[PATCH v4 14/15] 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.

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>
Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>
---
v2:
	- simplified and get rid of ->allow_idmap global VFS callback
v3:
	- now use a new SB_I_NOIDMAP flag
v4:
	- small rebase changes
---
 fs/fuse/inode.c           | 12 +++++++++---
 include/uapi/linux/fuse.h | 20 +++++++++++++++++++-
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 2e26810066e8..9f9456d3e466 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1343,6 +1343,12 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 			}
 			if (flags & FUSE_NO_EXPORT_SUPPORT)
 				fm->sb->s_export_op = &fuse_export_fid_operations;
+			if (flags & FUSE_ALLOW_IDMAP) {
+				if (fc->default_permissions)
+					fm->sb->s_iflags &= ~SB_I_NOIDMAP;
+				else
+					ok = false;
+			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1390,7 +1396,7 @@ void fuse_send_init(struct fuse_mount *fm)
 		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_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND;
+		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_ALLOW_IDMAP;
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		flags |= FUSE_MAP_ALIGNMENT;
@@ -1980,7 +1986,7 @@ static void fuse_kill_sb_anon(struct super_block *sb)
 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,
 	.kill_sb	= fuse_kill_sb_anon,
@@ -2001,7 +2007,7 @@ static struct file_system_type fuseblk_fs_type = {
 	.init_fs_context = fuse_init_fs_context,
 	.parameters	= fuse_fs_parameters,
 	.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 2ccf38181df2..f1e99458e29e 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -217,6 +217,9 @@
  *  - add backing_id to fuse_open_out, add FOPEN_PASSTHROUGH open flag
  *  - add FUSE_NO_EXPORT_SUPPORT init flag
  *  - add FUSE_NOTIFY_RESEND, add FUSE_HAS_RESEND init flag
+ *
+ *  7.41
+ *  - add FUSE_ALLOW_IDMAP
  */
 
 #ifndef _LINUX_FUSE_H
@@ -252,7 +255,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 40
+#define FUSE_KERNEL_MINOR_VERSION 41
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -421,6 +424,7 @@ struct fuse_file_lock {
  * FUSE_NO_EXPORT_SUPPORT: explicitly disable export support
  * FUSE_HAS_RESEND: kernel supports resending pending requests, and the high bit
  *		    of the request ID indicates resend requests
+ * FUSE_ALLOW_IDMAP: allow creation of idmapped mounts
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -466,6 +470,7 @@ struct fuse_file_lock {
 
 /* Obsolete alias for FUSE_DIRECT_IO_ALLOW_MMAP */
 #define FUSE_DIRECT_IO_RELAX	FUSE_DIRECT_IO_ALLOW_MMAP
+#define FUSE_ALLOW_IDMAP	(1ULL << 40)
 
 /**
  * CUSE INIT request/reply flags
@@ -984,6 +989,19 @@ struct fuse_fallocate_in {
  */
 #define FUSE_UNIQUE_RESEND (1ULL << 63)
 
+/**
+ * This value will be set by the kernel to
+ * (struct fuse_in_header).{uid,gid} fields in
+ * case when:
+ * - fuse daemon enabled FUSE_ALLOW_IDMAP
+ * - idmapping information is not available and uid/gid
+ *   can not be mapped in accordance with an idmapping.
+ *
+ * Note: an idmapping information always available
+ * for inode creation operations like:
+ * FUSE_MKNOD, FUSE_SYMLINK, FUSE_MKDIR, FUSE_TMPFILE,
+ * FUSE_CREATE and FUSE_RENAME2 (with RENAME_WHITEOUT).
+ */
 #define FUSE_INVALID_UIDGID ((uint32_t)(-1))
 
 struct fuse_in_header {
-- 
2.34.1