linux-kernel - Re: fix int_sqrt() for very large numbers

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Message-ID: <CAHk-=wgrTWpzTo56=mOT8f8RDAo=RvHsXp4d8GqbLwEZUQmJqw@mail.gmail.com>
Date:   Sun, 20 Jan 2019 22:00:07 +1200
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Crt Mori <cmo@...exis.com>
Cc:     Will Deacon <will.deacon@....com>,
        Florian La Roche <florian.laroche@...glemail.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Joe Perches <joe@...ches.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: fix int_sqrt() for very large numbers

On Sun, Jan 20, 2019 at 9:30 PM Crt Mori <cmo@...exis.com> wrote:
>
> I have just re-read the patch submit discussion and a sqrt of 64bit
> number can never be more than 32bit. That is why u32 return value is
> enough.

Right. And that's exactly why I thought it was so odd how the
mlx90632.c driver - which is the only user of int_sqrt64() - does a
nested one.

It would be sufficient to do

        int_sqrt(int_sqrt64(..))

because the domain on the outer one isn't actually 64 bits, it's only 32 bits.

But it's just a slight confusion and doesn't really matter much.

The bug in int_sqrt64() is real, but as mentioned, the calculations
done to generate the argument for that one user do seem to all be
*signed*. If it overflows into the high bit of an u64 (which is where
the off-by-one bug would matter), then the types in that one user are
already wrong.

So it would probably be a good idea to clarify the single user too while at it.

           Linus