path-util: introduce new safe_getcwd() wrapper

It's like get_current_dir_name() but protects us from
CVE-2018-1000001-style exploits:

https://www.halfdog.net/Security/2017/LibcRealpathBufferUnderflow/

src/basic/path-util.c

@@ -90,6 +90,24 @@ char *path_make_absolute(const char *p, const char *prefix) {
                 return strjoin(prefix, "/", p);
 }
 
+int safe_getcwd(char **ret) {
+        char *cwd;
+
+        cwd = get_current_dir_name();
+        if (!cwd)
+                return negative_errno();
+
+        /* Let's make sure the directory is really absolute, to protect us from the logic behind
+         * CVE-2018-1000001 */
+        if (cwd[0] != '/') {
+                free(cwd);
+                return -ENOMEDIUM;
+        }
+
+        *ret = cwd;
+        return 0;
+}
+
 int path_make_absolute_cwd(const char *p, char **ret) {
         char *c;
 

src/basic/path-util.h

@@ -41,6 +41,7 @@ bool is_path(const char *p) _pure_;
 int path_split_and_make_absolute(const char *p, char ***ret);
 bool path_is_absolute(const char *p) _pure_;
 char* path_make_absolute(const char *p, const char *prefix);
+int safe_getcwd(char **ret);
 int path_make_absolute_cwd(const char *p, char **ret);
 int path_make_relative(const char *from_dir, const char *to_path, char **_r);
 char* path_kill_slashes(char *path);