyway.

[...]

Cheers
---Dave