[RFC PATCH 1/2] arm64: fpsimd: Fix bad si_code for undiagnosed SIGFPE

[Dave Martin]

On Wed, Jan 24, 2018 at 10:47:36AM -0600, Eric W. Biederman wrote:
> Dave Martin <Dave.Martin at arm.com> writes:
> 
> > On Mon, Jan 22, 2018 at 03:13:08PM -0600, Eric W. Biederman wrote:
> >> 
> >> > diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> >> > index e447283..77edb00 100644
> >> > --- a/include/uapi/asm-generic/siginfo.h
> >> > +++ b/include/uapi/asm-generic/siginfo.h
> >> > @@ -193,7 +193,8 @@ typedef struct siginfo {
> >> >  #define FPE_FLTRES	6	/* floating point inexact result */
> >> >  #define FPE_FLTINV	7	/* floating point invalid operation */
> >> >  #define FPE_FLTSUB	8	/* subscript out of range */
> >> > -#define NSIGFPE		8
> >> > +#define FPE_UNKNOWN	9	/* undiagnosed floating-point exception */
> >> > +#define NSIGFPE		9
> >> 
> >> Minor nit here.
> >> 
> >> At least before this is final I would really appreciate if you could
> >> rebase this on top of my unificiation of siginfo.h that I posted on
> >> linux-arch and is in my siginfo-next branch.
> >> 
> >> As that already pushes NSIGFPE up to 13.
> >> 
> >> Which would make this patch change NSIGFPE to 14 and allocate the number
> >> 14 for FPE_UNKNOWN
> >
> > Looking at this, I note a few things:
> >
> >  * For consistent naming, FPE_FLTUNK might be a better name for
> >    FPE_UNKNOWN.
> >
> >    FPE_FLTUNK seems generic, tempting me to insert it as number 9
> >    (since only the numbers up to 8 are ABI just now).
> 
> Except on ia64 and frv.  And who knows we might need it on one of those
> architectures as well.

I thought those weren't actually upstreamed yet...

> >    (The temptation to call it FPE_FLUNK is strong, but I can't argue
> >    that's consistent...)
> 
> I totally understand the temptation.
> 
> >  * No distinction is drawn between generic and arch-dependent codes
> >    here, so NSIGFPE will typically be too big.  The generic siginfo
> >    handling code can detect random garbage in si_code this way, but
> >    off-by-ones or misused arch-specific codes may slip through.
> >
> >    In particular, new x86-specific FPE_* codes will likely be
> >    invisible to the BUILD_BUG_ON()s in arch/x86/kernel/signal_compat.c
> >    unless so many are added that x86 overtakes ia64.
> 
> Long ago in a far off time, we had arch dependent system call numbers
> and the like because that provided ABI compatibility with the existing
> unix on the platform.
> 
> I don't see any of that with the siginfo si_codes.   In most cases
> they are arch dependent extensions which is silly.  We should have
> unconditionally extended the si_codes for all architectures in case
> another architecture needs that si_code.
> 
> The fact we now have battling meanings for si_codes depending on the
> architecture is an unfortunate mess.
> 
> So to me it looks most maintainable going forward to declare that all
> si_codes should be allocated generically, from the same number space,
> in the same header file.  While we live with the existing historic
> mess.

I guess that's fair enough.  This also provides a consistent
interpretation for NSIGXXX.

> >  * Should we reserve space for future generic codes (say up to 15)?
> >    Downside: si_code validation is not a simple matter of checking
> >    <= NSIGFPE in that case.  (Though <= is still better than no
> >    check at all, and no worse than the current situation.)
> 
> I think new si_codes should be allocated where there are not conflicts
> on any architecture.  Just in case they are useful on another
> architecture in the future.
> 
> >  * What are NSIGFPE etc. doing in this header?  These aren't specified
> >    by POSIX and I'm not sure what userspace would legitimately use them
> >    for... though it may be too late to change this now.
> >
> >    Most instances on codeseaarch.debian.net are the kernel, copies
> >    of kernel headers, and translated versions of kernel headers.
> >    It's hard to be exhaustive though.
> >
> >
> > We could have something like this:
> >
> > #define FPE_FLTUNK	9
> > #define __NSIGFPE_GENERIC	9
> > #define NSIGFPE		__NSIGFPE_GENERIC
> >
> > /* si_code <= 15 reserved for arch-independent codes */
> >
> > #if defined(__frv__)
> >
> > # define FPE_MDAOF	16
> > # undef NSIGFPE
> > # define NSIGFPE	16
> >
> > #elif define(__ia64__)
> >
> > # define __FPE_DECOVF	16
> > # define __FPE_DECDIV	17
> > # define __FPE_DECERR	18
> > # define __FPR_INVASC	19
> > # undef NSIGFPE
> > # define NSIGFPE	19
> >
> > #endif
> >
> > (Avoiding a (base + offset) approach for the arch codes, since that
> > would make it look like the codes can be renumbered safely without
> > breaking anything).
> >
> > The generic vs. arch vs. NSIGFOO problem already exists for other
> > signals.  We could take a similar approach for those, but OTOH it
> > may just not be worth the effort.
> 
> What I have tried to do in my merger is discurage the idea that there
> are any arch specific si_codes.  To set NSIGXXX to the largest value
> from any of the architectures.  And to encourage new si_codes get
> allocated after the current NSIGXXX.  So that they will work on all
> architectures.
> 
> It is all a bit of a mess, but one unified mess seems like the best we
> can do right now.

That sounds fair, now that I have a better understanding of the context
for all this.

If the policy is that all the codes are generic (even if not all can
happen on all arches) then FPE_FLTUNK may as well be 14.

Cheers
---Dave