apparmor: fix unsigned len comparison with less than zero

The sanity check in macro update_for_len checks to see if len
is less than zero, however, len is a size_t so it can never be
less than zero, so this sanity check is a no-op.  Fix this by
making len a ssize_t so the comparison will work and add ulen
that is a size_t copy of len so that the min() macro won't
throw warnings about comparing different types.

Addresses-Coverity: ("Macro compares unsigned to 0")
Fixes: f1bd9041 ("apparmor: add the base fns() for domain labels")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/label.c

@@ -1462,11 +1462,13 @@ static inline bool use_label_hname(struct aa_ns *ns, struct aa_label *label,
 /* helper macro for snprint routines */
 #define update_for_len(total, len, size, str)	\
 do {					\
+	size_t ulen = len;		\
+					\
 	AA_BUG(len < 0);		\
-	total += len;			\
-	len = min(len, size);		\
-	size -= len;			\
-	str += len;			\
+	total += ulen;			\
+	ulen = min(ulen, size);		\
+	size -= ulen;			\
+	str += ulen;			\
 } while (0)
 
 /**
@@ -1601,7 +1603,7 @@ int aa_label_snxprint(char *str, size_t size, struct aa_ns *ns,
 	struct aa_ns *prev_ns = NULL;
 	struct label_it i;
 	int count = 0, total = 0;
-	size_t len;
+	ssize_t len;
 
 	AA_BUG(!str && size != 0);
 	AA_BUG(!label);