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

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

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:
> >
> > On Tue, Jan 12, 2021 at 3:09 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > >
> > > 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.
> > >
> >
> > See:
> >
> > https://sourceware.org/bugzilla/show_bug.cgi?id=14961
> >
> > --
> > H.J.
>
> Testing:
>
> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> index 93e63ee889..f4dd735555 100644
> --- a/include/libc-symbols.h
> +++ b/include/libc-symbols.h
> @@ -441,7 +441,8 @@ for linking")
>  #endif
>
>  /* 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.


-- 
H.J.