Re: [PATCH 1/2] exfat: fix iomap mapping when writeback races with truncate

[Chi Zhiling <chizhiling@163.com>]

On 11/13/26 4:06 PM, Youling Tang wrote:
> From: Youling Tang <tangyouling@kylinos.cn>
> 
> __exfat_iomap_begin() reads i_size twice: once to detect a range that
> starts beyond EOF, and once to clamp the mapping to EOF.  A concurrent
> truncate can shrink i_size between the two reads, so the clamp can
> produce a negative length.  The unsigned cluster index subtraction then
> underflows, making exfat_map_cluster() believe it has to allocate a huge
> number of clusters and report a hole.
> 
> Fix this by reading i_size once for both the EOF check and the
> block-aligned clamp.
> 
> Fixes: 82a81a7352bc ("exfat: add iomap buffered I/O support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>

Looks good to me:

Reviewed-by: Chi Zhiling <chizhiling@kylinos.cn>

> ---
>   fs/exfat/iomap.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/exfat/iomap.c b/fs/exfat/iomap.c
> index 4e6f3ed9dcc8..11823591b3a8 100644
> --- a/fs/exfat/iomap.c
> +++ b/fs/exfat/iomap.c
> @@ -51,8 +51,10 @@ static int __exfat_iomap_begin(struct inode *inode, loff_t offset, loff_t length
>   	bool balloc = false;
>   
>   	if (!may_alloc) {
> +		loff_t isize = i_size_read(inode);
> +
>   		/* Completely beyond EOF. Treat as hole */
> -		if (i_size_read(inode) <= offset) {
> +		if (offset >= isize) {
>   			iomap->type = IOMAP_HOLE;
>   			iomap->addr = IOMAP_NULL_ADDR;
>   			iomap->offset = offset;
> @@ -61,9 +63,8 @@ static int __exfat_iomap_begin(struct inode *inode, loff_t offset, loff_t length
>   		}
>   
>   		/* Clamp length if the requested range goes beyond i_size */
> -		if (offset + length > i_size_read(inode))
> -			length = round_up(i_size_read(inode),
> -					  i_blocksize(inode)) - offset;
> +		if (offset + length > isize)
> +			length = round_up(isize, i_blocksize(inode)) - offset;
>   	}
>   
>   	num_clusters = exfat_bytes_to_cluster_round_up(sbi,