glibc clears AT_HWCAP and set only HW_CAP_X86_64 (0x2)

[Baojun Wang]

Thanks a lot H.J & Carlos.

On Thu, Oct 17, 2019 at 17:06 H.J. Lu <hjl.tools@gmail.com> wrote:

> On Thu, Oct 17, 2019 at 1:31 PM Baojun Wang <wangbj@gmail.com> wrote:
> >
> > Yes, the values read from /proc/self/auxv is different (Linux x86_64,
> > glibc-2.27, tried glibc HEAD without any difference) with
> > getauxval(AT_HWCAP).
> >
> > Here is the program I wrote for testing:
> >
> > https://gist.github.com/wangbj/658da547437fe9b7eff2113cc9fc9741
> >
> > And here is the output (check AT_HWCAP):
> >
> > $ ./features
> >
> >          AT_BASE: getauxval = 0x7f861434a000    , proc_getauxval =
> > 0x7f861434a000
> >
> >         AT_ENTRY: getauxval = 0x4005c0          , proc_getauxval =
> > 0x4005c0
> >
> >         AT_HWCAP: getauxval = 0x2               , proc_getauxval =
> > 0xbfebfbff
> >
> > Best Regards,
> > baojun
> >
> > On Thu, Oct 17, 2019 at 4:26 PM Carlos O'Donell <carlos@redhat.com>
> wrote:
> > >
> > > On 10/17/19 11:29 AM, Baojun Wang wrote:
> > > > Hi libc,
> > > >
> > > > when calling getauxval(AT_HWCAP), it returns 0x2, because of below
> snippet:
> > > >
> > > >
> https://code.woboq.org/userspace/glibc/sysdeps/x86/cpu-features.c.html#510
> > > >
> > > > Is there any reason why glibc don't use the values passed from
> kernel auxv?
> > >
> > > I don't know of any good reason.
> > >
> > > The semantics of getauxval() is to provide the auxiliary vector data.
> > >
> > > In this case glibc *does* read AT_HWCAP, but then for x86 it overrides
> it
> > > with a computed value based on cpuid.
> > >
> > > HJ, Is there any reason we do this?
>
> dl_hwcap is used to load DSO optimized for a platform.  On i386, we use
> AT_HWCAP passed from kernel.  On x86-64, we use a different approach
> since AT_HWCAP passed from kernel isn't very useful:
>
> commit 1432d38ea04ab5e96f21a382101856db5b49ad8a
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Wed May 3 13:42:42 2017 -0700
>
>     x86: Set dl_platform and dl_hwcap from CPU features [BZ #21391]
>
>     dl_platform and dl_hwcap are set from AT_PLATFORM and AT_HWCAP very
>     early during startup.  They are used by dynamic linker to determine
>     platform and build an array of hardware capability names, which are
>     added to search path when loading shared object.  dl_platform and
>     dl_hwcap are unused on x86-64.  On i386, i386, i486, i586 and i686
>     platforms were supported and only SSE2 capability was used.
>
>     On x86, usage of AT_PLATFORM and AT_HWCAP to determine platform and
>     processor capabilities is obsolete since all information is available
>     in dl_x86_cpu_features.  This patch sets dl_platform and dl_hwcap from
>     dl_x86_cpu_features in dynamic linker.  On i386, the available plaforms
>     are changed to i586 and i686 since i386 has been deprecated.  On
> x86-64,
>     the available plaforms are haswell, which is for Haswell class
> processors
>     with BMI1, BMI2, LZCNT, MOVBE, POPCNT, AVX2 and FMA, and xeon_phi,
> which
>     is for Xeon Phi class processors with AVX512F, AVX512CD, AVX512ER and
>     AVX512PF.  A capability, avx512_1, is also added to x86-64 for AVX512
>     ISAs: AVX512F, AVX512CD, AVX512BW, AVX512DQ and AVX512VL.
>
> and
>
> commit 45ff34638f034877b6a490c217d6a0632ce263f4
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Mon Sep 11 08:18:11 2017 -0700
>
>     x86: Add x86_64 to x86-64 HWCAP [BZ #22093]
>
>     Before glibc 2.26, ld.so set dl_platform to "x86_64" and searched the
>     "x86_64" subdirectory when loading a shared library.  ld.so in glibc
>     2.26 was changed to set dl_platform to "haswell" or "xeon_phi", based
>     on supported ISAs.  This led to shared library loading failure for
>     shared libraries placed under the "x86_64" subdirectory.
>
>     This patch adds "x86_64" to x86-64 dl_hwcap so that ld.so will always
>     search the "x86_64" subdirectory when loading a shared library.
>
>     NB: We can't set x86-64 dl_platform to "x86-64" since ld.so will skip
>     the "haswell" and "xeon_phi" subdirectories on "haswell" and "xeon_phi"
>     machines.
>
> > > Baojun, Do you see a difference between getauxval() return and real
> AT_HWCAP?
> > >
> > > --
> > > Cheers,
> > > Carlos.
> > >
>
>
>
> --
> H.J.
>