Re: [PATCH v5 7/7] Make libc symbols hidden in static PIE

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 20/01/2021 12:31, Szabolcs Nagy via Libc-alpha wrote:
> Hidden visibility can avoid indirections and RELATIVE relocs in
> static PIE libc.
> 
> The check should use IS_IN_LIB instead of IS_IN(libc) since all
> symbols are defined locally in static PIE and the optimization is
> useful in all libraries not just libc. However the test system
> links objects from libcrypt.a into dynamic linked test binaries
> where hidden visibility does not work.  I think mixing static and
> shared libc components in the same binary should not be supported
> usage, but to be safe only use hidden in libc.a.

Why do we need this linkage for testing? Could we fix it so this
change could be used in all static libraries?

> 
> On some targets (i386) this optimization cannot be applied because
> hidden visibility PIE ifunc functions don't work, so it is gated by
> NO_HIDDEN_EXTERN_FUNC_IN_PIE.
> 
> From -static-pie linked 'int main(){}' this shaves off 71 relative
> relocs on aarch64 and reduces code size by about 2k.

Nice.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  include/libc-symbols.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> index ea126ae70c..f4dd735555 100644
> --- a/include/libc-symbols.h
> +++ b/include/libc-symbols.h
> @@ -434,13 +434,18 @@ for linking")
>    strong_alias(real, name)
>  #endif
>  
> -#if defined SHARED || defined LIBC_NONSHARED \
> -  || (BUILD_PIE_DEFAULT && IS_IN (libc))
> +#if defined SHARED || defined LIBC_NONSHARED
>  # define attribute_hidden __attribute__ ((visibility ("hidden")))
>  #else
>  # define attribute_hidden
>  #endif
>  
> +/* Mark all symbols hidden in static PIE libc to avoid GOT indirections.  */
> +#if BUILD_PIE_DEFAULT && !defined NO_HIDDEN_EXTERN_FUNC_IN_PIE \
> +    && IS_IN (libc) && !defined LIBC_NONSHARED
> +# pragma GCC visibility push(hidden)
> +#endif
> +
>  #define attribute_tls_model_ie __attribute__ ((tls_model ("initial-exec")))
>  
>  #define attribute_relro __attribute__ ((section (".data.rel.ro")))
> 

Ok.