queue-3.18/lib-div64.c-off-by-one-in-shift.patch

   1 From dd59dac3d4d81e5d9469bd20d48fe55e8cbd53a2 Mon Sep 17 00:00:00 2001
   2 From: Stanislaw Gruszka <sgruszka@redhat.com>
   3 Date: Thu, 7 Mar 2019 16:28:18 -0800
   4 Subject: lib/div64.c: off by one in shift
   5
   6 [ Upstream commit cdc94a37493135e355dfc0b0e086d84e3eadb50d ]
   7
   8 fls counts bits starting from 1 to 32 (returns 0 for zero argument).  If
   9 we add 1 we shift right one bit more and loose precision from divisor,
  10 what cause function incorect results with some numbers.
  11
  12 Corrected code was tested in user-space, see bugzilla:
  13    https://bugzilla.kernel.org/show_bug.cgi?id=202391
  14
  15 Link: http://lkml.kernel.org/r/1548686944-11891-1-git-send-email-sgruszka@redhat.com
  16 Fixes: 658716d19f8f ("div64_u64(): improve precision on 32bit platforms")
  17 Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
  18 Reported-by: Siarhei Volkau <lis8215@gmail.com>
  19 Tested-by: Siarhei Volkau <lis8215@gmail.com>
  20 Acked-by: Oleg Nesterov <oleg@redhat.com>
  21 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
  22 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
  23 Signed-off-by: Sasha Levin <sashal@kernel.org>
  24 ---
  25  lib/div64.c | 4 ++--
  26  1 file changed, 2 insertions(+), 2 deletions(-)
  27
  28 diff --git a/lib/div64.c b/lib/div64.c
  29 index 4382ad77777e..ce76dc3d674e 100644
  30 --- a/lib/div64.c
  31 +++ b/lib/div64.c
  32 @@ -100,7 +100,7 @@ u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
  33                 quot = div_u64_rem(dividend, divisor, &rem32);
  34                 *remainder = rem32;
  35         } else {
  36 -               int n = 1 + fls(high);
  37 +               int n = fls(high);
  38                 quot = div_u64(dividend >> n, divisor >> n);
  39
  40                 if (quot != 0)
  41 @@ -138,7 +138,7 @@ u64 div64_u64(u64 dividend, u64 divisor)
  42         if (high == 0) {
  43                 quot = div_u64(dividend, divisor);
  44         } else {
  45 -               int n = 1 + fls(high);
  46 +               int n = fls(high);
  47                 quot = div_u64(dividend >> n, divisor >> n);
  48
  49                 if (quot != 0)
  50 --
  51 2.19.1
  52