utils: improve get_ns_uid() and add get_ns_gid()

src/lxc/cgroups/cgfsng.c

@@ -1388,6 +1388,8 @@ static int chown_cgroup_wrapper(void *data)
 	}
 
 	destuid = get_ns_uid(arg->origuid);
+	if (destuid == LXC_INVALID_UID)
+		destuid = 0;
 
 	for (i = 0; arg->hierarchies[i]; i++) {
 		char *fullpath;

src/lxc/macro.h

@@ -340,4 +340,7 @@ extern int __build_bug_on_failed;
 #define PTR_TO_INTMAX(p) ((intmax_t)((intptr_t)(p)))
 #define INTMAX_TO_PTR(u) ((void *)((intptr_t)(u)))
 
+#define LXC_INVALID_UID ((uid_t)-1)
+#define LXC_INVALID_GID ((gid_t)-1)
+
 #endif /* __LXC_MACRO_H */

src/lxc/utils.c

@@ -544,7 +544,34 @@ uid_t get_ns_uid(uid_t orig)
 		}
 	}
 
-	nsid = 0;
+	nsid = LXC_INVALID_UID;
+
+found:
+	fclose(f);
+	free(line);
+	return nsid;
+}
+
+gid_t get_ns_gid(gid_t orig)
+{
+	char *line = NULL;
+	size_t sz = 0;
+	gid_t nsid, hostid, range;
+	FILE *f = fopen("/proc/self/gid_map", "r");
+	if (!f)
+		return 0;
+
+	while (getline(&line, &sz, f) != -1) {
+		if (sscanf(line, "%u %u %u", &nsid, &hostid, &range) != 3)
+			continue;
+
+		if (hostid <= orig && hostid + range > orig) {
+			nsid += orig - hostid;
+			goto found;
+		}
+	}
+
+	nsid = LXC_INVALID_GID;
 
 found:
 	fclose(f);

src/lxc/utils.h

@@ -328,6 +328,10 @@ inline static bool am_host_unpriv(void)
  * parse /proc/self/uid_map to find what @orig maps to
  */
 extern uid_t get_ns_uid(uid_t orig);
+/*
+ * parse /proc/self/gid_map to find what @orig maps to
+ */
+extern gid_t get_ns_gid(gid_t orig);
 
 extern bool dir_exists(const char *path);