[PATCH] x86/vdso: Emit a GNU hash

[Andy Lutomirski <luto@kernel.org>]

From: Andy Lutomirski <luto@amacapital.net>

Some dynamic loaders may be slightly faster if a GNU hash is
available.  Strangely, this seems to have no effect at all on the
vdso size.

This is unlikely to have any measurable effect on the time it takes
to resolve vdso symbols (since there are so few of them).  In some
contexts, it can be a win for a different reason: if every DSO has a
GNU hash section, then libc can avoid calculating SysV hashes at
all.  Both musl and glibc appear to have this optimization.

It's plausible that this breaks some ancient glibc version.  If so,
then, depending on what glibc versions break, we could either
require COMPAT_VDSO for them or consider reverting.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---

For review.  I'll add it to my (currently empty) 4.3 pull request if
people like it, unless Ingo feels like adding it to -tip directly.

 arch/x86/entry/vdso/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index e97032069f88..20f3ac53148d 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -175,7 +175,7 @@ quiet_cmd_vdso = VDSO    $@
 		       -Wl,-T,$(filter %.lds,$^) $(filter %.o,$^) && \
 		 sh $(srctree)/$(src)/checkundef.sh '$(NM)' '$@'
 
-VDSO_LDFLAGS = -fPIC -shared $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) \
+VDSO_LDFLAGS = -fPIC -shared $(call cc-ldoption, -Wl$(comma)--hash-style=both) \
 	$(call cc-ldoption, -Wl$(comma)--build-id) -Wl,-Bsymbolic $(LTO_CFLAGS)
 GCOV_PROFILE := n
 
-- 
2.4.3