Re: [PATCH] ntfs: fix undefined behavior in mft/index record size calculation

[Namjae Jeon]

On Tue, Aug 25, 2026 at 6:54 PM Hongling Zeng <zenghongling@xxxxxxxxxx> wrote:
>
> The boot sector validation allows clusters_per_mft_record and
> clusters_per_index_record to range from 0xE1 (-31) to 0xF7 (-9) when
> interpreted as signed values. When these are used as negative shift
> counts in expressions like `1 << -clusters_per_mft_record`, values
> like 0xE1 cause `1 << 31`, which shifts into the sign bit of a 32-bit
> signed integer, resulting in undefined behavior.
>
> Fix by using unsigned shift (1U << ...) instead of signed shift.
> This prevents undefined behavior while preserving the full valid
> range of negative values (-31 to -9) that may appear in NTFS boot
> sectors.
>
> The encoding scheme uses negative values to represent record sizes
> smaller than cluster_size: -log2(record_size). Common values include
> -10 (1024 bytes) for mft_record_size and -12 (4096 bytes) for
> index_record_size.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hongling Zeng <zenghongling@xxxxxxxxxx>
Applied it to #ntfs-next with Baolin's reviewed-by tag.
Thanks!