From fbfaf851902cd9293f392f3a1735e0543016d530 Mon Sep 17 00:00:00 2001
From: Florian La Roche <florian.laroche@googlemail.com>
Date: Sat, 19 Jan 2019 16:14:50 +0100
Subject: [PATCH] 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>
---
lib/int_sqrt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
index 14436f4ca6bd7..30e0f9770f88c 100644
--- 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;
--
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