From fc0f36b2ce201142a9137fa8ea6a55e454d1213f Mon Sep 17 00:00:00 2001

From: Austin Clements <[email protected]>

Date: Fri, 2 Oct 2015 18:17:54 -0400

Subject: [PATCH 29/63] [release-branch.go1.5] runtime: use 4 byte writes in

 amd64p32 memmove/memclr

Currently, amd64p32's memmove and memclr use 8 byte writes as much as

possible and 1 byte writes for the tail of the object. However, if an

object ends with a 4 byte pointer at an 8 byte aligned offset, this

may copy/zero the pointer field one byte at a time, allowing the

garbage collector to observe a partially copied pointer.

Fix this by using 4 byte writes instead of 8 byte writes.

Updates #12552.

Change-Id: I13324fd05756fb25ae57e812e836f0a975b5595c

Reviewed-on: https://go-review.googlesource.com/15370

Run-TryBot: Austin Clements <[email protected]>

TryBot-Result: Gobot Gobot <[email protected]>

Reviewed-by: Brad Fitzpatrick <[email protected]>

Reviewed-by: Keith Randall <[email protected]>

Reviewed-on: https://go-review.googlesource.com/16908

Reviewed-by: Russ Cox <[email protected]>

---

 src/runtime/asm_amd64p32.s          |  6 +++---

 src/runtime/memmove_nacl_amd64p32.s | 23 +++++++++++++----------

 2 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/src/runtime/asm_amd64p32.s b/src/runtime/asm_amd64p32.s

index 6e97256..a001a76 100644

--- a/src/runtime/asm_amd64p32.s

+++ b/src/runtime/asm_amd64p32.s

@@ -627,12 +627,12 @@ TEXT runtime·memclr(SB),NOSPLIT,$0-8

 	MOVL	ptr+0(FP), DI

 	MOVL	n+4(FP), CX

 	MOVQ	CX, BX

-	ANDQ	$7, BX

-	SHRQ	$3, CX

+	ANDQ	$3, BX

+	SHRQ	$2, CX

 	MOVQ	$0, AX

 	CLD

 	REP

-	STOSQ

+	STOSL

 	MOVQ	BX, CX

 	REP

 	STOSB

diff --git a/src/runtime/memmove_nacl_amd64p32.s b/src/runtime/memmove_nacl_amd64p32.s

index 373607a..be9e1e5 100644

--- a/src/runtime/memmove_nacl_amd64p32.s

+++ b/src/runtime/memmove_nacl_amd64p32.s

@@ -4,6 +4,9 @@

 #include "textflag.h"

+// This could use MOVSQ, but we use MOVSL so that if an object ends in

+// a 4 byte pointer, we copy it as a unit instead of byte by byte.

+

 TEXT runtime·memmove(SB), NOSPLIT, $0-12

 	MOVL	to+0(FP), DI

 	MOVL	from+4(FP), SI

@@ -14,9 +17,9 @@ TEXT runtime·memmove(SB), NOSPLIT, $0-12

 forward:

 	MOVL	BX, CX

-	SHRL	$3, CX

-	ANDL	$7, BX

-	REP; MOVSQ

+	SHRL	$2, CX

+	ANDL	$3, BX

+	REP; MOVSL

 	MOVL	BX, CX

 	REP; MOVSB

 	RET

@@ -32,13 +35,13 @@ back:

 	STD

 	MOVL	BX, CX

-	SHRL	$3, CX

-	ANDL	$7, BX

-	SUBL	$8, DI

-	SUBL	$8, SI

-	REP; MOVSQ

-	ADDL	$7, DI

-	ADDL	$7, SI

+	SHRL	$2, CX

+	ANDL	$3, BX

+	SUBL	$4, DI

+	SUBL	$4, SI

+	REP; MOVSL

+	ADDL	$3, DI

+	ADDL	$3, SI

 	MOVL	BX, CX

 	REP; MOVSB

 	CLD

-- 

2.6.1