Re: Seeking advise on most portable way to detect 64-bit off_t

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 07/03/24 15:31, Aliaksey Kandratsenka wrote:
> Thanks.
> 
> So it turns out I also needed to undef _TIME_BITS. Since glibc errors whenever _TIME_BITS are set and _FILE_OFFSET_BITS aren't. No complaints. IMHO it is a very reasonable thing that glibc does there. But it makes things slightly riskier on our end.
> 
> I have a question about a potentially longer term future. Do you guys plan to, say, default to _TIME_BITS=64 and _FILE_OFFSET_BITS=64 at any point? Asking because it would break my code (thankfully, it won't be silent breaking, but still).

It was on my plan when I we started to effectively work on 64 bit
time_t, but we decided this would case too much potential breakage
(as debian is finding out recently). So I think it is unlikely that
we will even change it.

> 
> Should I consider something else to make my code more robust ?
> 
> On Mon, Mar 4, 2024 at 7:04 AM Adhemerval Zanella Netto <adhemerval.zanella@linaro.org <mailto:adhemerval.zanella@linaro.org>> wrote:
> 
> 
> 
>     On 01/03/24 18:06, Aliaksey Kandratsenka via Libc-help wrote:
>     > Hi.
>     >
>     > In gperftools we have the feature to hook mmap calls which works by
>     > interposing over glibc's mmap. So it has to deal with off_t complexities.
>     > We don't use that offset argument, other than just passing it to real mmap
>     > implementation. I am also trying to support different Linux libc-s just for
>     > completeness. And musl being new enough and wise enough has always had
>     > 64-bit off_t (bionic hasn't and there is really no excuse...)
>     >
>     > So with that I need some means to detect 32-bit off_t that works across
>     > multiple libcs and has the highest hope of working in the future.
>     >
>     > By looking around, I choose to detect 32-bit-ness of off_t by looking at
>     > semi-obscure define _POSIX_V7_ILP32_OFF32:
>     > https://github.com/gperftools/gperftools/blob/37b59a206e36b405dcb2ac09d502875dd629a80b/src/mmap_hook.cc#L275 <https://github.com/gperftools/gperftools/blob/37b59a206e36b405dcb2ac09d502875dd629a80b/src/mmap_hook.cc#L275>
>     >
>     > It does seem to do the right thing across implementations I checked. Well
>     > mostly.
>     >
>     > These days, Debian folk are doing a massive 64-bit time_t transition (which
>     > also includes mass-enabling 64-bit off_t, at last) and they're rebuilding
>     > everything with _FILE_OFFSET_BITS=64. And my "approach" fails.
>     > https://buildd.debian.org/status/fetch.php?pkg=google-perftools&arch=armel&ver=2.15-1.1&stamp=1709166723&file=log <https://buildd.debian.org/status/fetch.php?pkg=google-perftools&arch=armel&ver=2.15-1.1&stamp=1709166723&file=log>
>     >
>     > Glibc doesn't adjust _POSIX_V7_ILP32_{BIGOFF,OFF32} defines based on
>     > _FILE_OFFSET_BITS. Perhaps rightly so. But with that situation I need some
>     > other, and hopefully more robust means to detect off_t width.
>     >
>     > I am thinking of 3 options:
>     >
>     > * Keep _POSIX_V7 thingy but also add a check for _FILE_OFFSET_BITS == 64.
>     > And then behave as if off_t is 32-bit.
>     > * #undef _FILE_OFFSET_BITS at the first line in mmap_hooks.cc. It will
>     > cause glibc on those legacy 32-bit off_t systems to define 32-bit off_t and
>     > give me right defines and my code will define mmap symbol with 32-bit
>     > offset arg and mmap64 symbol with 64-bit offset. And thus matching glibc.
>     > * just manually hardcode knowledge that glibc +
> 
>     You need to undefine _FILE_OFFSET_BITS to get the expected types for
>     off_t and off64_t on the mmap_hook.cc TU, since the whole idea is to
>     override the symbol.  You can also try to poke on glibc internals and
>     use __off_t and __off64_t, which would not be redefined depending the
>     the preprocessor (but tying to internal implementation is always a bad
>     idea).
> 
>     So I would recommend this option, which seems to what you did and it is
>     what sanitizers do also [1].
> 
>     > {i386,arm{el,hf},mips32,ppc32} has 32-bit off_t. But that leaves the
>     > question of how to deal with e.g. bionic (well, I could in principle say,
>     > bionic already being broken enough is only supported for 64-bit systems,
>     > which is where android systems are moving anyways)
>     >
>     > I am looking for any comments or suggestions. Particularly I'd like my fix
>     > to be robust w.r.t. any further glibc evolution.
> 
>     [1] https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp#L15 <https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp#L15>
>