Re: [PATCH] xfs: use a unique and persistent value for f_fsid

Darrick J. Wong
2021-03-23T06:36:15Z

On Tue, Mar 23, 2021 at 06:50:44AM +0200, Amir Goldstein wrote:
> On Tue, Mar 23, 2021 at 1:03 AM Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Mon, Mar 22, 2021 at 07:11:18PM +0200, Amir Goldstein wrote:
> > > Some filesystems on persistent storage backend use a digest of the
> > > filesystem's persistent uuid as the value for f_fsid returned by
> > > statfs(2).
> > >
> > > xfs, as many other filesystem provide the non-persistent block device
> > > number as the value of f_fsid.
> > >
> > > Since kernel v5.1, fanotify_init(2) supports the flag FAN_REPORT_FID
> > > for identifying objects using file_handle and f_fsid in events.
> >
> > The filesystem id is encoded into the VFS filehandle - it does not
> > need some special external identifier to identify the filesystem it
> > belongs to....
> >
>
> Let's take it from the start.
> There is no requirement for fanotify to get a persistent fs id, we just need
> a unique fs id that is known to userspace, so the statfs API is good enough
> for our needs.
>
> See quote from fanotify.7:
>
> " The fields of the fanotify_event_info_fid structure are as follows:
> ...
> fsid This is a unique identifier of the filesystem
> containing the object associated with the event. It is a structure of
> type __kernel_fsid_t and contains the same value as f_fsid when
> calling statfs(2).
>
> file_handle
> This is a variable length structure of type struct
> file_handle. It is an opaque handle that corresponds to a specified
> object on a filesystem as returned by name_to_handle_at(2). It
> can be used to uniquely identify a file on a
> filesystem and can be passed as an argument to open_by_handle_at(2).

Hmmmm.... so I guess you'd /like/ a file handle that will survive across
unmount/mount cycles, and possibly even a reboot?

I looked at the first commit, and I guess you use name_to_handle_at,
which returns a mount_id that is .... that weird number in the leftmost
column of /proc/mountinfo, which increments monotonically for each mount
and definitely doesn't survive a remount cycle, let alone a reboot?

Hence wanting to use something less volatile than mnt_id_ida...?

My natural inclination is "just use whatever NFS does", but ... then I
saw fh_compose and realized that the fsid part of an NFS handle depends
on the export options and probably isn't all that easy for someone who
isn't an nfs client to extract.

Was that how you arrived at using the statfs fsid field?

...except XFS doesn't guarantee that fsid is particularly unique or
stable, since a reboot can enumerate blockdevs in a different order and
hence the dev_t will change. UUIDs also aren't a great idea because you
can snapshot an fs and mount it with nouuid, and now a "unique" file
handle can map ambiguously to two different files.

Urgh, I'm gonna have to think about this one, all the options suck.
fanotify might be smart enough to handle ambiguous file handles but now
I wonder how dumb programs react to that. Also it's 23:30 here. :)

