Re: [PATCH 10/10] ext4: zero post-EOF partial block before appending write

[Zhang Yi]

On 3/24/2026 4:31 AM, Jan Kara wrote:
> On Tue 10-03-26 09:41:01, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>>
>> In cases of appending write beyond EOF, ext4_zero_partial_blocks() is
>> called within ext4_*_write_end() to zero out the partial block beyond
>> EOF. This prevents exposing stale data that might be written through
>> mmap.
>>
>> However, supporting only the regular buffered write path is
>> insufficient. It is also necessary to support the DAX path as well as
>> the upcoming iomap buffered write path. Therefore, move this operation
>> to ext4_write_checks().
>>
>> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> I'd note that this allows page fault to race in between the zeroing and
> actual write resulting in new possible result - previously for file size 8,
> pwrite('WWWW...', 8, 16) racing with mmap writes of 'MMMMMM...' at offset 8
> into the page you could see either:
> 
> DDDDDDDD00000000WWWWWWWW
> or
> DDDDDDDDMMMMMMMMMMMMMMMM
> 
> 
> now you can see both of the above an also
> 
> DDDDDDDMMMMMMMMWWWWWWWWW
> 
> But I don't think that's strictly invalid content and userspace that would
> depend on the outcome of such race would be silly. So feel free to add:

Yes exactly.  :-)

Thanks,
Yi.

> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
> 								Honza
> 
>> ---
>>  fs/ext4/file.c  | 14 ++++++++++++++
>>  fs/ext4/inode.c | 21 +++++++--------------
>>  2 files changed, 21 insertions(+), 14 deletions(-)
>>
>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>> index f1dc5ce791a7..b2e44601ab6a 100644
>> --- a/fs/ext4/file.c
>> +++ b/fs/ext4/file.c
>> @@ -271,6 +271,8 @@ static ssize_t ext4_generic_write_checks(struct kiocb *iocb,
>>  
>>  static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
>>  {
>> +	struct inode *inode = file_inode(iocb->ki_filp);
>> +	loff_t old_size = i_size_read(inode);
>>  	ssize_t ret, count;
>>  
>>  	count = ext4_generic_write_checks(iocb, from);
>> @@ -280,6 +282,18 @@ static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
>>  	ret = file_modified(iocb->ki_filp);
>>  	if (ret)
>>  		return ret;
>> +
>> +	/*
>> +	 * If the position is beyond the EOF, it is necessary to zero out the
>> +	 * partial block that beyond the existing EOF, as it may contains
>> +	 * stale data written through mmap.
>> +	 */
>> +	if (iocb->ki_pos > old_size && !ext4_verity_in_progress(inode)) {
>> +		ret = ext4_block_zero_eof(inode, old_size, iocb->ki_pos);
>> +		if (ret < 0)
>> +			return ret;
>> +	}
>> +
>>  	return count;
>>  }
>>  
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 5288d36b0f09..67a4d12fcb4d 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -1456,10 +1456,9 @@ static int ext4_write_end(const struct kiocb *iocb,
>>  	folio_unlock(folio);
>>  	folio_put(folio);
>>  
>> -	if (old_size < pos && !verity) {
>> +	if (old_size < pos && !verity)
>>  		pagecache_isize_extended(inode, old_size, pos);
>> -		ext4_block_zero_eof(inode, old_size, pos);
>> -	}
>> +
>>  	/*
>>  	 * Don't mark the inode dirty under folio lock. First, it unnecessarily
>>  	 * makes the holding time of folio lock longer. Second, it forces lock
>> @@ -1574,10 +1573,8 @@ static int ext4_journalled_write_end(const struct kiocb *iocb,
>>  	folio_unlock(folio);
>>  	folio_put(folio);
>>  
>> -	if (old_size < pos && !verity) {
>> +	if (old_size < pos && !verity)
>>  		pagecache_isize_extended(inode, old_size, pos);
>> -		ext4_block_zero_eof(inode, old_size, pos);
>> -	}
>>  
>>  	if (size_changed) {
>>  		ret2 = ext4_mark_inode_dirty(handle, inode);
>> @@ -3196,7 +3193,7 @@ static int ext4_da_do_write_end(struct address_space *mapping,
>>  	struct inode *inode = mapping->host;
>>  	loff_t old_size = inode->i_size;
>>  	bool disksize_changed = false;
>> -	loff_t new_i_size, zero_len = 0;
>> +	loff_t new_i_size;
>>  	handle_t *handle;
>>  
>>  	if (unlikely(!folio_buffers(folio))) {
>> @@ -3240,19 +3237,15 @@ static int ext4_da_do_write_end(struct address_space *mapping,
>>  	folio_unlock(folio);
>>  	folio_put(folio);
>>  
>> -	if (pos > old_size) {
>> +	if (pos > old_size)
>>  		pagecache_isize_extended(inode, old_size, pos);
>> -		zero_len = pos - old_size;
>> -	}
>>  
>> -	if (!disksize_changed && !zero_len)
>> +	if (!disksize_changed)
>>  		return copied;
>>  
>> -	handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
>> +	handle = ext4_journal_start(inode, EXT4_HT_INODE, 1);
>>  	if (IS_ERR(handle))
>>  		return PTR_ERR(handle);
>> -	if (zero_len)
>> -		ext4_block_zero_eof(inode, old_size, pos);
>>  	ext4_mark_inode_dirty(handle, inode);
>>  	ext4_journal_stop(handle);
>>  
>> -- 
>> 2.52.0
>>