Re: [PATCH] btrfs: Avoid potential integer overflow when left-shifting 32-bit int

[Qu Wenruo]

On 2023/4/7 03:24, Nur Hussein wrote:
> In scrub_stripe(), the 32-bit signed value returned by the
> nr_data_stripes(map) function call should be cast to u64
> before being shifted left by BTRFS_STRIPE_LEN_SHIFT (16),
> as a cautionary measure to avoid potential overflows. We
> then assign it to a u64 value anyway, so a cast before a
> shift seems prudent.

I'd say it's a little overkilled.

For nr_data_stripes(), it's at most hundreds of stripes (which is 
already insane).
Even with 16 bits left shift, we need to get 2 ** 16 stripes to overflow 
32bits.

Thanks,
Qu
> Signed-off-by: Nur Hussein <hussein@xxxxxxxxxxx>
> ---
>   fs/btrfs/scrub.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> index ccb4f58ae307..4de1665fcd52 100644
> --- a/fs/btrfs/scrub.c
> +++ b/fs/btrfs/scrub.c
> @@ -2187,7 +2187,7 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
>   
>   	/* Initialize @offset in case we need to go to out: label */
>   	get_raid56_logic_offset(physical, stripe_index, map, &offset, NULL);
> -	increment = nr_data_stripes(map) << BTRFS_STRIPE_LEN_SHIFT;
> +	increment = (u64)nr_data_stripes(map) << BTRFS_STRIPE_LEN_SHIFT;
>   
>   	/*
>   	 * Due to the rotation, for RAID56 it's better to iterate each stripe