[PATCH 4.4 230/312] power: test_power: correctly handle empty writes

[Greg Kroah-Hartman]

From: Sasha Levin <sasha.le...@oracle.com>

commit 6b9140f39c2aaf76791197fbab0839c0e4af56e8 upstream.

Writing 0 length data into test_power makes it access an invalid array
location and kill the system.

Fixes: f17ef9b2d ("power: Make test_power driver more dynamic.")
Signed-off-by: Sasha Levin <sasha.le...@oracle.com>
Signed-off-by: Sebastian Reichel <s...@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/power/test_power.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/power/test_power.c
+++ b/drivers/power/test_power.c
@@ -301,6 +301,8 @@ static int map_get_value(struct battery_
        buf[MAX_KEYLENGTH-1] = '\0';
 
        cr = strnlen(buf, MAX_KEYLENGTH) - 1;
+       if (cr < 0)
+               return def_val;
        if (buf[cr] == '\n')
                buf[cr] = '\0';