Messages in this thread | | | Date | Thu, 20 Sep 2012 18:27:54 +0530 | From | Raghavendra K T <> | Subject | Re: 3.6rc6 slab corruption. |
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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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