[Question] Crash of kmem_cache_cpu->freelist access

[Question] Crash of kmem_cache_cpu->freelist access

[Gavin Guo]

Hi all,

I'm currently debugging a bug and found out the problem is the general
protection fault of the following access:

static inline void *get_freepointer(struct kmem_cache *s, void *object)
{
        return *(void **)(object + s->offset);
}

I tried to disassembly and found that the object is from c->freelist
and it has an abnormal value which caused the fault. My first thought
is to try to add slub_debug in the kernel command line. But, the
kernel is a production kernel and may not have the chance to add
kernel parameters. The other way is to "echo 1 >
/sys/kernel/slab/<object name>/poison." But, I found the allocation is
bound to kmalloc-1024. So, it may not have a chance to enable the
sysfs poison debugging.

I tried to debug for a long time and can't find any clue. Is there
anyone has efficient debugging methods to deal with the c->freelist
crashing if the slub_debug doesn't have a chance to be added to kernel
parameters.

Really thanks for your time reading the mail.

Thanks,
Tuffkid

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Re: [Question] Crash of kmem_cache_cpu->freelist access

[Christoph Lameter]

On Sun, 14 Dec 2014, Gavin Guo wrote:

> I tried to disassembly and found that the object is from c->freelist
> and it has an abnormal value which caused the fault. My first thought
> is to try to add slub_debug in the kernel command line. But, the
> kernel is a production kernel and may not have the chance to add
> kernel parameters. The other way is to "echo 1 >

the slub_debug option is there so that production kernels *can* run with
debugging and that we can locate these issues.

If you cannot get to a boot prompt then please change the grub
configuration and add the slub_debug parameter.

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