[PATCH v3] ktime: Fix ktime_divns to do signed division

[John Stultz]

It was noted that the 32bit implementation of ktime_divns()
was doing unsigned division and didn't properly handle
negative values.

And when a ktime helper was changed to utilize
ktime_divns, it caused a regression on some IR blasters.
See the following bugzilla for details:
  https://bugzilla.redhat.com/show_bug.cgi?id=1200353

This patch fixes the problem in ktime_divns by checking
and preserving the sign bit, and then reapplying it if
appropriate after the division, it also changes the return
type to a s64 to make it more obvious this is expected.

Nicolas also pointed out that negative dividers would
cause infinite loops on 32bit systems, negative dividers
is unlikely for users of this function, but out of caution
this patch adds checks for negative dividers for both
32-bit (BUG_ON) and 64-bit(WARN_ON) versions to make sure
no such use cases creep in.

Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Josh Boyer <jwboyer@xxxxxxxxxx>
Cc: One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx>
Cc: Trevor Cordes <trevor@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.17+ for regression
Tested-by: Trevor Cordes <trevor@xxxxxxxxxxxxx>
Reported-by: Trevor Cordes <trevor@xxxxxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---

New in v3:
* Fix casting issue Nicolas pointed out
* Use WARN_ON for 64bit case

 include/linux/ktime.h | 27 +++++++++++++++++++++++----
 kernel/time/hrtimer.c | 11 ++++++++---
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/include/linux/ktime.h b/include/linux/ktime.h
index 5fc3d10..ab2de1c7 100644
--- a/include/linux/ktime.h
+++ b/include/linux/ktime.h
@@ -166,19 +166,38 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
 }
 
 #if BITS_PER_LONG < 64
-extern u64 __ktime_divns(const ktime_t kt, s64 div);
-static inline u64 ktime_divns(const ktime_t kt, s64 div)
+extern s64 __ktime_divns(const ktime_t kt, s64 div);
+static inline s64 ktime_divns(const ktime_t kt, s64 div)
 {
+	/*
+	 * Negative divisors could cause an inf loop,
+	 * so bug out here.
+	 */
+	BUG_ON(div < 0);
 	if (__builtin_constant_p(div) && !(div >> 32)) {
-		u64 ns = kt.tv64;
+		s64 ns = kt.tv64;
+		int neg = (ns < 0);
+
+		if (neg)
+			ns = -ns;
 		do_div(ns, div);
+		if (neg)
+			ns = -ns;
 		return ns;
 	} else {
 		return __ktime_divns(kt, div);
 	}
 }
 #else /* BITS_PER_LONG < 64 */
-# define ktime_divns(kt, div)		(u64)((kt).tv64 / (div))
+static inline s64 ktime_divns(const ktime_t kt, s64 div)
+{
+	/*
+	 * 32-bit implementation cannot handle negative divisors,
+	 * so catch them on 64bit as well.
+	 */
+	WARN_ON(div < 0);
+	return kt.tv64 / div;
+}
 #endif
 
 static inline s64 ktime_to_us(const ktime_t kt)
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 76d4bd9..c98ce4d 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -266,12 +266,15 @@ lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
 /*
  * Divide a ktime value by a nanosecond value
  */
-u64 __ktime_divns(const ktime_t kt, s64 div)
+s64 __ktime_divns(const ktime_t kt, s64 div)
 {
-	u64 dclc;
-	int sft = 0;
+	s64 dclc;
+	int neg, sft = 0;
 
 	dclc = ktime_to_ns(kt);
+	neg = (dclc < 0);
+	if (neg)
+		dclc = -dclc;
 	/* Make sure the divisor is less than 2^32: */
 	while (div >> 32) {
 		sft++;
@@ -279,6 +282,8 @@ u64 __ktime_divns(const ktime_t kt, s64 div)
 	}
 	dclc >>= sft;
 	do_div(dclc, (unsigned long) div);
+	if (neg)
+		dclc = -dclc;
 
 	return dclc;
 }
-- 
1.9.1

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