namespace: introduce SB_I_NOIDMAP flag

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()")

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>

diff --git a/fs/namespace.c b/fs/namespace.c
index 328087a4df8a6e19f5a64cff8fe53423a04f6df9..d1702285c9153a13ea982ff24e5f821d851adc77 100644 (file)
--- 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 fd34b5755c0b52988a7131493cd56fa8ea640ad9..6ff547ef21f2e6aa2d00d0a58644ee0e04794ee9 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1189,6 +1189,7 @@ 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 {