From: Zhang Yi <yi.zhang@huawei.com>

In __iomap_write_begin(), if we unaligned buffered write data to a hole
of a regular file, we only zero out the place where aligned to block
size that we don't want to write, but mark the whole range uptodate if
block size < folio size. This is wrong since the not zeroed part will
contains stale data and can be accessed by a concurrent buffered read
easily (on the filesystem may not hold inode->i_rwsem) once we mark the
range uptodate. Fix this by drop iomap_set_range_uptodate() in the
zeroing out branch.

Fixes: 9dc55f1389f9 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
Reported-by: Matthew Wilcox <willy@infradead.org>
Closes: https://lore.kernel.org/all/ZqsN5ouQTEc1KAzV@casper.infradead.org/
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
 fs/iomap/buffered-io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index ac762de9a27f..96600405dbb5 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -744,8 +744,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
 					poff, plen, srcmap);
 			if (status)
 				return status;
+			iomap_set_range_uptodate(folio, poff, plen);
 		}
-		iomap_set_range_uptodate(folio, poff, plen);
 	} while ((block_start += plen) < block_end);
 
 	return 0;
-- 
2.39.2

On Mon, Aug 12, 2024 at 08:11:58PM +0800, Zhang Yi wrote:
> +++ b/fs/iomap/buffered-io.c
> @@ -744,8 +744,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  					poff, plen, srcmap);
>  			if (status)
>  				return status;
> +			iomap_set_range_uptodate(folio, poff, plen);
>  		}
> -		iomap_set_range_uptodate(folio, poff, plen);
>  	} while ((block_start += plen) < block_end);

Um, what I meant was to just delete the iomap_set_range_uptodate()
call in __iomap_write_begin() altogether.  We'll call it soon enough in
__iomap_write_end().

On 2024/8/17 12:48, Matthew Wilcox wrote:
> On Mon, Aug 12, 2024 at 08:11:58PM +0800, Zhang Yi wrote:
>> +++ b/fs/iomap/buffered-io.c
>> @@ -744,8 +744,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>>  					poff, plen, srcmap);
>>  			if (status)
>>  				return status;
>> +			iomap_set_range_uptodate(folio, poff, plen);
>>  		}
>> -		iomap_set_range_uptodate(folio, poff, plen);
>>  	} while ((block_start += plen) < block_end);
> 
> Um, what I meant was to just delete the iomap_set_range_uptodate()
> call in __iomap_write_begin() altogether.  We'll call it soon enough in
> __iomap_write_end().
> 

Yeah! Looks reasonable to me.

Thanks,
Yi.

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

On Mon, Aug 12, 2024 at 08:11:58PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> In __iomap_write_begin(), if we unaligned buffered write data to a hole
> of a regular file, we only zero out the place where aligned to block
> size that we don't want to write, but mark the whole range uptodate if
> block size < folio size. This is wrong since the not zeroed part will
> contains stale data and can be accessed by a concurrent buffered read
> easily (on the filesystem may not hold inode->i_rwsem) once we mark the
> range uptodate. Fix this by drop iomap_set_range_uptodate() in the
> zeroing out branch.
> 
> Fixes: 9dc55f1389f9 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
> Reported-by: Matthew Wilcox <willy@infradead.org>
> Closes: https://lore.kernel.org/all/ZqsN5ouQTEc1KAzV@casper.infradead.org/
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> ---
>  fs/iomap/buffered-io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index ac762de9a27f..96600405dbb5 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -744,8 +744,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  					poff, plen, srcmap);
>  			if (status)
>  				return status;
> +			iomap_set_range_uptodate(folio, poff, plen);
>  		}
> -		iomap_set_range_uptodate(folio, poff, plen);

Don't we need to iomap_set_range_uptodate for the bytes that we zeroed
with folio_zero_segments?

--D

>  	} while ((block_start += plen) < block_end);
>  
>  	return 0;
> -- 
> 2.39.2
> 
>

On 2024/8/13 0:49, Darrick J. Wong wrote:
> On Mon, Aug 12, 2024 at 08:11:58PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>>
>> In __iomap_write_begin(), if we unaligned buffered write data to a hole
>> of a regular file, we only zero out the place where aligned to block
>> size that we don't want to write, but mark the whole range uptodate if
>> block size < folio size. This is wrong since the not zeroed part will
>> contains stale data and can be accessed by a concurrent buffered read
>> easily (on the filesystem may not hold inode->i_rwsem) once we mark the
>> range uptodate. Fix this by drop iomap_set_range_uptodate() in the
>> zeroing out branch.
>>
>> Fixes: 9dc55f1389f9 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
>> Reported-by: Matthew Wilcox <willy@infradead.org>
>> Closes: https://lore.kernel.org/all/ZqsN5ouQTEc1KAzV@casper.infradead.org/
>> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
>> ---
>>  fs/iomap/buffered-io.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
>> index ac762de9a27f..96600405dbb5 100644
>> --- a/fs/iomap/buffered-io.c
>> +++ b/fs/iomap/buffered-io.c
>> @@ -744,8 +744,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>>  					poff, plen, srcmap);
>>  			if (status)
>>  				return status;
>> +			iomap_set_range_uptodate(folio, poff, plen);
>>  		}
>> -		iomap_set_range_uptodate(folio, poff, plen);
> 
> Don't we need to iomap_set_range_uptodate for the bytes that we zeroed
> with folio_zero_segments?
> 

We must do partial block zeroing here, hence we don't need to set update
bit.

Thanks,
Yi.

> --D
> 
>>  	} while ((block_start += plen) < block_end);
>>  
>>  	return 0;
>> -- 
>> 2.39.2
>>
>>