Re: [PATCH 1/2] erofs-utils: fix undefined behavior shift in erofs_init_devices

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2026/4/19 21:16, Nithurshen wrote:
> In erofs_init_devices(), roundup_pow_of_two() can potentially trigger
> an undefined behavior shift if the incremented 'ondisk_extradevs'
> value results in an overflow or an input that leads to an
> out-of-bounds shift.

I wonder why there is "out-of-bounds shift"? why you all
think it's an issue? Can you please explain in details?

ondisk_extradevs is 65535 at most.

Thanks,
Gao Xiang


> 
> Promote the argument to u64 before the increment to ensure the
> rounding logic operates on a safe bit-width.
> 
> Signed-off-by: Nithurshen <nithurshen.dev@gmail.com>
> ---
>   lib/super.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/super.c b/lib/super.c
> index 088c9a0..10831a7 100644
> --- a/lib/super.c
> +++ b/lib/super.c
> @@ -49,7 +49,7 @@ static int erofs_init_devices(struct erofs_sb_info *sbi,
>   		return 0;
>   
>   	sbi->extra_devices = ondisk_extradevs;
> -	sbi->device_id_mask = roundup_pow_of_two(ondisk_extradevs + 1) - 1;
> +	sbi->device_id_mask = roundup_pow_of_two((u64)ondisk_extradevs + 1) - 1;
>   	sbi->devs = calloc(ondisk_extradevs, sizeof(*sbi->devs));
>   	if (!sbi->devs)
>   		return -ENOMEM;