Re: [PATCH V8 15/19] xfs: Directory's data fork extent counter can never overflow

Chandan Babu R
2022-03-29T05:22:27Z

On 25 Mar 2022 at 03:44, Dave Chinner wrote:
> On Mon, Mar 21, 2022 at 10:47:46AM +0530, Chandan Babu R wrote:
>> The maximum file size that can be represented by the data fork extent counter
>> in the worst case occurs when all extents are 1 block in length and each block
>> is 1KB in size.
>>
>> With XFS_MAX_EXTCNT_DATA_FORK_SMALL representing maximum extent count and with
>> 1KB sized blocks, a file can reach upto,
>> (2^31) * 1KB = 2TB
>>
>> This is much larger than the theoretical maximum size of a directory
>> i.e. 32GB * 3 = 96GB.
>>
>> Since a directory's inode can never overflow its data fork extent counter,
>> this commit replaces checking the return value of
>> xfs_iext_count_may_overflow() with calls to ASSERT(error == 0).
>
> I'd really prefer that we don't add noise like this to a bunch of
> call sites. If directories can't overflow the extent count in
> normal operation, then why are we even calling
> xfs_iext_count_may_overflow() in these paths? i.e. an overflow would
> be a sign of an inode corruption, and we should have flagged that
> long before we do an operation that might overflow the extent count.
>
> So, really, I think you should document the directory size
> constraints at the site where we define all the large extent count
> values in xfs_format.h, remove the xfs_iext_count_may_overflow()
> checks from the directory code and replace them with a simple inode
> verifier check that we haven't got more than 100GB worth of
> individual extents in the data fork for directory inodes....

I don't think that we could trivially verify if the extents in a directory's
data fork add up to more than 96GB.

xfs_dinode->di_size tracks the size of XFS_DIR2_DATA_SPACE. This also includes
holes that could be created by freeing directory entries in a single directory
block. Also, there is no easy method to determine the space occupied by
XFS_DIR2_LEAF_SPACE and XFS_DIR2_FREE_SPACE segments of a directory.

May be the following can be added to xfs_dinode_verify(),

if (S_ISDIR(mode) && ((xfs_dinode->di_size + 2 * 32GB) > 96GB))
return __this_address

as a rough check against possible directory inode corruption.

--
chandan