--D

> ..."
>
> So the main objective is to "uniquely identify an object" which was observed
> before (i.e. at the time of setting a watch) and a secondary objective is to
> resolve a path from the identifier, which requires extra privileges.
>
> This definition does not specify the lifetime of the identifier and
> indeed, in most
> cases, uniqueness in the system while filesystem is mounted should suffice
> as that is also the lifetime of the fanotify mark.
>
> But the fanotify group can outlive the mounted filesystem and it can be used
> to watch multiple filesystems. It's not really a problem per-se that
> xfs filesystems
> can change and reuse f_fsid, it is just less friendly that's all.
>
> I am trying to understand your objection to making this "friendly" change.
>
> > > The xfs specific ioctl XFS_IOC_PATH_TO_FSHANDLE similarly attaches an
> > > fsid to exported file handles, but it is not the same fsid exported
> > > via statfs(2) - it is a persistent fsid based on the filesystem's uuid.
> >
> > To actually use that {fshandle,fhandle} tuple for anything
> > requires CAP_SYS_ADMIN. A user can read the fshandle, but it can't
> > use it for anything useful.
>
> It is a unique identifier and that is a useful thing - see demo code:
> * Index watches by fanotify fid:
> https://github.com/amir73il/inotify-tools/commit/ed82098b54b847e3c2d46b32d35b2aa537a9ba94
> * Handle global watches on several filesystems:
> https://github.com/amir73il/inotify-tools/commit/1188ef00dc84964de58afb32c91e19930ad1e2e8
>
> > i.e. it's use is entirely isolated to
> > the file handle interface for identifying the filesystem the handle
> > belongs to. This is messy, but XFS inherited this "fixed fsid"
> > interface from Irix filehandles and was needed to port
> > xfsdump/xfsrestore to Linux. Realistically, it is not functionality
> > that should be duplicated/exposed more widely on Linux...
> >
>
> Other filesystems expose a uuid digest as f_fsid: ext4, btrfs, ocfs2
> and many more. XFS is really the exception among the big local fs.
> This is not exposing anything new at all.
> I would say it is more similar to the way that the generation part of
> the file handle has improved over the years in different filesystems
> to provide better uniqueness guarantees.
>
> > IMO, if fanotify needs a persistent filesystem ID on Linux, it
>
> It does not *need* that. It's just nicer for f_fsid to use a persistent
> fs identifier if one is anyway available.
>
> > should be using something common across all filesystems from the
> > linux superblock, not deep dark internal filesystem magic. The
> > export interfaces that generate VFS (and NFS) filehandles already
> > have a persistent fsid associated with them, which may in fact be
> > the filesystem UUID in it's entirety.
> >
>
> Yes, nfsd is using dark internal and AFAIK undocumnetd magic to
> pick that identifier (Bruce, am I wrong?).
>
> > The export-derived "filesystem ID" is what should be exported to
> > userspace in combination with the file handle to identify the fs the
> > handle belongs to because then you have consistent behaviour and a
> > change that invalidates the filehandle will also invalidate the
> > fshandle....
> >
>
> nfsd has a much stronger persistent file handles requirement than
> fanotify. There is no need to make things more complicated than
> they need to be.
>
> > > Use the same persistent value for f_fsid, so object identifiers in
> > > fanotify events will describe the objects more uniquely.
> >
> > It's not persistent as in "will never change". The moment a user
> > changes the XFS filesystem uuid, the f_fsid changes.
> >
>
> Yes. I know. But it's still much better than the bdev number IMO.
>
> > However, changing the uuid on XFS is an offline (unmounted)
> > operation, so there will be no fanotify marks present when it is
> > changed. Hence when it is remounted, there will be a new f_fsid
> > returned in statvfs(), just like what happens now, and all
> > applications dependent on "persistent" fsids (and persistent
> > filehandles for that matter) will now get ESTALE errors...
> >
> > And, worse, mp->m_fixed_fsid (and XFS superblock UUIDs in general)
> > are not unique if you've got snapshots and they've been mounted via
> > "-o nouuid" to avoid XFS's duplicate uuid checking. This is one of
> > the reasons that the duplicate checking exists - so that fshandles
> > are unique and resolve to a single filesystem....
> >
>
> Both of the caveats of uuid you mentioned are not a big concern for
> fanotify because the nature of f_fsid can be understood by the event
> listener before setting the multi-fs watch (i.e. in case of fsid collision).
>
> > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > > ---
> > >
> > > Guys,
> > >
> > > This change would be useful for fanotify users.
> > > Do you see any problems with that minor change of uapi?
> >
> > Yes.
> >
> > IMO, we shouldn't be making a syscall interface rely on the
> > undefined, filesystem specific behaviour a value some other syscall
> > exposes to userspace. This means the fsid has no defined or
> > standardised behaviour applications can rely on and can't be
> > guaranteed unique and unchanging by fanotify. This seems like a
> > lose-lose situation for everyone...
> >
>
> The fanotify uapi guarantee is to provide the same value of f_fsid
> observed by statfs() uapi. The statfs() uapi guarantee about f_fsid is
> a bit vague, but it's good enough for our needs:
>
> "...The general idea is that f_fsid contains some random stuff such that the
> pair (f_fsid,ino) uniquely determines a file. Some operating systems use
> (a variation on) the device number, or the device number combined with the
> filesystem type..."
>
> Regardless of the fanotify uapi and whether it's good or bad, do you insist
> that the value of f_fsid exposed by xfs needs to be the bdev number and
> not derived from uuid?
>
> One thing we could do is in the "-o nouuid" case that you mentioned
> we continue to use the bdev number for f_fsid.
> Would you like me to make that change?
>
> Thanks,
> Amir.