tools/testing/selftests/sysctl: Add pre-check to the value of writes_strict

Sysctl test will fail in some items if the value of /proc/sys/kernel
/sysctrl_writes_strict is 0 as the default value in kernel older than v4.5.

Make this test more robus and compatible with older kernel by checking and
update writes_strict value and restore it when test is done.
Signed-off-by: NOrson Zhai <orson.zhai@linaro.org>
Reviewed-by: NSumit Semwal <sumit.semwal@linaro.org>
Tested-by: NSumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/sysctl/common_tests

@@ -24,6 +24,14 @@ verify()
 	return 0
 }
 
+exit_test()
+{
+	if [ ! -z ${old_strict} ]; then
+		echo ${old_strict} > ${WRITES_STRICT}
+	fi
+	exit $rc
+}
+
 trap 'set_orig; rm -f "${TEST_FILE}"' EXIT
 
 rc=0
@@ -63,6 +71,20 @@ else
 	echo "ok"
 fi
 
+echo -n "Checking write strict setting ... "
+WRITES_STRICT="${SYSCTL}/kernel/sysctl_writes_strict"
+if [ ! -e ${WRITES_STRICT} ]; then
+	echo "FAIL, but skip in case of old kernel" >&2
+else
+	old_strict=$(cat ${WRITES_STRICT})
+	if [ "$old_strict" = "1" ]; then
+		echo "ok"
+	else
+		echo "FAIL, strict value is 0 but force to 1 to continue" >&2
+		echo "1" > ${WRITES_STRICT}
+	fi
+fi
+
 # Now that we've validated the sanity of "set_test" and "set_orig",
 # we can use those functions to set starting states before running
 # specific behavioral tests.

tools/testing/selftests/sysctl/run_numerictests

@@ -7,4 +7,4 @@ TEST_STR=$(( $ORIG + 1 ))
 
 . ./common_tests
 
-exit $rc
+exit_test

tools/testing/selftests/sysctl/run_stringtests

@@ -74,4 +74,4 @@ else
 	echo "ok"
 fi
 
-exit $rc
+exit_test