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    SubjectRe: 3.6rc6 slab corruption.
    On 09/20/2012 03:19 AM, David Rientjes wrote:
    > On Wed, 19 Sep 2012, David Rientjes 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>
    >>
    >
    > An alternative to this, though, might be to never test for *ppos == 0 in
    > u32_array_read() and do the format_array_alloc() in u32_array_open() to
    > initialize file->private_data. If that allocation fails, just return
    > -ENOMEM. Then you never need to add a mutex in the read path.
    >

    Tested-by: Raghavendra <raghavendra.kt@linux.vnet.ibm.com>



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