tree 2e37e06899951dacc0e4ad0a91aedba167b8094b
parent 89a9f049f8b9755d8422c8e6ac6adb6fef437908
author Florian La Roche <florian.laroche@googlemail.com> 1547910890 +0100
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1548189634 +0100

fix int_sqrt64() for very large numbers

commit fbfaf851902cd9293f392f3a1735e0543016d530 upstream.

If an input number x for int_sqrt64() has the highest bit set, then
fls64(x) is 64.  (1UL << 64) is an overflow and breaks the algorithm.

Subtracting 1 is a better guess for the initial value of m anyway and
that's what also done in int_sqrt() implicitly [*].

[*] Note how int_sqrt() uses __fls() with two underscores, which already
    returns the proper raw bit number.

    In contrast, int_sqrt64() used fls64(), and that returns bit numbers
    illogically starting at 1, because of error handling for the "no
    bits set" case. Will points out that he bug probably is due to a
    copy-and-paste error from the regular int_sqrt() case.

Signed-off-by: Florian La Roche <Florian.LaRoche@googlemail.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>