On Wed, Aug 14, 2024 at 4:19 PM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > On Wed, Aug 14, 2024 at 01:40:34PM GMT, Alexander Mikhalitsyn wrote: > > 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 Hi Christian, > > Hm, I'm confused why won't the following (uncompiled) work? I believe that your way should work. Sorry about that. It's my bad that I didn't consider setting SB_I_NOIDMAP in fill_super and unsetting it later on once we had enough information. Huge thanks for pointing this out! I'll drop -v3 soon and also add support for virtiofs in the same series. Kind regards, Alex > > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c > index ed4c2688047f..8ead1cacdd2f 100644 > --- a/fs/fuse/inode.c > +++ b/fs/fuse/inode.c > @@ -1346,10 +1346,12 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args, > 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) > + if (fc->owner_uid_gid_ext && fc->default_permissions) { > fc->allow_idmap = 1; > - else > + fm->sb->s_iflags &= ~SB_I_NOIDMAP; > + } else { > ok = false; > + } > } > } else { > ra_pages = fc->max_read / PAGE_SIZE; > @@ -1576,6 +1578,7 @@ static void fuse_sb_defaults(struct super_block *sb) > sb->s_time_gran = 1; > sb->s_export_op = &fuse_export_operations; > sb->s_iflags |= SB_I_IMA_UNVERIFIABLE_SIGNATURE; > + sb->s_iflags |= SB_I_NOIDMAP; > if (sb->s_user_ns != &init_user_ns) > sb->s_iflags |= SB_I_UNTRUSTED_MOUNTER; > sb->s_flags &= ~(SB_NOSEC | SB_I_VERSION); > 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 fd34b5755c0b..185004c41a5e 100644 > --- 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 {