[PATCH v1 9/9] fs/fuse: allow idmapped mounts

[Alexander Mikhalitsyn]

Now we have everything in place and we can allow idmapped mounts
by setting the FS_ALLOW_IDMAP flag. Notice that real availability
of idmapped mounts will depend on the fuse daemon. Fuse daemon
have to set FUSE_ALLOW_IDMAP flag in the FUSE_INIT reply.

To discuss:
- we enable idmapped mounts support only if "default_permissions" mode is enabled,
because otherwise we would need to deal with UID/GID mappings in the userspace side OR
provide the userspace with idmapped req->in.h.uid/req->in.h.gid values which is not
something that we probably want to. Idmapped mounts phylosophy is not about faking
caller uid/gid.

- We have a small offlist discussion with Christian around adding fs_type->allow_idmap
hook. Christian pointed that it would be nice to have a superblock flag instead like
SB_I_NOIDMAP and we can set this flag during mount time if we see that filesystem does not
support idmappings. But, unfortunately I didn't succeed here because the kernel will
know if the filesystem supports idmapping or not after FUSE_INIT request, but FUSE_INIT request
is being sent at the end of mounting process, so mount and superblock will exist and
visible by the userspace in that time. It seems like setting SB_I_NOIDMAP flag in this
case is too late as user may do the trick with creating a idmapped mount while it wasn't
restricted by SB_I_NOIDMAP. Alternatively, we can introduce a "positive" version SB_I_ALLOWIDMAP
and "weak" version of FS_ALLOW_IDMAP like FS_MAY_ALLOW_IDMAP. So if FS_MAY_ALLOW_IDMAP is set,
then SB_I_ALLOWIDMAP has to be set on the superblock to allow creation of an idmapped mount.
But that's a matter of our discussion.

Some extra links and examples:

- libfuse support
https://github.com/mihalicyn/libfuse/commits/idmap_support

- fuse-overlayfs support:
https://github.com/mihalicyn/fuse-overlayfs/commits/idmap_support

- cephfs-fuse conversion example
https://github.com/mihalicyn/ceph/commits/fuse_idmap

- glusterfs conversion example
https://github.com/mihalicyn/glusterfs/commits/fuse_idmap

Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Seth Forshee <sforshee@xxxxxxxxxx>
Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Cc: Bernd Schubert <bschubert@xxxxxxx>
Cc: <linux-fsdevel@xxxxxxxxxxxxxxx>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
---
 fs/fuse/fuse_i.h          |  3 +++
 fs/fuse/inode.c           | 22 +++++++++++++++++++---
 include/uapi/linux/fuse.h |  5 ++++-
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 94b25ea5344a..9317b8c35191 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -809,6 +809,9 @@ struct fuse_conn {
 	/* Add owner_{u,g}id info when creating a new inode */
 	unsigned int owner_uid_gid_ext:1;
 
+	/* Allow creation of idmapped mounts */
+	unsigned int allow_idmap:1;
+
 	/* Does the filesystem support per inode DAX? */
 	unsigned int inode_dax:1;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 08cd3714b32d..47e32a8baed3 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1286,6 +1286,12 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 				fc->direct_io_allow_mmap = 1;
 			if (flags & FUSE_OWNER_UID_GID_EXT)
 				fc->owner_uid_gid_ext = 1;
+			if (flags & FUSE_ALLOW_IDMAP) {
+				if (fc->owner_uid_gid_ext && fc->default_permissions)
+					fc->allow_idmap = 1;
+				else
+					ok = false;
+			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1332,7 +1338,8 @@ void fuse_send_init(struct fuse_mount *fm)
 		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
 		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
 		FUSE_SECURITY_CTX | FUSE_CREATE_SUPP_GROUP |
-		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP | FUSE_OWNER_UID_GID_EXT;
+		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP |
+		FUSE_OWNER_UID_GID_EXT | FUSE_ALLOW_IDMAP;
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		flags |= FUSE_MAP_ALIGNMENT;
@@ -1915,12 +1922,20 @@ static void fuse_kill_sb_anon(struct super_block *sb)
 	fuse_mount_destroy(get_fuse_mount_super(sb));
 }
 
+static bool fuse_allow_idmap(struct super_block *sb)
+{
+	struct fuse_conn *fc = get_fuse_conn_super(sb);
+
+	return fc->allow_idmap;
+}
+
 static struct file_system_type fuse_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "fuse",
-	.fs_flags	= FS_HAS_SUBTYPE | FS_USERNS_MOUNT,
+	.fs_flags	= FS_HAS_SUBTYPE | FS_USERNS_MOUNT | FS_ALLOW_IDMAP,
 	.init_fs_context = fuse_init_fs_context,
 	.parameters	= fuse_fs_parameters,
+	.allow_idmap	= fuse_allow_idmap,
 	.kill_sb	= fuse_kill_sb_anon,
 };
 MODULE_ALIAS_FS("fuse");
@@ -1938,8 +1953,9 @@ static struct file_system_type fuseblk_fs_type = {
 	.name		= "fuseblk",
 	.init_fs_context = fuse_init_fs_context,
 	.parameters	= fuse_fs_parameters,
+	.allow_idmap	= fuse_allow_idmap,
 	.kill_sb	= fuse_kill_sb_blk,
-	.fs_flags	= FS_REQUIRES_DEV | FS_HAS_SUBTYPE,
+	.fs_flags	= FS_REQUIRES_DEV | FS_HAS_SUBTYPE | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("fuseblk");
 
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index ebe82104b172..d8e1235d9796 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -215,6 +215,7 @@
  *  7.40
  *  - add FUSE_EXT_OWNER_UID_GID
  *  - add FUSE_OWNER_UID_GID_EXT
+ *  - add FUSE_ALLOW_IDMAP
  */
 
 #ifndef _LINUX_FUSE_H
@@ -250,7 +251,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 39
+#define FUSE_KERNEL_MINOR_VERSION 40
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -416,6 +417,7 @@ struct fuse_file_lock {
  * FUSE_DIRECT_IO_ALLOW_MMAP: allow shared mmap in FOPEN_DIRECT_IO mode.
  * FUSE_OWNER_UID_GID_EXT: add inode owner UID/GID info to create, mkdir,
  *			   symlink and mknod
+ * FUSE_ALLOW_IDMAP: allow creation of idmapped mounts
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -459,6 +461,7 @@ struct fuse_file_lock {
 /* Obsolete alias for FUSE_DIRECT_IO_ALLOW_MMAP */
 #define FUSE_DIRECT_IO_RELAX	FUSE_DIRECT_IO_ALLOW_MMAP
 #define FUSE_OWNER_UID_GID_EXT	(1ULL << 37)
+#define FUSE_ALLOW_IDMAP	(1ULL << 38)
 
 /**
  * CUSE INIT request/reply flags
-- 
2.34.1