Re: [PATCH] ntfs: widen the sub-block offsets in ntfs_decompress()

[liubaolin <liubaolin12138@163.com>]

在 2026/10/9 15:55, Zhan Xusheng 写道:
> ntfs_decompress() tracks where the current sub-block starts and ends
> inside the destination page with two u16s:
> 
> 	u16 do_sb_start;
> 	u16 do_sb_end;
> 	...
> 	do_sb_start = *dest_ofs;
> 	do_sb_end = do_sb_start + NTFS_SB_SIZE;
> 
> *dest_ofs is an int page offset masked with ~PAGE_MASK, and
> NTFS_SB_SIZE is 0x1000, so do_sb_end needs PAGE_SIZE + 0xfff of range.
> That exceeds u16 once PAGE_SIZE reaches 0x10000: the last sub-block of a
> page starts at 0xf000 and do_sb_end wraps from 0x10000 to 0.
> 
> The zero-fill that pads a short sub-block then computes a negative
> length:
> 
> 	if (dp_addr < dp_sb_end) {
> 		int nr_bytes = do_sb_end - *dest_ofs;
> 
> 		memset(dp_addr, 0, nr_bytes);
> 
> A sub-block made only of symbol tokens reaches this without consulting
> do_sb_end on the way, because that path just does
> 
> 	*dp_addr++ = *cb++;
> 	++*dest_ofs;
> 
> and is bounded by dp_addr against dp_sb_end.  With do_sb_start at 0xf000
> and eight literal bytes emitted, nr_bytes is -61448 and the memset()
> runs for 18446744073709490168 bytes.
> 
> Getting there is cheap: fifteen sub-blocks of eleven bytes each advance
> *dest_ofs to 0xf000, since the same zero-fill pads each of them out to
> 0x1000 while do_sb_end is still exact, and the sixteenth one triggers
> the wrap.  A compression block is 0x10000 bytes with 4 KiB clusters and
> the fixed compression unit of 4, so all sixteen fit with room to spare.
> 
> Phrase tokens are bounded against do_sb_end and so return -EOVERFLOW
> instead, and the
> 
> 	if (*dest_index == dest_max_index && do_sb_end > dest_max_ofs)
> 
> check at the top of the sub-block stops firing for the same reason.
> 
> Use int for both, matching *dest_ofs and dest_max_ofs.  The narrow type
> came in with the original import and survived the removal and revert of
> the driver.
> 
> Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
> Fixes: 1e9ea7e04472 ("Revert "fs: Remove NTFS classic"")
> ---
>   fs/ntfs/compress.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ntfs/compress.c b/fs/ntfs/compress.c
> index 075b57fc1de61..82c7c7d290737 100644
> --- a/fs/ntfs/compress.c
> +++ b/fs/ntfs/compress.c
> @@ -182,8 +182,8 @@ static int ntfs_decompress(struct page *dest_pages[], int completed_pages[],
>   	u8 *dp_addr;		/* Current pointer into dp. */
>   	u8 *dp_sb_start;	/* Start of current sub-block in dp. */
>   	u8 *dp_sb_end;		/* End of current sb in dp (dp_sb_start + NTFS_SB_SIZE). */
> -	u16 do_sb_start;	/* @dest_ofs when starting this sub-block. */
> -	u16 do_sb_end;		/* @dest_ofs of end of this sb (do_sb_start + NTFS_SB_SIZE). */
> +	int do_sb_start;	/* @dest_ofs when starting this sub-block. */
> +	int do_sb_end;		/* @dest_ofs of end of this sb (do_sb_start + NTFS_SB_SIZE). */
>   
>   	/* Variables for tag and token parsing. */
>   	u8 tag;			/* Current tag. */
> 
> base-commit: 6c377d19d4a5116d9bec5203aa3c6c11523e7898

Looks good to me.

Reviewed-by: Baolin Liu <liubaolin@kylinos.cn>