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

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Message-ID: <ZDAf8Cv3HR728on3@gandalf>
Date:   Fri, 7 Apr 2023 21:51:44 +0800
From:   Nur Hussein <hussein@...xcat.org>
To:     Qu Wenruo <quwenruo.btrfs@....com>
Cc:     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 Fri, Apr 07, 2023 at 08:35:40AM +0800, Qu Wenruo wrote:
> 
> 
> 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.

Perhaps so, but it was flagged by Coverity, and it's a little safer with
the cast, with no cost. It's up to y'all if you want it though.

- Nur