util: Simplify input validation

It appears that snprintf(3) is being used for input validation.
However, this seems like overkill because it causes szPath to be
copied an extra time.  The mostly likely protections being sought
here, according to https://cwe.mitre.org/data/definitions/20.html,
look to be DoS attacks involving CPU and memory usage.  A simpler
check that uses strnlen(3) can mitigate against both of these and is
simpler.
Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Bjoern Jacke <bjacke@samba.org>
(cherry picked from commit 922bce26)

lib/util/util_paths.c

@@ -69,21 +69,20 @@ static char *get_user_home_dir(TALLOC_CTX *mem_ctx)
 	struct passwd pwd = {0};
 	struct passwd *pwdbuf = NULL;
 	char buf[NSS_BUFLEN_PASSWD] = {0};
+	size_t len;
 	int rc;
 
 	rc = getpwuid_r(getuid(), &pwd, buf, NSS_BUFLEN_PASSWD, &pwdbuf);
 	if (rc != 0 || pwdbuf == NULL ) {
-		int len_written;
 		const char *szPath = getenv("HOME");
 		if (szPath == NULL) {
 			return NULL;
 		}
-		len_written = snprintf(buf, sizeof(buf), "%s", szPath);
-		if (len_written >= sizeof(buf) || len_written < 0) {
-			/* Output was truncated or an error. */
+		len = strnlen(szPath, PATH_MAX);
+		if (len >= PATH_MAX) {
 			return NULL;
 		}
-		return talloc_strdup(mem_ctx, buf);
+		return talloc_strdup(mem_ctx, szPath);
 	}
 
 	return talloc_strdup(mem_ctx, pwd.pw_dir);