linux-kernel - [PATCH 5.0 86/93] lib/div64.c: off by one in shift

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Date:   Thu, 18 Apr 2019 19:58:04 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Stanislaw Gruszka <sgruszka@...hat.com>,
        Siarhei Volkau <lis8215@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.0 86/93] lib/div64.c: off by one in shift

[ Upstream commit cdc94a37493135e355dfc0b0e086d84e3eadb50d ]

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

Link: http://lkml.kernel.org/r/1548686944-11891-1-git-send-email-sgruszka@redhat.com
Fixes: 658716d19f8f ("div64_u64(): improve precision on 32bit platforms")
Signed-off-by: Stanislaw Gruszka <sgruszka@...hat.com>
Reported-by: Siarhei Volkau <lis8215@...il.com>
Tested-by: Siarhei Volkau <lis8215@...il.com>
Acked-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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)
-- 
2.19.1