Hello Lianbo,
I wanted to keep the story short, but here is the long version.
This problem has been hiding for a long time due to a compiler alignment padding on 64bit machine.
For example, on RHEL 8.0, “include/linux/slub_def.h?v=4.18.0-80”:
0101 void (*ctor)(void *);
0102 unsigned int inuse; /* Offset to metadata */
0103 unsigned int align; /* Alignment */
0104 unsigned int red_left_pad; /* Left redzone padding size */
0105 const char *name; /* Name (only for display!) */
We have 3 “unsigned int” followed by “char*name” (64bit aligned), hence the compiler will add a 32bit padding after red_left_pad.
With such padding (and a on little-endian machine, ie. x86_64), a bogus load_ULONG() instead of a load_UINT() can’t really be noticed if the padding is zero-filled.
Now, on releases where we don’t have such compiler padding after red_left_pad, a bogus load_ULONG() instead of a load_UINT()
will produce noticeable garbage.
For example, on RHEL 7.8, “include/linux/slub_def.h?v=3.10.0-1127”:
0084 void (*ctor)(void *);
0085 int inuse; /* Offset to metadata */
0086 int align; /* Alignment */
0087 int reserved; /* Reserved bytes at the end of slabs */
0088 RH_KABI_FILL_HOLE(int red_left_pad) /* Left redzone padding size */
0089 const char *name; /* Name (only for display!) */
Here there won’t be any 32bit padding after red_left_pad as we have 4 “int” before “char*name”,
so here a load_ULONG() instead of load_UINT() we will get bogus data.
From a RHEL 7.8 crash dump, where the machine was booted with “slub_debug=FZP” (ie. Debugging all caches):
crash> sys | grep REL
RELEASE: 3.10.0-1127.el7.x86_64
crash> kmem -S names_cache
CACHE OBJSIZE ALLOCATED TOTAL SLABS SSIZE NAME
ffff8838ffc2c040 4096 1 336 48 32k names_cache
[…]
KMEM_CACHE_NODE NODE SLABS PARTIAL PER-CPU
ffff88017fdcfcc0 0 6 6 0
NODE 0 PARTIAL:
SLAB MEMORY NODE TOTAL ALLOCATED FREE
ffffea0021799000 ffff88085e640000 0 7 0 7
ffffea002179a600 ffff88085e698000 0 7 0 7
ffffea00217fce00 ffff88085ff38000 0 7 0 7
ffffea00216df000 ffff88085b7c0000 0 7 0 7
ffffea00216dc400 ffff88085b710000 0 7 0 7
ffffea0021799c00 ffff88085e670000 0 7 0 7
[…]
crash> struct kmem_cache ffff8838ffc2c040 | grep red
red_left_pad = 64,
crash> kmem -s ffff88085e640040 <<< trying first object, should be free
CACHE OBJSIZE ALLOCATED TOTAL SLABS SSIZE NAME
ffff8838ffc2c040 4096 1 336 48 32k names_cache
SLAB MEMORY NODE TOTAL ALLOCATED FREE
ffffea0021799000 ffff88085e640000 0 7 0 7
FREE / [ALLOCATED]
[ffff88085e640000] <<< getting expected red zone address, but not free !!
crash> set redzone off
redzone: off
crash> kmem -s ffff88085e640040 <<< trying again with redzone off
CACHE OBJSIZE ALLOCATED TOTAL SLABS SSIZE NAME
ffff8838ffc2c040 4096 1 336 48 32k names_cache
SLAB MEMORY NODE TOTAL ALLOCATED FREE
ffffea0021799000 ffff88085e640000 0 7 0 7
FREE / [ALLOCATED]
[ffc128105e640040] <<< getting a bogus object address, still not free
crash> rd -32 0xffff8838ffc2c09c
ffff8838ffc2c09c: 00000040 @...
crash> rd -64 0xffff8838ffc2c09c
ffff8838ffc2c09c: ffc1a00800000040 @.......
crash> p /x 0xffff88085e640000+0xffc1a00800000040
$1 = 0xffc128105e640040
Here above we can see that red_left_pad was read by crash as 0xffc1a00800000040 instead of 0x40 !!
