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

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

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