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commit fbfaf851902cd9293f392f3a1735e0543016d530
parent 6e693b3ffecb0b478c7050b44a4842854154f715
Author: Florian La Roche <florian.laroche@googlemail.com>
Date:   Sat, 19 Jan 2019 16:14:50 +0100

fix int_sqrt64() for very large numbers

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>

Diffstat:
Mlib/int_sqrt.c | 2+-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c @@ -52,7 +52,7 @@ u32 int_sqrt64(u64 x) if (x <= ULONG_MAX) return int_sqrt((unsigned long) x); - m = 1ULL << (fls64(x) & ~1ULL); + m = 1ULL << ((fls64(x) - 1) & ~1ULL); while (m != 0) { b = y + m; y >>= 1;