[PATCH v4 7/8] userns: Replace strncmp with str_has_prefix

[Chuhong Yuan <hslester96@gmail.com>]

strncmp(str, const, len) is error-prone because len
is easy to have typo.
The example is the hard-coded len has counting error
or sizeof(const) forgets - 1.
So we prefer using newly introduced str_has_prefix()
to substitute such strncmp to make code better.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
---
Changes in v4:
  - Eliminate assignments in if conditions.

 kernel/user_namespace.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 8eadadc478f9..bd5702f9273a 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -1138,6 +1138,7 @@ ssize_t proc_setgroups_write(struct file *file, const char __user *buf,
 	char kbuf[8], *pos;
 	bool setgroups_allowed;
 	ssize_t ret;
+	size_t len;
 
 	/* Only allow a very narrow range of strings to be written */
 	ret = -EINVAL;
@@ -1153,16 +1154,19 @@ ssize_t proc_setgroups_write(struct file *file, const char __user *buf,
 
 	/* What is being requested? */
 	ret = -EINVAL;
-	if (strncmp(pos, "allow", 5) == 0) {
-		pos += 5;
+
+	len = str_has_prefix(pos, "allow");
+	if (len) {
+		pos += len;
 		setgroups_allowed = true;
+	} else {
+		len = str_has_prefix(pos, "deny");
+		if (len) {
+			pos += len;
+			setgroups_allowed = false;
+		} else
+			goto out;
 	}
-	else if (strncmp(pos, "deny", 4) == 0) {
-		pos += 4;
-		setgroups_allowed = false;
-	}
-	else
-		goto out;
 
 	/* Verify there is not trailing junk on the line */
 	pos = skip_spaces(pos);
-- 
2.20.1