Increase slab redzone to 64bits

There are two problems with the existing redzone implementation.

Firstly, it's causing misalignment of structures which contain a 64-bit
integer, such as netfilter's 'struct ipt_entry' -- causing netfilter
modules to fail to load because of the misalignment.  (In particular, the
first check in
net/ipv4/netfilter/ip_tables.c::check_entry_size_and_hooks())

On ppc32 and sparc32, amongst others, __alignof__(uint64_t) == 8.

With slab debugging, we use 32-bit redzones. And allocated slab objects
aren't sufficiently aligned to hold a structure containing a uint64_t.

By _just_ setting ARCH_KMALLOC_MINALIGN to __alignof__(u64) we'd disable
redzone checks on those architectures.  By using 64-bit redzones we avoid that
loss of debugging, and also fix the other problem while we're at it.

When investigating this, I noticed that on 64-bit platforms we're using a
32-bit value of RED_ACTIVE/RED_INACTIVE in the 64-bit memory location set
aside for the redzone.  Which means that the four bytes immediately before
or after the allocated object at 0x00,0x00,0x00,0x00 for LE and BE
machines, respectively.  Which is probably not the most useful choice of
poison value.

One way to fix both of those at once is just to switch to 64-bit
redzones in all cases.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/poison.h b/include/linux/poison.h
index 95f518b17684..d93c300a3449 100644
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -15,8 +15,8 @@
  * Magic nums for obj red zoning.
  * Placed in the first word before and the first word after an obj.
  */
-#define RED_INACTIVE    0x5A2CF071UL    /* when obj is inactive */
-#define RED_ACTIVE      0x170FC2A5UL    /* when obj is active */
+#define RED_INACTIVE    0x09F911029D74E35BULL   /* when obj is inactive */
+#define RED_ACTIVE      0xD84156C5635688C0ULL   /* when obj is active */
 
 #define SLUB_RED_INACTIVE       0xbb
 #define SLUB_RED_ACTIVE         0xcc