[PATCH] Fix missing of last user while dumping slab corruption log

[ShiYong LI <a22381@xxxxxxxxxxxx>]

Hi,

Even with SLAB_RED_ZONE and SLAB_STORE_USER enabled, kernel would NOT
store redzone and last user data around allocated memory space if arch
cache line > sizeof(unsigned long long). As a result, last user information
is unexpectedly MISSED while dumping slab corruption log.

This patch makes sure that redzone and last user tags get stored whatever
arch cache line.

Compared to original codes, the change surely affects head redzone (redzone1).
Actually, with SLAB_RED_ZONE and SLAB_STORE_USER enabled,
allocated memory layout is as below:

[ redzone1 ]   <--------- Affected area.
[ real object space ]
[ redzone2 ]
[ last user ]
[ ... ]

Let's do some analysis: (whatever SLAB_STORE_USER is).

1) With SLAB_RED_ZONE on, "align" >= sizeof(unsigned long long) according to
    the following codes:
	/* 2) arch mandated alignment */
	if (ralign < ARCH_SLAB_MINALIGN) {
		ralign = ARCH_SLAB_MINALIGN;
	}
	/* 3) caller mandated alignment */
	if (ralign < align) {
		ralign = align;
	}
	...
	/*
	 * 4) Store it.
	 */
	align = ralign;

    That's to say, could guarantee that redzone1 does NOT get broken
at all. Meanwhile,
    Real object space could meet the need of cache line size by using
"align"  argument.

2) With SLAB_RED_ZONE off, the change has no impact.


>From 03b28964311090533643acd267abe0cbc3c9b0a5 Mon Sep 17 00:00:00 2001
From: Shiyong Li <shi-yong.li@xxxxxxxxxxxx>
Date: Fri, 2 Apr 2010 14:50:30 +0800
Subject: [PATCH] Fix missing of last user info while getting
DEBUG_SLAB config enabled.

Even with SLAB_RED_ZONE and SLAB_STORE_USER enabled, kernel would NOT
store redzone and last user data around allocated memory space if arch
cache line > sizeof(unsigned long long). As a result, last user information
is unexpectedly MISSED while dumping slab corruption log.

This fix makes sure that redzone and last user tags get stored whatever
cache line.

Signed-off-by: Shiyong Li <shi-yong.li@xxxxxxxxxxxx>
---
 mm/slab.c |    7 ++-----
 1 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index a8a38ca..84af997 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2267,9 +2267,6 @@ kmem_cache_create (const char *name, size_t
size, size_t align,
 	if (ralign < align) {
 		ralign = align;
 	}
-	/* disable debug if necessary */
-	if (ralign > __alignof__(unsigned long long))
-		flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
 	/*
 	 * 4) Store it.
 	 */
@@ -2289,8 +2286,8 @@ kmem_cache_create (const char *name, size_t
size, size_t align,
 	 */
 	if (flags & SLAB_RED_ZONE) {
 		/* add space for red zone words */
-		cachep->obj_offset += sizeof(unsigned long long);
-		size += 2 * sizeof(unsigned long long);
+		cachep->obj_offset += align;
+		size += align + sizeof(unsigned long long);
 	}
 	if (flags & SLAB_STORE_USER) {
 		/* user store requires one word storage behind the end of
-- 
1.6.0.4


Thanks & Best Regards
Shiyong

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