[PATCH v4 14/15] fs/fuse: allow idmapped mounts

Alexander Mikhalitsyn
2024-09-03T15:17:23Z

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@kernel.org>
Cc: Seth Forshee <sforshee@kernel.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Bernd Schubert <bschubert@ddn.com>
Cc: <linux-fsdevel@vger.kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
---
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