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Message-Id: <20190206001107.16488-15-dima@arista.com>
Date: Wed, 6 Feb 2019 00:10:48 +0000
From: Dmitry Safonov <dima@...sta.com>
To: linux-kernel@...r.kernel.org
Cc: Dmitry Safonov <dima@...sta.com>, Adrian Reber <adrian@...as.de>,
Andrei Vagin <avagin@...nvz.org>,
Andrei Vagin <avagin@...il.com>,
Andy Lutomirski <luto@...nel.org>,
Andy Tucker <agtucker@...gle.com>,
Arnd Bergmann <arnd@...db.de>,
Christian Brauner <christian.brauner@...ntu.com>,
Cyrill Gorcunov <gorcunov@...nvz.org>,
Dmitry Safonov <0x7f454c46@...il.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>,
Pavel Emelyanov <xemul@...tuozzo.com>,
Shuah Khan <shuah@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
containers@...ts.linux-foundation.org, criu@...nvz.org,
linux-api@...r.kernel.org, x86@...nel.org
Subject: [PATCH 14/32] x86/VDSO: Build VDSO with -ffunction-sections
As it has been discussed on timens RFC, adding a new conditional branch
`if (inside_time_ns)` on VDSO for all processes is undesirable.
It will add a penalty for everybody as branch predictor may mispredict
the jump. Also there are instruction cache lines wasted on cmp/jmp.
Those effects of introducing time namespace are very much unwanted
having in mind how much work have been spent on micro-optimisation
vdso code.
Addressing those problems, there are two versions of VDSO's .so:
for host tasks (without any penalty) and for processes inside of time
namespace with clk_to_ns() that subtracts offsets from host's time.
Unfortunately, to allow changing VDSO VMA on a running process,
the entry points to VDSO should have the same offsets (addresses).
That's needed as i.e. application that calls setns() may have already
resolved VDSO symbols in GOT/PLT.
Compile VDSO images with -ffunction-sections so that VDSO entries can be
aligned on the same addresses with linker script magic.
Put ia32 functions those are written in assembly into corresponding
sections.
Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
arch/x86/entry/vdso/Makefile | 2 +-
arch/x86/entry/vdso/vdso32/sigreturn.S | 2 ++
arch/x86/entry/vdso/vdso32/system_call.S | 2 +-
3 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 2433ed9342fd..55ba81d4415c 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -3,7 +3,7 @@
# Building vDSO images for x86.
#
-KBUILD_CFLAGS += $(DISABLE_LTO)
+KBUILD_CFLAGS += $(DISABLE_LTO) -ffunction-sections
KASAN_SANITIZE := n
UBSAN_SANITIZE := n
OBJECT_FILES_NON_STANDARD := y
diff --git a/arch/x86/entry/vdso/vdso32/sigreturn.S b/arch/x86/entry/vdso/vdso32/sigreturn.S
index c3233ee98a6b..b641ccf8d664 100644
--- a/arch/x86/entry/vdso/vdso32/sigreturn.S
+++ b/arch/x86/entry/vdso/vdso32/sigreturn.S
@@ -11,6 +11,7 @@
.globl __kernel_sigreturn
.type __kernel_sigreturn,@function
nop /* this guy is needed for .LSTARTFDEDLSI1 below (watch for HACK) */
+ .section .text.__kernel_sigreturn, "ax"
ALIGN
__kernel_sigreturn:
.LSTART_sigreturn:
@@ -21,6 +22,7 @@ __kernel_sigreturn:
nop
.size __kernel_sigreturn,.-.LSTART_sigreturn
+ .section .text.__kernel_rt_sigreturn, "ax"
.globl __kernel_rt_sigreturn
.type __kernel_rt_sigreturn,@function
ALIGN
diff --git a/arch/x86/entry/vdso/vdso32/system_call.S b/arch/x86/entry/vdso/vdso32/system_call.S
index 263d7433dea8..13ec05287f63 100644
--- a/arch/x86/entry/vdso/vdso32/system_call.S
+++ b/arch/x86/entry/vdso/vdso32/system_call.S
@@ -8,7 +8,7 @@
#include <asm/cpufeatures.h>
#include <asm/alternative-asm.h>
- .text
+ .section .text.__kernel_vsyscall, "ax"
.globl __kernel_vsyscall
.type __kernel_vsyscall,@function
ALIGN
--
2.20.1
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