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

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

On Thu, Jan 14, 2021 at 3:18 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>
> The 01/13/2021 09:50, Szabolcs Nagy via Libc-alpha wrote:
> > The 01/12/2021 17:19, H.J. Lu wrote:
> > > On Tue, Jan 12, 2021 at 4:33 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > > On Tue, Jan 12, 2021 at 4:02 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > > > See:
> > > > >
> > > > > https://sourceware.org/bugzilla/show_bug.cgi?id=14961
> > > > >
> > > >  /* Mark all symbols hidden in static PIE libc to avoid GOT indirections.  */
> > > > -#if BUILD_PIE_DEFAULT && IS_IN (libc) && !defined LIBC_NONSHARED
> > > > +#if BUILD_PIE_DEFAULT && !defined NO_HIDDEN_EXTERN_FUNC_IN_PIE \
> > > > +    && IS_IN (libc) && !defined LIBC_NONSHARED
> > > >  # pragma GCC visibility push(hidden)
> > > >  #endif
> > > >
> > >
> > > This works on i686.
>
> The series i plan to commit today is in nsz/bug27072 now,
>
> This is the v4 of this patch:
>
> 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. Hidden visibility 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.
>
> There are targets (i686) where hidden visibility functions are
> problematic in PIE code so hidden cannot be applied to all symbols.
> Then static PIE requires extern object access without relocations
> (e.g. by relying on copy relocations in shared libraries instead of
> GOT access in PIE code). See bug 14961.
>
> From -static-pie linked 'int main(){}' this shaves off 73 relative
> relocs on aarch64 and reduces code size too.
> ---
>  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")))
> --
> 2.17.1
>

This generates bad static PIE on i386.   This patch is needed:

diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index f4dd735555..72276a5c48 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -434,7 +434,9 @@ for linking")
   strong_alias(real, name)
 #endif

-#if defined SHARED || defined LIBC_NONSHARED
+#if defined SHARED || defined LIBC_NONSHARED \
+  || !defined NO_HIDDEN_EXTERN_FUNC_IN_PIE \
+  || (BUILD_PIE_DEFAULT && IS_IN (libc))
 # define attribute_hidden __attribute__ ((visibility ("hidden")))
 #else
 # define attribute_hidden

-- 
H.J.