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

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Sep 28, 2020 at 6:08 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu via Libc-alpha:
>
> > X86 CPU features in ld.so are initialized by init_cpu_features, which is
> > invoked by DL_PLATFORM_INIT from _dl_sysdep_start.  But when ld.so is
> > loaded by static executable, DL_PLATFORM_INIT is never called.  Also
> > x86 cache info in libc.o and libc.a is initialized by a constructor
> > which may be called too late.  Since _rtld_global_ro in ld.so is
> > initialized by dynamic relocation,  we can also initialize x86 CPU
> > features in _rtld_global_ro in ld.so and cache info in libc.so by
> > initializing dummy function pointers in ld.so and libc.so via IFUNC
> > relocation.
>
> _rtld_global_ro is *partially* initialized by relocation.  Most of it is
> not initialized (see the need for rtld_active).
>
> Please make this a little bit clearer in the commit message.

Will do

> > diff --git a/sysdeps/i386/dl-machine.h b/sysdeps/i386/dl-machine.h
> > index 0f08079e48..5e22e795cc 100644
> > --- a/sysdeps/i386/dl-machine.h
> > +++ b/sysdeps/i386/dl-machine.h
> > @@ -25,7 +25,6 @@
> >  #include <sysdep.h>
> >  #include <tls.h>
> >  #include <dl-tlsdesc.h>
> > -#include <cpu-features.c>
> >
> >  /* Return nonzero iff ELF header is compatible with the running host.  */
> >  static inline int __attribute__ ((unused))
> > @@ -250,7 +249,7 @@ dl_platform_init (void)
> >  #if IS_IN (rtld)
> >    /* init_cpu_features has been called early from __libc_start_main in
> >       static executable.  */
> > -  init_cpu_features (&GLRO(dl_x86_cpu_features));
> > +  _dl_x86_init_cpu_features ();
>
> Is the commented outdated?

I will fix it.

> > 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.

> relocation processing is correctly ordered?

cpu_features is also initialized by IFUNC relocation in ld.so which
is relocated before libc.so.

> > +/* NB: Call init_cacheinfo by initializing a dummy function pointer via
> > +   IFUNC relocation.  */
> > +extern void __x86_cacheinfo (void) attribute_hidden;
> > +const void (*__x86_cacheinfo_p) (void) attribute_hidden
> > +  = __x86_cacheinfo;
> > +
> > +__ifunc (__x86_cacheinfo, __x86_cacheinfo, NULL, void, init_cacheinfo);
> >  #endif
>
> Please expand the comment and mention that is used to initialize the
> dormant copy of ld.so after static dlopen.  The comment in
> sysdeps/x86/dl-get-cpu-features.c is good.

Will do.

> > 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 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.

-- 
H.J.