linux-kernel - [PATCH v3 8/8] platform: vivid-cec: use 64-bit arithmetic instead of 32-bit

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Message-ID: <cca3c728f123d714dc8e4ed87510aeb2e2d63db6.1517929336.git.gustavo@embeddedor.com>
Date:   Tue, 6 Feb 2018 10:53:44 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Subject: [PATCH v3 8/8] platform: vivid-cec: use 64-bit arithmetic instead of
 32-bit

Add suffix ULL to constant 10 in order to give the compiler complete
information about the proper arithmetic to use. Notice that this
constant is used in a context that expects an expression of type
u64 (64 bits, unsigned).

The expression len * 10 * CEC_TIM_DATA_BIT_TOTAL is currently being
evaluated using 32-bit arithmetic.

Also, remove unnecessary parentheses and add a code comment to make it
clear what is the reason of the code change.

Addresses-Coverity-ID: 1454996 ("Unintentional integer overflow")
Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
Changes in v2:
 - Update subject and changelog to better reflect the proposed code changes.
 - Add suffix ULL to constant instead of casting a variable.
 - Remove unnecessary parentheses.
 - Add code comment.

Changes in v3:
 - Mention the specific Coverity report in the commit message.

 drivers/media/platform/vivid/vivid-cec.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/media/platform/vivid/vivid-cec.c b/drivers/media/platform/vivid/vivid-cec.c
index b55d278..614787b 100644
--- a/drivers/media/platform/vivid/vivid-cec.c
+++ b/drivers/media/platform/vivid/vivid-cec.c
@@ -82,8 +82,15 @@ static void vivid_cec_pin_adap_events(struct cec_adapter *adap, ktime_t ts,
 
 	if (adap == NULL)
 		return;
-	ts = ktime_sub_us(ts, (CEC_TIM_START_BIT_TOTAL +
-			       len * 10 * CEC_TIM_DATA_BIT_TOTAL));
+
+	/*
+	 * Suffix ULL on constant 10 makes the expression
+	 * CEC_TIM_START_BIT_TOTAL + 10ULL * len * CEC_TIM_DATA_BIT_TOTAL
+	 * to be evaluated using 64-bit unsigned arithmetic (u64), which
+	 * is what ktime_sub_us expects as second argument.
+	 */
+	ts = ktime_sub_us(ts, CEC_TIM_START_BIT_TOTAL +
+			       10ULL * len * CEC_TIM_DATA_BIT_TOTAL);
 	cec_queue_pin_cec_event(adap, false, ts);
 	ts = ktime_add_us(ts, CEC_TIM_START_BIT_LOW);
 	cec_queue_pin_cec_event(adap, true, ts);
-- 
2.7.4