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SubjectRe: 3.6rc6 slab corruption.
On Wed, 19 Sep 2012, Konrad Rzeszutek Wilk wrote:

> From 0806b133b5b28081adf23d0d04a99636ed3b861b Mon Sep 17 00:00:00 2001
> From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Date: Wed, 19 Sep 2012 11:23:01 -0400
> Subject: [PATCH 1/2] debugfs: Add lock for u32_array_read
>
> Dave Jones spotted that the u32_array_read was doing something funny:
>
> =============================================================================
> BUG kmalloc-64 (Not tainted): Redzone overwritten
> -----------------------------------------------------------------------------
>
> INFO: 0xffff88001f4b4970-0xffff88001f4b4977. First byte 0xbb instead of 0xcc
> INFO: Allocated in u32_array_read+0xd1/0x110 age=0 cpu=6 pid=32767
> __slab_alloc+0x516/0x5a5
> __kmalloc+0x213/0x2c0
> u32_array_read+0xd1/0x110
> .. snip..
> INFO: Freed in u32_array_read+0x99/0x110 age=0 cpu=0 pid=32749
> __slab_free+0x3f/0x3bf
> kfree+0x2d5/0x310
> u32_array_read+0x99/0x110
>
> Linus tracked it down and found out that "debugfs is racy for that case
> [read calls in parallel on the debugfs]. At least the file->private_data
> accesses are, for the case of that "u32_array" case.
>
> In fact it is racy in ... the whole "file->private_data" access ..
> If you have multiple readers on the same file, the whole
>
> if (file->private_data) {
> kfree(file->private_data);
> file->private_data = NULL;
> }
>
> file->private_data = format_array_alloc("%u", data->array,
> data->elements);
>
> thing is just a disaster waiting to happen." He suggested
> putting a lock which this patch does.
>

Since these are non-seekable files, it must also race to find *ppos == 0.

> The consequence of this is that it will trigger more spinlock usage,
> as this particular debugfs is used to provide a histogram of spinlock
> contention. But memory corruption is a worst offender then that.
>
> Reported-by: Dave Jones <davej@redhat.com>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>

Tested-by: David Rientjes <rientjes@google.com>


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