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

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Message-ID: <353b44f3-fb95-ac43-53ba-0d3b45fff574@gmx.com>
Date:   Fri, 7 Apr 2023 08:35:40 +0800
From:   Qu Wenruo <quwenruo.btrfs@....com>
To:     Nur Hussein <hussein@...xcat.org>, clm@...com,
        josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: Avoid potential integer overflow when
 left-shifting 32-bit int



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@...xcat.org>
> ---
>   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