Re: [PATCH 1/4] x86: Initialize CPU info via IFUNC relocation [BZ 26203]

[Florian Weimer <fweimer@redhat.com>]

* H. J. Lu:

>> > diff --git a/sysdeps/x86/cacheinfo.c b/sysdeps/x86/cacheinfo.c
>> > index 217c21c34f..7a325ab70e 100644
>> > --- a/sysdeps/x86/cacheinfo.c
>> > +++ b/sysdeps/x86/cacheinfo.c
>>
>> > +  assert (cpu_features->basic.kind != arch_kind_unknown);
>>
>> Why doesn't this assert fire occasionally?  How do you ensure that
>
> See
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=26203
>
> It only happens in dlopen from a static executable.

Sorry, I don't understand how this answers my question.

Do you mean that for the non-static case, initialization has already
happened.

>> relocation processing is correctly ordered?
>
> cpu_features is also initialized by IFUNC relocation in ld.so which
> is relocated before libc.so.

Is that really true in all cases?  Even if libc.so is preloaded?
(Static dlopen probably ignores LD_PRELOAD.)

Maybe put this information as a comment next to the assert?

But since cacheinfo.os is linked into libc.so, I don't really think the
assert is correct.

>> > diff --git a/sysdeps/x86/dl-get-cpu-features.c b/sysdeps/x86/dl-get-cpu-features.c
>> > index 5f9e46b0c6..da4697b895 100644
>> > --- a/sysdeps/x86/dl-get-cpu-features.c
>> > +++ b/sysdeps/x86/dl-get-cpu-features.c
>> > @@ -1,4 +1,4 @@
>> > -/* This file is part of the GNU C Library.
>> > +/* Initialize CPU feature data via IFUNC relocation.
>> >     Copyright (C) 2015-2020 Free Software Foundation, Inc.
>> >
>> >     The GNU C Library is free software; you can redistribute it and/or
>> > @@ -18,6 +18,29 @@
>> >
>> >  #include <ldsodefs.h>
>> >
>> > +#ifdef SHARED
>> > +# include <cpu-features.c>
>> > +
>> > +/* NB: Normally, DL_PLATFORM_INIT calls init_cpu_features to initialize
>> > +   CPU features.  But when loading ld.so inside of static executable,
>> > +   DL_PLATFORM_INIT isn't called.  Call init_cpu_features by initializing
>> > +   a dummy function pointer via IFUNC relocation for ld.so.  */
>> > +extern void __x86_cpu_features (void) attribute_hidden;
>> > +const void (*__x86_cpu_features_p) (void) attribute_hidden
>> > +  = __x86_cpu_features;
>> > +
>> > +void
>> > +_dl_x86_init_cpu_features (void)
>> > +{
>> > +  struct cpu_features *cpu_features = __get_cpu_features ();
>> > +  if (cpu_features->basic.kind == arch_kind_unknown)
>> > +    init_cpu_features (cpu_features);
>> > +}
>> > +
>> > +__ifunc (__x86_cpu_features, __x86_cpu_features, NULL, void,
>> > +      _dl_x86_init_cpu_features);
>> > +#endif
>> > +
>> >  #undef __x86_get_cpu_features
>>
>> Why do we need both the conditional check and the function pointer hack?
>
> Because _dl_x86_init_cpu_features is called both indirectly and by IFUNC
> reloc in dynamic executable, but it is only called by IFUNC reloc when
> dlopen in static executable.

I think we always need to call it eventually, as a dependency of filling
in the cacheinfo data?

>> I expect that one of the function pointers can go, probably the one
>> here.  The cache hierarchy data might be used by a string function that
>> has not been selected by IFUNC.
>>
>
> There are one IFUNC reloc in ld.so and the other in libc.so.  We need
> both.

libc.so should not need the relocation hack because we have
__libc_early_init, which is also called after static dlopen and before
constructors.

Thanks,
Florian
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