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lib/div64.c: off by one in shift 

commit cdc94a37493135e355dfc0b0e086d84e3eadb50d upstream.

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: 658716d19f ("div64_u64(): improve precision on 32bit platforms")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Reported-by: Siarhei Volkau <lis8215@gmail.com>
Tested-by: Siarhei Volkau <lis8215@gmail.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
This commit is contained in:
Stanislaw Gruszka 2019-03-07 16:28:18 -08:00 committed by syphyr
parent 8cac138b0d
commit a1da08993f
1 changed files with 2 additions and 2 deletions
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lib/div64.c
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@ -100,7 +100,7 @@ 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)
@ -138,7 +138,7 @@ 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)