Re: [PATCH 15/28] ext4: Calculate and verify block bitmap checksum

[Andreas Dilger]

On 2011-11-09, at 5:57 PM, Darrick J. Wong wrote:
> On Mon, Nov 07, 2011 at 02:44:02PM -0700, Andreas Dilger wrote:
>> On 2011-11-07, at 1:00 PM, Darrick J. Wong wrote:
>>> On Thu, Oct 13, 2011 at 12:16:31AM -0700, Darrick J. Wong wrote:
>>>> On Wed, Oct 12, 2011 at 06:00:40PM -0600, Andreas Dilger wrote:
>>>>> I've been thinking a while that we should add per-group error flags
>>>>> for the block and inode bitmaps.  That way, if we detect errors with
>>>>> either one, we can set the flag in the group descriptor and avoid
>>>>> using it for any allocations in the future.  Otherwise, we try to
>>>>> read the bitmap in repeatedly.
>>>> 
>>>> I think there's some code in ext4 somewhere that does that.  I also
>>>> wonder if the possibility that we're seeing a transient corruption
>>>> error is worth rechecking the block until it fails?  (I suspect not,
>>>> but I decided to throw that out there anyway.)
>>> 
>>> There's a bit of code in ext4_init_block_bitmap that makes a block group
>>> unwritable if the bg checksum fails to verify:
>>> 
>>> /* If checksum is bad mark all blocks used to prevent allocation
>>>  * essentially implementing a per-group read-only flag. */
>>> if (!ext4_group_desc_csum_verify(sbi, block_group, gdp)) {
>>> 	ext4_error(sb, "Checksum bad for group %u",
>>> 			block_group);
>>> 	ext4_free_blks_set(sb, gdp, 0);
>>> 	ext4_free_inodes_set(sb, gdp, 0);
>>> 	ext4_itable_unused_set(sb, gdp, 0);
>>> 	memset(bh->b_data, 0xff, sb->s_blocksize);
>>> 	ext4_block_bitmap_csum_set(sb, block_group, gdp, bh,
>>> 				   EXT4_BLOCKS_PER_GROUP(sb) /
>>> 				   8);
>>> 	return 0;
>>> }
>>> 
>>> Do people think that doing this in the event of a block/inode bitmap
>>> checksum failure is a good idea?
>> 
>> For me, yes.  The sanity checks we do on the block bitmaps are only very
>> basic (e.g. bits for bitmaps themselves are set, for inode table).  Blocking
>> any allocation from a single group with a bad checksum is not harmful in the
>> long term, and can avoid an explosion of corruption if blocks would otherwise
>> be allocated multiple times.
> 
> On second thought, let's see what happens with groups that fail checksums ...
> it seems that ext4_check_descriptors() fails the mount in the event of a group
> descriptor failing checksum.  Both ext4_read_inode_bitmap() and
> ext4_read_block_bitmap() return NULL if the respective bitmap fails checksum.
> It looks like most of ext4's block {de,}allocate functions will fail on NULL
> bitmap pointer, so it shouldn't be possible to allocate (or deallocate) from a
> broken group.
> 

> There is one small deficiency: we need an explicit flag that marks the group
> dead.  That way if either bitmap fails checksum, the other _bitmap_read()
> function will know to just return NULL, and the group becomes totally frozen to
> allocation activity.

These could be stored in the group flags, bg_flags, like the UNINIT bits.
Since the group descriptors are pinned in memory there is no need to
re-read the group (and fail) again.  If it is written to disk, then e2fsck
can clear it.

> I think ext4_new_inode() need to be taught to continue scanning groups if it
> encounters a "dead" group?  I'm not sure yet if the same thing needs to be
> applied to the block allocation code.

It would be nice, for the case of bitmap checksum errors at least that the
allocator just chose the next block group and continued on running, though
it should also mark the superblock error flag.

If things are so bad that the device is completely unreadable, all of the block
groups would quickly be marked in error and the filesystem is now aborted like
it would have been previously.

> I'm actually wondering how it is that a group could pass checksum verification
> at mount time but fail it later, which is what that code snippet seems designed
> to catch.  Maybe something related to online resize?   Either way, I'm now
> wondering if we really need something like that for the simple case of bitmaps
> failing checksum.  I think we're already covered, but maybe I missed something?

Hmm, I guess the only way that this might be hit is via memory corruption?
I never really thought about it.  I guess it isn't harmful to check, and in
case of an in-memory corruption, it does call ext4_error() and typically
marks the filesystem read-only.

For inode and block bitmaps, however, I've seen corruption in a lot of cases.
Sometimes the simple checks catch this, but having a proper checksum would be
a lot more reassuring.

Cheers, Andreas





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