linux-kernel - [PATCH] lib/div64: off by one in shift

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Message-Id: <1548686944-11891-1-git-send-email-sgruszka@redhat.com>
Date:   Mon, 28 Jan 2019 15:49:04 +0100
From:   Stanislaw Gruszka <sgruszka@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Behlendorf <behlendorf1@...l.gov>,
        Siarhei Volkau <lis8215@...il.com>
Subject: [PATCH] lib/div64: off by one in shift

fls counts bits starting from 1 to 32 (returns 0 for zero argument).
If we add 1 we shift right one bit more and loose precision from
divisor, what cause function incorect results with some numbers.

Corrected code was tested in user-space, see bugzilla:
https://bugzilla.kernel.org/show_bug.cgi?id=202391

Fixes: 658716d19f8f ("div64_u64(): improve precision on 32bit platforms")
Reported-and-tested-by: Siarhei Volkau <lis8215@...il.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@...hat.com>
---
 lib/div64.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/div64.c b/lib/div64.c
index 01c8602bb6ff..ee146bb4c558 100644
--- a/lib/div64.c
+++ b/lib/div64.c
@@ -109,7 +109,7 @@ u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
 		quot = div_u64_rem(dividend, divisor, &rem32);
 		*remainder = rem32;
 	} else {
-		int n = 1 + fls(high);
+		int n = fls(high);
 		quot = div_u64(dividend >> n, divisor >> n);
 
 		if (quot != 0)
@@ -147,7 +147,7 @@ u64 div64_u64(u64 dividend, u64 divisor)
 	if (high == 0) {
 		quot = div_u64(dividend, divisor);
 	} else {
-		int n = 1 + fls(high);
+		int n = fls(high);
 		quot = div_u64(dividend >> n, divisor >> n);
 
 		if (quot != 0)
-- 
1.9.3