[PATCH] timekeeping: improve arithmetic division in scale64_check_overflow()

[PATCH] timekeeping: improve arithmetic division in scale64_check_overflow()

[Wen Yang]

do_div() does a 64-by-32 division. Use div64_u64() instead of it
if the divisor is u64, to avoid truncation to 32-bit.
And as a nice side effect also cleans up the function a bit.

Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 kernel/time/timekeeping.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ca69290bee2a..4fc2af4367a7 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1005,9 +1005,8 @@ static int scale64_check_overflow(u64 mult, u64 div, u64 *base)
 	    ((int)sizeof(u64)*8 - fls64(mult) < fls64(rem)))
 		return -EOVERFLOW;
 	tmp *= mult;
-	rem *= mult;
 
-	do_div(rem, div);
+	rem = div64_u64(rem * mult, div);
 	*base = tmp + rem;
 	return 0;
 }
-- 
2.23.0

[tip: timers/core] timekeeping: Prevent 32bit truncation in scale64_check_overflow()

[tip-bot2 for Wen Yang]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     4cbbc3a0eeed675449b1a4d080008927121f3da3
Gitweb:        https://git.kernel.org/tip/4cbbc3a0eeed675449b1a4d080008927121f3da3
Author:        Wen Yang <wenyang@linux.alibaba.com>
AuthorDate:    Mon, 20 Jan 2020 18:05:23 +08:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 04 Mar 2020 10:17:51 +01:00

timekeeping: Prevent 32bit truncation in scale64_check_overflow()

While unlikely the divisor in scale64_check_overflow() could be >= 32bit in
scale64_check_overflow(). do_div() truncates the divisor to 32bit at least
on 32bit platforms.

Use div64_u64() instead to avoid the truncation to 32-bit.

[ tglx: Massaged changelog ]

Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200120100523.45656-1-wenyang@linux.alibaba.com

---
 kernel/time/timekeeping.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ca69290..4fc2af4 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1005,9 +1005,8 @@ static int scale64_check_overflow(u64 mult, u64 div, u64 *base)
 	    ((int)sizeof(u64)*8 - fls64(mult) < fls64(rem)))
 		return -EOVERFLOW;
 	tmp *= mult;
-	rem *= mult;
 
-	do_div(rem, div);
+	rem = div64_u64(rem * mult, div);
 	*base = tmp + rem;
 	return 0;
 }