userns: use union in {g,u}idmap struct

- Add a struct containing two pointer to extents and wrap both the static extent
  array and the struct into a union. This is done in preparation for bumping the
  {g,u}idmap limits for user namespaces.
- Add brackets around anonymous union when using designated initializers to
  initialize members in order to please gcc <= 4.4.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

include/linux/user_namespace.h

@@ -12,13 +12,21 @@
 
 #define UID_GID_MAP_MAX_EXTENTS 5
 
-struct uid_gid_map {	/* 64 bytes -- 1 cache line */
-	u32 nr_extents;
-	struct uid_gid_extent {
+struct uid_gid_extent {
 	u32 first;
 	u32 lower_first;
 	u32 count;
-	} extent[UID_GID_MAP_MAX_EXTENTS];
+};
+
+struct uid_gid_map {	/* 64 bytes -- 1 cache line */
+	u32 nr_extents;
+	union {
+		struct uid_gid_extent extent[UID_GID_MAP_MAX_EXTENTS];
+		struct {
+			struct uid_gid_extent *forward;
+			struct uid_gid_extent *reverse;
+		};
+	};
 };
 
 #define USERNS_SETGROUPS_ALLOWED 1UL

kernel/user.c

@@ -26,28 +26,34 @@
 struct user_namespace init_user_ns = {
 	.uid_map = {
 		.nr_extents = 1,
+		{
 			.extent[0] = {
 				.first = 0,
 				.lower_first = 0,
 				.count = 4294967295U,
 			},
 		},
+	},
 	.gid_map = {
 		.nr_extents = 1,
+		{
 			.extent[0] = {
 				.first = 0,
 				.lower_first = 0,
 				.count = 4294967295U,
 			},
 		},
+	},
 	.projid_map = {
 		.nr_extents = 1,
+		{
 			.extent[0] = {
 				.first = 0,
 				.lower_first = 0,
 				.count = 4294967295U,
 			},
 		},
+	},
 	.count = ATOMIC_INIT(3),
 	.owner = GLOBAL_ROOT_UID,
 	.group = GLOBAL_ROOT_GID,