Re: [PATCH 4/4] xfs: fix sign handling problem in xfs_bmbt_diff_two_keys

[Eric Sandeen <sandeen@sandeen.net>]

On 8/26/19 6:15 PM, Dave Chinner wrote:
> On Mon, Aug 26, 2019 at 02:49:09PM -0700, Darrick J. Wong wrote:
>> From: Darrick J. Wong <darrick.wong@oracle.com>
>>
>> In xfs_bmbt_diff_two_keys, we perform a signed int64_t subtraction with
>> two unsigned 64-bit quantities.  If the second quantity is actually the
>> "maximum" key (all ones) as used in _query_all, the subtraction
>> effectively becomes addition of two positive numbers and the function
>> returns incorrect results.  Fix this with explicit comparisons of the
>> unsigned values.  Nobody needs this now, but the online repair patches
>> will need this to work properly.
>>
>> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
>> ---
>>  fs/xfs/libxfs/xfs_bmap_btree.c |   16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>
>>
>> diff --git a/fs/xfs/libxfs/xfs_bmap_btree.c b/fs/xfs/libxfs/xfs_bmap_btree.c
>> index fbb18ba5d905..3c1a805b3775 100644
>> --- a/fs/xfs/libxfs/xfs_bmap_btree.c
>> +++ b/fs/xfs/libxfs/xfs_bmap_btree.c
>> @@ -400,8 +400,20 @@ xfs_bmbt_diff_two_keys(
>>  	union xfs_btree_key	*k1,
>>  	union xfs_btree_key	*k2)
>>  {
>> -	return (int64_t)be64_to_cpu(k1->bmbt.br_startoff) -
>> -			  be64_to_cpu(k2->bmbt.br_startoff);
>> +	uint64_t		a = be64_to_cpu(k1->bmbt.br_startoff);
>> +	uint64_t		b = be64_to_cpu(k2->bmbt.br_startoff);
>> +
>> +	/*
>> +	 * Note: This routine previously casted a and b to int64 and subtracted
>> +	 * them to generate a result.  This lead to problems if b was the
>> +	 * "maximum" key value (all ones) being signed incorrectly, hence this
>> +	 * somewhat less efficient version.
>> +	 */
>> +	if (a > b)
>> +		return 1;
>> +	else if (b > a)
>> +		return -1;
> 
> No need for an else here, but otherwise OK.
> 
> Reviewed-by: Dave Chinner <dchinner@redhat.com>

In fact having the else means the a == b case isn't handled, even if it
should never happen, so might a static checker eventually complain about
reaching the end of a non-void function?

-Eric