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linux-efi@vger.kernel.org, John Johansen , "jk@ozlabs.org" , linux-fsdevel@vger.kernel.org Date: Tue, 11 Mar 2025 12:20:05 -0400 In-Reply-To: <20250311-trunk-farben-fe36bebe233a@brauner> References: <465d1d23-3b36-490e-b0dd-74889d17fa4c@tnxip.de> <20250311-visite-rastplatz-d1fdb223dc10@brauner> <814a257530ad5e8107ce5f48318ab43a3ef1f783.camel@HansenPartnership.com> <20250311-trunk-farben-fe36bebe233a@brauner> Autocrypt: addr=James.Bottomley@HansenPartnership.com; prefer-encrypt=mutual; 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 8mT2Eh9SkH+YVOmaKsajgZgrBxA7fWmGoxXswEVxJIFj3vW7yNc0C5HaUdYa5iGOMs4kg2ht4s7yy7NRQuh7BifWjo6BQ6k4S1H+6axZucxhSV1L6zN9d+lr3Xo/vy1unzA== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.3 Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2025-03-11 at 16:55 +0100, Christian Brauner wrote: > On Tue, Mar 11, 2025 at 09:01:36AM -0400, James Bottomley wrote: > > On Tue, 2025-03-11 at 09:45 +0100, Christian Brauner wrote: [...] > > > efivars as written only has a single global superblock and it > > > doesn't support idmapped mounts and I don't see why it ever > > > would. > >=20 > > So that's not quite true: efivarfs currently supports uid and gid > > mapping as mount options, which certainly looks like they were > > designed to allow a second mount in a user directory.=C2=A0 I've no ide= a > > what the actual use case for this is, but if I go for a single > > superblock, any reconfigure with new uid/gid would become globally > > effective (change every current mount) because they're stored in > > the superblock information. > >=20 > > So what is the use case for this uid/gid parameter?=C2=A0 If no-one can > > remember and no-one actually uses it, perhaps the whole config path > > can be simplified by getting rid of the options?=C2=A0 Even if there is > > a use case, if it's single mount only then we can still go with a > > global superblock. >=20 > So efivarfs uses get_tree_single(). That means that only a single > superblock of the filesystem type efivarfs can ever exist on the > system. >=20 > If efivars is mounted multiple times it will be the exact same > superblock that's used. IOW, mounting efivars multiple times is akin > to a bind-mount. It would be a bit ugly but it could be done by > making sure that any uid/gid changes are reflected. But see below. OK, so it's a fair bet that either the uid/gid option is never used or only used once globally. > > > But since efivars does only ever have a single global superblock, > > > one possibility is to an internal superblock that always exits > > > and is resurfaced whenever userspace mounts efivarfs. That's > > > essentially the devtmpfs model. > > >=20 > > > Then you can stash: > > >=20 > > > static struct vfsmount *efivarfs_mnt; > > >=20 > > > globally and use that in efivarfs_pm_notify() to fill in struct > > > path. > >=20 > > I didn't see devtmpfs when looking for examples, since it's hiding > > outside of the fs/ directory.=C2=A0 However, it does seem to be a bit > > legacy nasty as an example to copy.=C2=A0 However, I get the basics: > > we'd instantiate the mnt and superblock on init (stashing mnt in > > the sfi so the notifier gets it).=C2=A0 Then we can do the variable > > population on reconfigure, just in case an EFI system doesn't want > > to mount efivarfs to save memory. > >=20 > > I can code that up if I can get an answer to the uid/gid parameter > > question above. >=20 > I have some questions. efivarfs registers efivarfs_pm_notify even > before a superblock exists in efivarfs_init_fs_context(). That's > called during > fd_context =3D fsopen("efivarfs") before a superblock even exists: >=20 > (1) Is it guaranteed that efivarfs_pm_notify() is only called once a > =C2=A0=C2=A0=C2=A0 superblock exists? Yes (there was a fix that ensured it) >=20 > (2) Is it guaranteed that efivarfs_pm_notify() is only called when =20 > and while a mount for the superblock exists? That's the intention, but I thought the last unmount would trigger the superblock kill. However, I was thinking I'd need a new mechanism based on reconfiguration to do the dentries only if the filesystem got mounted, so I think I can cope with that. > If the question to either one of those is "no" then the global > vfsmount hack will not help. >=20 > From reading efivarfs_pm_notify() it looks like the answer to (1) is > "yes" because you're dereferencing sfi->sb->s_root in > efivarfs_pm_notify(). >=20 > But I'm not at all certain that (2) isn't a "no" and that > efivarfs_pm_notify() can be called before a mount exists. IOW, once > fsconfig(FSCONFIG_CMD_CREATE) is called the notifier seems ready and > registered but userspace isn't forced to call fsmount(fd_fs) at all. I'd like to have efivarfs so that if no-one's actually mounted anything then the reflection of the variables isn't taking up any space, so I was planning a new mechanism for that anyway. > They could just not to do it for whatever reason but the notifier > should already be able to run. >=20 > Another question is whether the superblock can be freed while > efivarfs_pm_notify() is running? I think that can't happen because > blocking_notifier_chain_unregister(&efivar_ops_nh, &sfi->nb) will > block in efivarfs_kill_sb() until all outstanding calls to > efivarfs_pm_notify() are finished? That's the way it's supposed to work, yes. However, if we move to an always persistent superblock and mnt, I was thinking there'd have to be an indicator in the sfi about whether the variables were reflected or not. Regards, James > If (2) isn't guranteed then efivarfs_pm_notify() needs to be > rewritten without relying on files because there's no guarantee that > a mount exists at all. >=20