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

[TAO HU]

Hi, Pekka Enberg

We verified on OMAP3 (ARM V7).
1 thing we're sure
a) cachep->obj_offset will be aligned with "align" passed to
kmem_cache_create ()

Are you concerned about alignment for red-zone obj?
We'll double check.

-- 
Best Regards
Hu Tao

On Wed, Apr 7, 2010 at 11:25 PM, Pekka Enberg <penberg@xxxxxxxxxxxxxx> wrote:
> Hi,
>
> (I'm cc'ing David who did some fixes in this area previously.)
>
> On Fri, Apr 2, 2010 at 10:21 AM, ShiYong LI <a22381@xxxxxxxxxxxx> wrote:
>> 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
>
> The problem I have with this patch is that I am unable to convince
> myself that it's correct.
>
> The code in question is pretty hard to follow and it can break on
> architectures with strict alignment requirements. I'm not sure I fully
> understand why we've been disabling debugging in the first place.
>
> On which architectures have you verified this fix?
>
>                        Pekka
>
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