fs/mnt_idmapping: introduce an invalid_mnt_idmap

Link: https://lore.kernel.org/linux-fsdevel/20240904-baugrube-erhoben-b3c1c49a2645@brauner/Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/mnt_idmapping.c

@@ -32,6 +32,15 @@ struct mnt_idmap nop_mnt_idmap = {
 };
 EXPORT_SYMBOL_GPL(nop_mnt_idmap);
 
+/*
+ * Carries the invalid idmapping of a full 0-4294967295 {g,u}id range.
+ * This means that all {g,u}ids are mapped to INVALID_VFS{G,U}ID.
+ */
+struct mnt_idmap invalid_mnt_idmap = {
+	.count	= REFCOUNT_INIT(1),
+};
+EXPORT_SYMBOL_GPL(invalid_mnt_idmap);
+
 /**
  * initial_idmapping - check whether this is the initial mapping
  * @ns: idmapping to check
@@ -75,6 +84,8 @@ vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
 
 	if (idmap == &nop_mnt_idmap)
 		return VFSUIDT_INIT(kuid);
+	if (idmap == &invalid_mnt_idmap)
+		return INVALID_VFSUID;
 	if (initial_idmapping(fs_userns))
 		uid = __kuid_val(kuid);
 	else
@@ -112,6 +123,8 @@ vfsgid_t make_vfsgid(struct mnt_idmap *idmap,
 
 	if (idmap == &nop_mnt_idmap)
 		return VFSGIDT_INIT(kgid);
+	if (idmap == &invalid_mnt_idmap)
+		return INVALID_VFSGID;
 	if (initial_idmapping(fs_userns))
 		gid = __kgid_val(kgid);
 	else
@@ -140,6 +153,8 @@ kuid_t from_vfsuid(struct mnt_idmap *idmap,
 
 	if (idmap == &nop_mnt_idmap)
 		return AS_KUIDT(vfsuid);
+	if (idmap == &invalid_mnt_idmap)
+		return INVALID_UID;
 	uid = map_id_up(&idmap->uid_map, __vfsuid_val(vfsuid));
 	if (uid == (uid_t)-1)
 		return INVALID_UID;
@@ -167,6 +182,8 @@ kgid_t from_vfsgid(struct mnt_idmap *idmap,
 
 	if (idmap == &nop_mnt_idmap)
 		return AS_KGIDT(vfsgid);
+	if (idmap == &invalid_mnt_idmap)
+		return INVALID_GID;
 	gid = map_id_up(&idmap->gid_map, __vfsgid_val(vfsgid));
 	if (gid == (gid_t)-1)
 		return INVALID_GID;
@@ -296,7 +313,7 @@ struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns)
  */
 struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap)
 {
-	if (idmap != &nop_mnt_idmap)
+	if (idmap != &nop_mnt_idmap && idmap != &invalid_mnt_idmap)
 		refcount_inc(&idmap->count);
 
 	return idmap;
@@ -312,7 +329,8 @@ EXPORT_SYMBOL_GPL(mnt_idmap_get);
  */
 void mnt_idmap_put(struct mnt_idmap *idmap)
 {
-	if (idmap != &nop_mnt_idmap && refcount_dec_and_test(&idmap->count))
+	if (idmap != &nop_mnt_idmap && idmap != &invalid_mnt_idmap &&
+	    refcount_dec_and_test(&idmap->count))
 		free_mnt_idmap(idmap);
 }
 EXPORT_SYMBOL_GPL(mnt_idmap_put);

include/linux/mnt_idmapping.h

@@ -9,6 +9,7 @@ struct mnt_idmap;
 struct user_namespace;
 
 extern struct mnt_idmap nop_mnt_idmap;
+extern struct mnt_idmap invalid_mnt_idmap;
 extern struct user_namespace init_user_ns;
 
 typedef struct {