With the patch:
crash> kmem -s ffff88085e640040
CACHE OBJSIZE ALLOCATED TOTAL SLABS SSIZE NAME
ffff8838ffc2c040 4096 1 336 48 32k names_cache
SLAB MEMORY NODE TOTAL ALLOCATED FREE
ffffea0021799000 ffff88085e640000 0 7 0 7
FREE / [ALLOCATED]
ffff88085e640000 <<< getting expected red zone address, and object free
crash> set redzone off
redzone: off
crash> kmem -s ffff88085e640040
CACHE OBJSIZE ALLOCATED TOTAL SLABS SSIZE NAME
ffff8838ffc2c040 4096 1 336 48 32k names_cache
SLAB MEMORY NODE TOTAL ALLOCATED FREE
ffffea0021799000 ffff88085e640000 0 7 0 7
FREE / [ALLOCATED]
ffff88085e640040 <<< getting expected object address, and object free
Now, with respect to:
> And also please add the kernel commit to patch log:
> kernel commit 2ca6d39b3102 ("slub: make ->red_left_pad unsigned int")
I don’t agree with this suggestion, this problem has nothing to do with “int” versus “unsigned int”.
It is only related to doing a load_ULONG() (64bit) instead of a load_INT() or load_UINT() (32bit).
For some releases, where we have a 32bit padding after red_left_pad, the problem won’t be noticed.
But without a 32bit padding after red_left_pad, crash will get a bogus red_left_pad causing all objects
to be reported as [ALLOCATED], and it will print bogus object addresses when using “set redzone off”.
Cheers,
Georges
From: lijiang <lijiang@xxxxxxxxxx>
Sent: Wednesday, September 4, 2024 5:13 AM
To: devel@xxxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Aureau, Georges (Kernel Tools ERT) <georges.aureau@xxxxxxx>
Subject: Re: [PATCH] “kmem address” not working properly when redzone is enabled
Hi, Aureau
Thank you for the fix.
On Thu, Aug 29, 2024 at 5:56 PM <devel-request@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Date: Thu, 29 Aug 2024 09:15:36 +0000
From: "Aureau, Georges (Kernel Tools ERT)" <georges.aureau@xxxxxxx>
Subject: [PATCH] “kmem address” not working properly
when redzone is enabled
To: "devel@xxxxxxxxxxxxxxxxxxxxxxxxxxx"
<devel@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
Message-ID: <SJ0PR84MB1482E72F9E168B3B0CE885C89F962@xxxxxxxxxxxxxxxxxx
RD84.PROD.OUTLOOK.COM>
Content-Type: text/plain; charset="Windows-1252"
Crash “kmem address” not working properly when redzone is enabled.
When "slub_debug" is enabled with redzoning, "kmem address" does not work properly.
The "red_left_pad" member within "struct kmem_cache" is currently an "unsigned int",
it used to be an "int", but it never was a "long", hence "red_left_pad" in do_slab_slub()
was not initialized properly. This "red_left_pad" issue resulted in reporting free objects
as "[ALLOCATED]", and in reporting bogus object addresses when using "set redzone off".
Can you help add the result of the 'kmem address' command here? We can clearly see what error it is.
And also please add the kernel commit to patch log:
kernel commit 2ca6d39b3102 ("slub: make ->red_left_pad unsigned int")
Signed-off-by: Georges Aureau <georges.aureau@xxxxxxx>
--
diff --git a/memory.c b/memory.c
index a74ebaf..967a9cf 100644
--- a/memory.c
+++ b/memory.c
@@ -19637,7 +19637,8 @@ do_slab_slub(struct meminfo *si, int verbose)
int i, free_objects, cpu_slab, is_free, node;
ulong p, q;
#define SLAB_RED_ZONE 0x00000400UL
- ulong flags, red_left_pad;
+ ulong flags;
+ uint red_left_pad;
if (!si->slab) {
if (CRASHDEBUG(1))
@@ -19727,7 +19728,7 @@ do_slab_slub(struct meminfo *si, int verbose)
if (VALID_MEMBER(kmem_cache_red_left_pad)) {
flags = ULONG(si->cache_buf + OFFSET(kmem_cache_flags));
if (flags & SLAB_RED_ZONE)
- red_left_pad = ULONG(si->cache_buf + OFFSET(kmem_cache_red_left_pad));
+ red_left_pad = UINT(si->cache_buf + OFFSET(kmem_cache_red_left_pad));
}
This change looks good to me, but I still have a question:
I can not reproduce the current issue, how did you reproduce this one? Can you help list the steps to reproduce?
Thanks
Lianbo
for (p = vaddr; p < vaddr + objects * si->size; p += si->size) {
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