From: Linus Torvalds <torvalds@linux-foundation.org>
To: Florian La Roche <florian.laroche@googlemail.com>
Cc: Will Deacon <will.deacon@arm.com>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
Crt Mori <cmo@melexis.com>, Joe Perches <joe@perches.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: fix int_sqrt() for very large numbers
Date: Sun, 20 Jan 2019 17:11:14 +1200 [thread overview]
Message-ID: <CAHk-=wjwt4M7wGcMDkVLmSCsFP3xj29pAVgsC6-zbc_XPN1yiw@mail.gmail.com> (raw)
In-Reply-To: <CANZrA2YLtnv+PRBYzcLdFUg4uktx82G8dkoZCic8-ANK-3-7fA@mail.gmail.com>
On Sun, Jan 20, 2019 at 5:03 PM Florian La Roche
<florian.laroche@googlemail.com> wrote:
>
> The real bug is that we compute 1 to 64 for bit 0 to bit 63, whereas
> the algorithm expects 0 to 63 for the value of m.
Florian, you seem to be in denial.
__fls() returns 0-63. Your patch is *wrong* for the __fls() use,
because when you subtract 1, you get -1 to 62, and the -1 now
introduces the very undefined behavior you claim your patch fixes.
So your patch fixes one real case (int_sqrt64(), that has one user
that doesn't care) but it *BREAKS* the other case that is actually
much more widely used (int_sqrt()).
See what Will and I are complainig about?
Linus
next prev parent reply other threads:[~2019-01-20 5:11 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-19 15:14 fix int_sqrt() for very large numbers Florian La Roche
2019-01-20 0:01 ` Will Deacon
2019-01-20 3:48 ` Linus Torvalds
2019-01-20 5:03 ` Florian La Roche
2019-01-20 5:11 ` Linus Torvalds [this message]
2019-01-20 8:31 ` Crt Mori
2019-01-20 9:29 ` Crt Mori
2019-01-20 10:00 ` Linus Torvalds
2019-01-21 0:20 ` Linus Torvalds
2019-01-21 20:25 ` Crt Mori
2019-02-02 6:11 ` [LKP] 32bd07585d: kernel_selftests.lib.prime_numbers.sh.fail kernel test robot
2019-02-02 13:10 ` Florian La Roche
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