chg: verify XDG_RUNTIME_DIR

According to the specification [1], $XDG_RUNTIME_DIR should be ignored
unless:

  The directory MUST be owned by the user, and he MUST be the only one
  having read and write access to it. Its Unix access mode MUST be 0700.

This patch adds a check and ignores it if it does not meet part of the
criteria.

[1]: https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

contrib/chg/chg.c

@@ -128,6 +128,24 b' static void preparesockdir(const char *s'
 		abortmsg("insecure sockdir %s", sockdir);
 }
 
+/*
+ * Check if a socket directory exists and is only owned by the current user.
+ * Return 1 if so, 0 if not. This is used to check if XDG_RUNTIME_DIR can be
+ * used or not. According to the specification [1], XDG_RUNTIME_DIR should be
+ * ignored if the directory is not owned by the user with mode 0700.
+ * [1]: https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
+ */
+static int checkruntimedir(const char *sockdir)
+{
+	struct stat st;
+	int r = lstat(sockdir, &st);
+	if (r < 0) /* ex. does not exist */
+		return 0;
+	if (!S_ISDIR(st.st_mode)) /* ex. is a file, not a directory */
+		return 0;
+	return st.st_uid == geteuid() && (st.st_mode & 0777) == 0700;
+}
+
 static void getdefaultsockdir(char sockdir[], size_t size)
 {
 	/* by default, put socket file in secure directory
@@ -135,7 +153,7 b' static void getdefaultsockdir(char sockd'
 	 * (permission of socket file may be ignored on some Unices) */
 	const char *runtimedir = getenv("XDG_RUNTIME_DIR");
 	int r;
-	if (runtimedir) {
+	if (runtimedir && checkruntimedir(runtimedir)) {
 		r = snprintf(sockdir, size, "%s/chg", runtimedir);
 	} else {
 		const char *tmpdir = getenv("TMPDIR");