Date:	Sat, 8 Mar 2014 02:25:43 +0530
From:	Nilesh More <nilesh99999@...il.com>
To:	"Theodore Ts'o" <tytso@....edu>, linux-kernel@...r.kernel.org
Subject: Re: Reporting a bug - Memory corruption in Linux kernel

Adding three more findings -

4. The memory pages thar are getting allocated in blkdev_get call in
step#1 are in msdos_partition() (1 page is alloacted here) and in
efi_partition() (2 pages are allocated here) function calls. I traced
the 'bdev->bd_inode->i_mapping->nrpages' to track the page
allocations. I could see this getting updated in above two function
calls. The call stack for reference:
blkdev_get->__blkdev_get->rescan_partitions-> check_partition->
check_part[i++](state)->efi_partition/msdos_partition

5. Now if I prevent the page allocations in efi_partition call by
doing an early return(anyway USB drive does not have efi compliant
partition table), then I could see the No-repro for this issue. This
suggests that page allocations from efi_partition function call are
running into already allocated ext4 fs allocations.

6. One more thing that I noticed is even with prevention of page
allocations in efi_partition, In valid access to FAT prints are
present. These pritnts won't be there if pages are not invalidated.
This means, even msdos_partition function call does not allocate the
correct/clean pages and when these pages are written back in while
invalidating them, incorrect data gets written to fat disk
inodes(directory inodes) which results in the "invalid access to FAT"
error prints.


I guess next step would be to try and understand the page allocations
in efi_partition and msdos_partition calls.

Thank you,
Nilesh

On Sat, Mar 8, 2014 at 1:48 AM, Nilesh More <nilesh99999@...il.com> wrote:
> Thanks Theodore for your quick reply.
>
> To make few things clear, USB drive has FAT file system in it. And the
> ext4 file system is of internal sdcard present in android device. The
> ext4 corruption in /data partition occurs when USB drive is
> hotplugged/hotunplugged. The bug may repro with first hotplug or with
> couple of hotplug/unplugs.
>
> The main concern here is even if USB drive is corrupted, that should
> not result into the native file system corruption.
>
> Today, I digged in further to see if I can get some more clues.
> Following are my findngs -
>
> 1. When the USB is hotplugged, in the call stack of add_disk( ),
> while registering disk blkdev_get(bdev, FMODE_READ, NULL) gets called
> which I guess scans the partition table, initializes part array and
> registers the partitions in the driver model.
>
> 2. To release the ownership of bdev obtained in step#1,
> blkdev_put(bdev, FMODE_READ) is called. This invalidates the pages
> cached for bdev in above blkdev_get call by first doing a writeback of
> these pages to disk.
>
> 3. Now if I prevent the invalidate page call in step# 2, I see that
> ext4 file system remains intact without any correction. That suggests,
> some part of cached pages obtained in step#1 blkdev_get call is
> already being used by ext4 file system and once these pages are
> invalidated we have a corruption in ext4 file system.
>
> My query now is, has anybody seen similar kind of issue before ? Could
> this be a known bug ?
>
> Thank you,
> Nilesh
>
>
> On Fri, Mar 7, 2014 at 9:30 AM, Theodore Ts'o <tytso@....edu> wrote:
>> On Fri, Mar 07, 2014 at 01:39:45AM +0530, Nilesh More wrote:
>>> Hi all,
>>>
>>> I am working on android bug wherein directory entries of ext4 file
>>> system get corrupted when USB is hotplugged (with auto mount support
>>> enabled).
>>>
>>> The logs as below:
>>> [ 413.607849] usb 2-1.1: USB disconnect, device number 12
>>                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>> Hot plugged or hot unplugged?  It looks like the problem is that the
>> block device disappeared out from under ext4.  Maybe you have a flaky
>> SD/MMC drive (i.e., funky contacts, etc.)?  Or maybe when you plug in
>> one USB device, the eMMC device where you have the mounted file system
>> disappeared?
>>
>>> If I prevent kill_bdev from invalidating pages, I see a No-Repro for
>>> this bug. Also there are no prints saying invalid access to FAT
>>> entry(which were present when bug reproduces). Earlier we had no-repro
>>> when added delay(1) before _getblk.
>>>
>>> This points out to the loss of sync between _getblk and kill_bdev and
>>> ALSO looks like kill_bdev inadvertently invalidates pages which are
>>> Ext4 owned.
>>
>> This looks like it's much more of a hardware issue than a software
>> issue.  If you are plugging in a USB device, you should *not* be
>> getting a USB disconnect message.  And the fact that the pages being
>> used by ext4 are getting invalidated would be consistent with the
>> theory that the USB device on which the ext4 file system was on is
>> somehow getting disconnected, per the message in you've shown in the
>> logs.
>>
>>                                                 - Ted
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