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

[Florian Weimer <fweimer@redhat.com>]

* 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.

> 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Is the commented outdated?

> 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
relocation processing is correctly ordered?

> +/* 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.

> 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?

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.

Thanks,
Florian
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