Add support for referencing specific symbol versions

This is needed for writing tests of compat symbols.

2016-06-22  Florian Weimer  <fweimer@redhat.com>

	Support linking against compatibility symbols, for use in tests.
	* include/libc-symbols.h (symbol_version_reference): New macro.
	(symbol_version): Use it.
	* include/shlib-compat.h: Unconditionally include <abi-versions.h>.
	(compat_symbol): Use compat_symbol_reference.
	(compat_symbol_1, compat_symbol_2): Remove.
	(compat_symbol_reference, compat_symbol_reference_1)
	(compat_symbol_reference_2): New macro.  Use
	symbol_version_reference.

On 06/22/2016 11:18 AM, Florian Weimer wrote:
> This is needed for writing tests of compat symbols.
> 
> 2016-06-22  Florian Weimer  <fweimer@redhat.com>
> 
> 	Support linking against compatibility symbols, for use in tests.
> 	* include/libc-symbols.h (symbol_version_reference): New macro.
> 	(symbol_version): Use it.
> 	* include/shlib-compat.h: Unconditionally include <abi-versions.h>.
> 	(compat_symbol): Use compat_symbol_reference.
> 	(compat_symbol_1, compat_symbol_2): Remove.
> 	(compat_symbol_reference, compat_symbol_reference_1)
> 	(compat_symbol_reference_2): New macro.  Use
> 	symbol_version_reference.

This is awesome. Please check this in. I'm happy to see this kind
of compat symbol testing framework go in!
 
> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> index 4548e09..c2b499a 100644
> --- a/include/libc-symbols.h
> +++ b/include/libc-symbols.h
> @@ -282,19 +282,28 @@ for linking")
>     past the last element in SET.  */
>  #define symbol_set_end_p(set, ptr) ((ptr) >= (void *const *) &__stop_##set)
>  
> +/* Use symbol_version_reference to specify the version a symbol
> +   reference should link to.  Use symbol_version or
> +   default_symbol_version for the definition of a versioned symbol.
> +   The difference is that the latter is a no-op in non-shared
> +   builds.  */
> +#ifdef __ASSEMBLER__
> +# define symbol_version_reference(real, name, version) \
> +     .symver real, name##@##version
> +#else  /* !__ASSEMBLER__ */
> +# define symbol_version_reference(real, name, version) \
> +  __asm__ (".symver " #real "," #name "@" #version)
> +#endif

OK. Wow. To be honest I was surprised this was
possible. We've always talked about using prebuilt binaries for
testing, but using this to call the compat symbol seems like a
perfect solution for now and much easier. Perhaps we'll never need
anything more.

> +
>  #ifdef SHARED
>  # define symbol_version(real, name, version) \
> -     _symbol_version(real, name, version)
> +  symbol_version_reference(real, name, version)
>  # define default_symbol_version(real, name, version) \
>       _default_symbol_version(real, name, version)
>  # ifdef __ASSEMBLER__
> -#  define _symbol_version(real, name, version) \
> -     .symver real, name##@##version
>  #  define _default_symbol_version(real, name, version) \
>       .symver real, name##@##@##version
>  # else
> -#  define _symbol_version(real, name, version) \
> -     __asm__ (".symver " #real "," #name "@" #version)
>  #  define _default_symbol_version(real, name, version) \
>       __asm__ (".symver " #real "," #name "@@" #version)
>  # endif
> diff --git a/include/shlib-compat.h b/include/shlib-compat.h
> index c1c5e2c..025c731 100644
> --- a/include/shlib-compat.h
> +++ b/include/shlib-compat.h
> @@ -19,10 +19,10 @@
>  #ifndef _SHLIB_COMPAT_H
>  #define _SHLIB_COMPAT_H	1
>  
> -#ifdef SHARED
> -
>  # include <abi-versions.h>
>  
> +#ifdef SHARED
> +
>  /* The file abi-versions.h (generated by scripts/abi-versions.awk) defines
>     symbols like `ABI_libm_GLIBC_2_0' for each version set in the source
>     code for each library.  For a version set that is subsumed by a later
> @@ -62,11 +62,7 @@
>    default_symbol_version (local, symbol, name)
>  
>  # define compat_symbol(lib, local, symbol, version) \
> -  compat_symbol_1 (lib, local, symbol, version)
> -# define compat_symbol_1(lib, local, symbol, version) \
> -  compat_symbol_2 (local, symbol, VERSION_##lib##_##version)
> -# define compat_symbol_2(local, symbol, name) \
> -  symbol_version (local, symbol, name)
> +  compat_symbol_reference (lib, local, symbol, version)
>  
>  #else
>  
> @@ -82,6 +78,14 @@
>  
>  #endif
>  
> +/* Use compat_symbol_reference for a reference to a specific version
> +   of a symbol.  Use compat_symbol to define such a symbol.  */
> +#define compat_symbol_reference(lib, local, symbol, version) \
> +  compat_symbol_reference_1 (lib, local, symbol, version)
> +#define compat_symbol_reference_1(lib, local, symbol, version) \
> +  compat_symbol_reference_2 (local, symbol, VERSION_##lib##_##version)
> +#define compat_symbol_reference_2(local, symbol, name) \
> +  symbol_version_reference (local, symbol, name)
>  
>  # ifdef LINK_OBSOLETE_RPC
>  /* Export the symbol for both static and dynamic linking.  */
> 

Cheers,
Carlos.

On 11/07/2016 04:23, Carlos O'Donell wrote:
> On 06/22/2016 11:18 AM, Florian Weimer wrote:
>> This is needed for writing tests of compat symbols.
>>
>> 2016-06-22  Florian Weimer  <fweimer@redhat.com>
>>
>> 	Support linking against compatibility symbols, for use in tests.
>> 	* include/libc-symbols.h (symbol_version_reference): New macro.
>> 	(symbol_version): Use it.
>> 	* include/shlib-compat.h: Unconditionally include <abi-versions.h>.
>> 	(compat_symbol): Use compat_symbol_reference.
>> 	(compat_symbol_1, compat_symbol_2): Remove.
>> 	(compat_symbol_reference, compat_symbol_reference_1)
>> 	(compat_symbol_reference_2): New macro.  Use
>> 	symbol_version_reference.
> 
> This is awesome. Please check this in. I'm happy to see this kind
> of compat symbol testing framework go in!

It could be also useful if you add a small snippet oh how to use such
as the one in include/libc-symbols.h:317.