linux-kernel - [PATCH 18/32] x86/vdso.lds: Align !timens (host's) vdso.so entries

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Date:   Wed,  6 Feb 2019 00:10:52 +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 18/32] x86/vdso.lds: Align !timens (host's) vdso.so entries

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.

Align VDSO entries for host with addresses generated from timens VDSO
(which is bigger as it has code for adding offsets).

Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
 arch/x86/entry/vdso/vdso-layout.lds.S | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/vdso/vdso-layout.lds.S b/arch/x86/entry/vdso/vdso-layout.lds.S
index ba216527e59f..e529ee3ec9e8 100644
--- a/arch/x86/entry/vdso/vdso-layout.lds.S
+++ b/arch/x86/entry/vdso/vdso-layout.lds.S
@@ -70,7 +70,17 @@ SECTIONS
 	 * stuff that isn't used at runtime in between.
 	 */

-	.text		: { *(.text*) }			:text	=0x90909090,
+	.text		: {
+#if defined(CONFIG_TIME_NS) && !defined(UNALIGNED_ENTRIES)
+#ifdef BUILD_VDSO32
+# include "vdso32.entries"
+#endif
+#ifdef BUILD_VDSO64
+# include "vdso64.entries"
+#endif
+#endif
+		*(.text*)
+	}						:text	=0x90909090,

 	.altinstructions	: { *(.altinstructions) }	:text
 	.altinstr_replacement	: { *(.altinstr_replacement) }	:text
-- 
2.20.1