linux-kernel - [LINUX PATCH v3] iio: core: Fix IIO_VAL_FRACTIONAL calculation for negative values

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Date:   Mon,  5 Oct 2020 08:05:16 -0700
From:   Anand Ashok Dumbre <anand.ashok.dumbre@...inx.com>
To:     jic23@...nel.org, knaack.h@....de, lars@...afoo.de,
        pmeerw@...erw.net, michal.simek@...inx.com, git@...inx.com,
        linux-iio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Cc:     anandash@...inx.com,
        Anand Ashok Dumbre <anand.ashok.dumbre@...inx.com>
Subject: [LINUX PATCH v3] iio: core: Fix IIO_VAL_FRACTIONAL calculation for negative values

Fixes IIO_VAL_FRACTIONAL for case when the result is negative and
exponent is 0.

example: if the result is -0.75, tmp0 will be 0 and tmp1 = 75
This causes the output to lose sign because of %d in snprintf
which works for tmp0 <= -1.

Signed-off-by: Anand Ashok Dumbre <anand.ashok.dumbre@...inx.com>
Reported-by: kernel test robot <lkp@...el.com> #error: uninitialized symbol tmp
Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
---

Changes in v3:
	Fixed a bug caught by kernel test robot and used correct variable

---
 drivers/iio/industrialio-core.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index cdcd16f1..ffd5176 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -592,6 +592,7 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
 {
 	unsigned long long tmp;
 	int tmp0, tmp1;
+	s64 tmp2;
 	bool scale_db = false;
 
 	switch (type) {
@@ -614,10 +615,13 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
 		else
 			return scnprintf(buf, len, "%d.%09u", vals[0], vals[1]);
 	case IIO_VAL_FRACTIONAL:
-		tmp = div_s64((s64)vals[0] * 1000000000LL, vals[1]);
+		tmp2 = div_s64((s64)vals[0] * 1000000000LL, vals[1]);
 		tmp1 = vals[1];
-		tmp0 = (int)div_s64_rem(tmp, 1000000000, &tmp1);
-		return scnprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
+		tmp0 = (int)div_s64_rem(tmp2, 1000000000, &tmp1);
+		if ((tmp2 < 0) && (tmp0 == 0))
+			return snprintf(buf, len, "-0.%09u", abs(tmp1));
+		else
+			return snprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
 	case IIO_VAL_FRACTIONAL_LOG2:
 		tmp = shift_right((s64)vals[0] * 1000000000LL, vals[1]);
 		tmp0 = (int)div_s64_rem(tmp, 1000000000LL, &tmp1);
-- 
2.7.4