Dave Martin <Dave.Martin@xxxxxxx> writes:
> On Wed, Jan 24, 2018 at 10:47:36AM -0600, Eric W. Biederman wrote:
>> Dave Martin <Dave.Martin@xxxxxxx> 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...
Oh no. Those have been upstream for a decade or so. They just have not
been in one unified file.
>> > (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.
Exactly.
Eric
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