[tip:x86/asm] x86/vdso: Emit a GNU hash

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  6b7e26547fad7ace3dcb27a5babd2317fb9d1e12
Gitweb:     http://git.kernel.org/tip/6b7e26547fad7ace3dcb27a5babd2317fb9d1e12
Author:     Andy Lutomirski <luto@amacapital.net>
AuthorDate: Thu, 6 Aug 2015 14:45:45 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Sat, 8 Aug 2015 10:42:07 +0200

x86/vdso: Emit a GNU hash

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>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Isaac Dunham <ibid.ag@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nathan Lynch <nathan_lynch@mentor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rich Felker <dalias@libc.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: musl@lists.openwall.com <musl@lists.openwall.com>
Link: http://lkml.kernel.org/r/fd56cc057a2d62ab31c56a48d04fccb435b3fd4f.1438897382.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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 96c0617..a3d0767 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