Re: [PATCHv3 4/6] ext4: Make ext4_ind_map_blocks work with fiemap

[Ritesh Harjani <riteshh@linux.ibm.com>]

On 2/26/20 9:41 PM, Darrick J. Wong wrote:
> On Wed, Feb 26, 2020 at 03:27:06PM +0530, Ritesh Harjani wrote:
>> For indirect block mapping if the i_block > max supported block in inode
>> then ext4_ind_map_blocks may return a -EIO error. But in case of fiemap
>> this could be a valid query to ext4_map_blocks.
>> So in case if !create then return 0. This also makes ext4_warning to
>> ext4_debug in ext4_block_to_path() for the same reason.
>>
>> Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
>> ---
>>   fs/ext4/indirect.c | 11 +++++++++--
>>   1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
>> index 3a4ab70fe9e0..e1ab495dd900 100644
>> --- a/fs/ext4/indirect.c
>> +++ b/fs/ext4/indirect.c
>> @@ -102,7 +102,11 @@ static int ext4_block_to_path(struct inode *inode,
>>   		offsets[n++] = i_block & (ptrs - 1);
>>   		final = ptrs;
>>   	} else {
>> -		ext4_warning(inode->i_sb, "block %lu > max in inode %lu",
>> +		/*
>> +		 * It's not yet an error to just query beyond max
>> +		 * block in inode. Fiemap callers may do so.
>> +		 */
>> +		ext4_debug("block %lu > max in inode %lu",
>>   			     i_block + direct_blocks +
>>   			     indirect_blocks + double_blocks, inode->i_ino);
> 
> Does that mean fiemap callers can spamflood dmesg with this message just
> by setting the query start range to a huge value?

Not in the old implementation. But This could happen with indirect
block mapping with new implementation in iomap (as there is no check in 
place before calling ext4_map_blocks()).
Previously __generic_block_fiemap() used to not query beyond
i_size_read(), so we were safe there.

So yes now as Jan also suggested, will add a check in place in
ext4_iomap_begin_report() itself, so that this flooding wont happen.


Thanks for the review!!

-ritesh

> 
> --D
> 
>>   	}
>> @@ -537,8 +541,11 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
>>   	depth = ext4_block_to_path(inode, map->m_lblk, offsets,
>>   				   &blocks_to_boundary);
>>   
>> -	if (depth == 0)
>> +	if (depth == 0) {
>> +		if (!(flags & EXT4_GET_BLOCKS_CREATE))
>> +			err = 0;
>>   		goto out;
>> +	}
>>   
>>   	partial = ext4_get_branch(inode, depth, offsets, chain, &err);
>>   
>> -- 
>> 2.21.0
>>