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.
> (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.
> * 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.
Eric
--
To unsubscribe from this list: send the line "unsubscribe linux-api" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
