[PATCH v3 2/5] Make libc symbols hidden in static PIE

[H.J. Lu]

On Tue, Jan 12, 2021 at 9:26 AM Szabolcs Nagy via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Hidden matters with static PIE: extern symbol access in position
> independent code usually involves GOT indirections which needs
> RELATIVE relocs in a static linked PIE. Using So hidden avoids
> indirections and RELATIVE relocs on targets that can access symbols
> pc-relative.
>
> The check should use IS_IN_LIB instead of IS_IN(libc) since all
> static libraries can use hidden visibility to avoid indirections,
> however the test system links objects from libcrypt.a into dynamic
> linked test binaries so hidden does not work there.  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.
>
> From -static-pie linked 'int main(){}' this shaves off 73 relative
> relocs on aarch64 and reduces code size too.
> ---
>  include/libc-symbols.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> index ea126ae70c..93e63ee889 100644
> --- a/include/libc-symbols.h
> +++ b/include/libc-symbols.h
> @@ -434,13 +434,17 @@ 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 && 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")))
> --
> 2.17.1
>

Unfortunately it doesn't work on i686 with --enable-static-pie:

/usr/local/bin/ld:
/export/build/gnu/tools-build/glibc-32bit-static-pie-gitlab/build-i686-linux/libc.a(dl-lookup-direct.o):
unsupported non-PIC call to IFUNC `strcmp'

Please make it opt-out.

-- 
H.J.