linux-kernel - Re: [PATCH] lib/div64: off by one in shift

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Message-ID: <20190129152105.GA18383@redhat.com>
Date:   Tue, 29 Jan 2019 16:21:06 +0100
From:   Stanislaw Gruszka <sgruszka@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        Brian Behlendorf <behlendorf1@...l.gov>,
        Siarhei Volkau <lis8215@...il.com>
Subject: Re: [PATCH] lib/div64: off by one in shift

On Mon, Jan 28, 2019 at 09:45:03AM -0800, Andrew Morton wrote:
> On Mon, 28 Jan 2019 15:49:04 +0100 Stanislaw Gruszka <sgruszka@...hat.com> wrote:
> 
> > fls counts bits starting from 1 to 32 (returns 0 for zero argument).
> > If we add 1 we shift right one bit more and loose precision from
> > divisor, what cause function incorect results with some numbers.
> > 
> > Corrected code was tested in user-space, see bugzilla:
> > https://bugzilla.kernel.org/show_bug.cgi?id=202391
> 
> What are the usersoace-visible runtime effects of this change?

The bug is rather theoretical and for most cases divisor is within
32 bits, so problem is not visible. Moreover the bug is only for
32-bit systems and various users of it like btrfs are unlikely
to be run on such systems. So I do not consider this as very
important fix.

Thanks
Stanislaw