[PATCH v4 01/15] fs/namespace: introduce SB_I_NOIDMAP flag

Alexander Mikhalitsyn
2024-09-03T15:16:54Z

Right now we determine if filesystem support vfs idmappings
or not basing on the FS_ALLOW_IDMAP flag presence. This
"static" way works perfecly well for local filesystems
like ext4, xfs, btrfs, etc. But for network-like filesystems
like fuse, cephfs this approach is not ideal, because sometimes
proper support of vfs idmaps requires some extensions for the on-wire
protocol, which implies that changes have to be made not only
in the Linux kernel code but also in the 3rd party components like
libfuse, cephfs MDS server and so on.

We have seen that issue during our work on cephfs idmapped mounts [1]
with Christian, but right now I'm working on the idmapped mounts
support for fuse/virtiofs and I think that it is a right time for this extension.

[1] 5ccd8530dd7 ("ceph: handle idmapped mounts in create_request_message()")

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>
Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
---
v3:
- this commit added
---
fs/namespace.c | 4 ++++
include/linux/fs.h | 1 +
2 files changed, 5 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 328087a4df8a..d1702285c915 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4436,6 +4436,10 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
return -EINVAL;

+ /* The filesystem has turned off idmapped mounts. */
+ if (m->mnt_sb->s_iflags & SB_I_NOIDMAP)
+ return -EINVAL;
+
/* We're not controlling the superblock. */
if (!ns_capable(fs_userns, CAP_SYS_ADMIN))
return -EPERM;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 6ca11e241a24..8756f84d627c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1190,6 +1190,7 @@ extern int send_sigurg(struct fown_struct *fown);
#define SB_I_TS_EXPIRY_WARNED 0x00000400 /* warned about timestamp range expiry */
#define SB_I_RETIRED 0x00000800 /* superblock shouldn't be reused */
#define SB_I_NOUMASK 0x00001000 /* VFS does not apply umask */
+#define SB_I_NOIDMAP 0x00002000 /* No idmapped mounts on this superblock */

/* Possible states of 'frozen' field */
enum {
--
2.34.1