Message-ID: <20250718-flitzen-imker-4874d797877e@brauner>
Date: Fri, 18 Jul 2025 10:54:32 +0200
From: Christian Brauner <brauner@...nel.org>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: linux-fsdevel <linux-fsdevel@...r.kernel.org>, John@...ves.net, 
	bernd@...ernd.com, miklos@...redi.hu, joannelkoong@...il.com, 
	Josef Bacik <josef@...icpanda.com>, linux-ext4 <linux-ext4@...r.kernel.org>, 
	Theodore Ts'o <tytso@....edu>, Neal Gompa <neal@...pa.dev>
Subject: Re: [RFC v3] fuse: use fs-iomap for better performance so we can
 containerize ext4

On Thu, Jul 17, 2025 at 04:10:38PM -0700, Darrick J. Wong wrote:
> Hi everyone,
> 
> DO NOT MERGE THIS, STILL!
> 
> This is the third request for comments of a prototype to connect the
> Linux fuse driver to fs-iomap for regular file IO operations to and from
> files whose contents persist to locally attached storage devices.
> 
> Why would you want to do that?  Most filesystem drivers are seriously
> vulnerable to metadata parsing attacks, as syzbot has shown repeatedly
> over almost a decade of its existence.  Faulty code can lead to total
> kernel compromise, and I think there's a very strong incentive to move
> all that parsing out to userspace where we can containerize the fuse
> server process.
> 
> willy's folios conversion project (and to a certain degree RH's new
> mount API) have also demonstrated that treewide changes to the core
> mm/pagecache/fs code are very very difficult to pull off and take years
> because you have to understand every filesystem's bespoke use of that
> core code.  Eeeugh.
> 
> The fuse command plumbing is very simple -- the ->iomap_begin,
> ->iomap_end, and iomap ->ioend calls within iomap are turned into
> upcalls to the fuse server via a trio of new fuse commands.  Pagecache
> writeback is now a directio write.  The fuse server is now able to
> upsert mappings into the kernel for cached access (== zero upcalls for
> rereads and pure overwrites!) and the iomap cache revalidation code
> works.
> 
> With this RFC, I am able to show that it's possible to build a fuse
> server for a real filesystem (ext4) that runs entirely in userspace yet
> maintains most of its performance.  At this stage I still get about 95%
> of the kernel ext4 driver's streaming directio performance on streaming
> IO, and 110% of its streaming buffered IO performance.  Random buffered
> IO is about 85% as fast as the kernel.  Random direct IO is about 80% as
> fast as the kernel; see the cover letter for the fuse2fs iomap changes
> for more details.  Unwritten extent conversions on random direct writes
> are especially painful for fuse+iomap (~90% more overhead) due to upcall
> overhead.  And that's with debugging turned on!
> 
> These items have been addressed since the first RFC:
> 
> 1. The iomap cookie validation is now present, which avoids subtle races
> between pagecache zeroing and writeback on filesystems that support
> unwritten and delalloc mappings.
> 
> 2. Mappings can be cached in the kernel for more speed.
> 
> 3. iomap supports inline data.
> 
> 4. I can now turn on fuse+iomap on a per-inode basis, which turned out
> to be as easy as creating a new ->getattr_iflags callback so that the
> fuse server can set fuse_attr::flags.
> 
> 5. statx and syncfs work on iomap filesystems.
> 
> 6. Timestamps and ACLs work the same way they do in ext4/xfs when iomap
> is enabled.
> 
> 7. The ext4 shutdown ioctl is now supported.
> 
> There are some major warts remaining:
> 
> a. ext4 doesn't support out of place writes so I don't know if that
> actually works correctly.
> 
> b. iomap is an inode-based service, not a file-based service.  This
> means that we /must/ push ext2's inode numbers into the kernel via
> FUSE_GETATTR so that it can report those same numbers back out through
> the FUSE_IOMAP_* calls.  However, the fuse kernel uses a separate nodeid
> to index its incore inode, so we have to pass those too so that
> notifications work properly.  This is related to #3 below:
> 
> c. Hardlinks and iomap are not possible for upper-level libfuse clients
> because the upper level libfuse likes to abstract kernel nodeids with
> its own homebrew dirent/inode cache, which doesn't understand hardlinks.
> As a result, a hardlinked file results in two distinct struct inodes in
> the kernel, which completely breaks iomap's locking model.  I will have
> to rewrite fuse2fs for the lowlevel libfuse library to make this work,
> but on the plus side there will be far less path lookup overhead.
> 
> d. There are too many changes to the IO manager in libext2fs because I
> built things needed to stage the direct/buffered IO paths separately.
> These are now unnecessary but I haven't pulled them out yet because
> they're sort of useful to verify that iomap file IO never goes through
> libext2fs except for inline data.
> 
> e. If we're going to use fuse servers as "safe" replacements for kernel
> filesystem drivers, we need to be able to set PF_MEMALLOC_NOFS so that
> fuse2fs memory allocations (in the kernel) don't push pagecache reclaim.
> We also need to disable the OOM killer(s) for fuse servers because you
> don't want filesystems to unmount abruptly.
> 
> f. How do we maximally contain the fuse server to have safe filesystem
> mounts?  It's very convenient to use systemd services to configure
> isolation declaratively, but fuse2fs still needs to be able to open
> /dev/fuse, the ext4 block device, and call mount() in the shared
> namespace.  This prevents us from using most of the stronger systemd

I'm happy to help you here.

First, I think using a character device for namespaced drivers is always
a mistake. FUSE predates all that ofc. They're incredibly terrible for
delegation because of devtmpfs not being namespaced as well as devices
in general. And having device nodes on anything other than tmpfs is just
wrong (TM).

In systemd I ultimately want a bpf LSM program that prevents the
creation of device nodes outside of tmpfs. They don't belong on
persistent storage imho. But anyway, that's besides the point.

Opening the block device should be done by systemd-mountfsd but I think
/dev/fuse should really be openable by the service itself.

So we can try and allowlist /dev/fuse in vfs_mknod() similar to
whiteouts. That means you can do mknod() in the container to create
/dev/fuse (Personally, I would even restrict this to tmpfs right off the
bat so that containers can only do this on their private tmpfs mount at
/dev.)

The downside of this would be to give unprivileged containers access to
FUSE by default. I don't think that's a problem per se but it is a uapi
change.

Let me think a bit about alternatives. I have one crazy idea but I'm not
sure enough about it to spill it.

> protections because they tend to run in a private mount namespace with
> various parts of the filesystem either hidden or readonly.
> 
> In theory one could design a socket protocol to pass mount options,
> block device paths, fds, and responsibility for the mount() call between
> a mount helper and a service:

This isn't a problem really. This should just be an extension to
systemd-mountfsd.

> 
> e2fsprogs would define as a systemd socket service for fuse2fs that sets
> up a dynamic unprivileged user, no network access, and no access to the
> host's filesystem aside from readonly access to the root filesystem.
> 
> The mount helper (e.g. mount.safe) would then connect to the magic
> socket and pass the CLI arguments to the fuse2fs service.  The service
> would parse the arguments, find the block device paths, and feed them
> back through the socket to mount.safe.  mount.safe would open them and
> pass fds back to the fuse2fs service.  The service would then open the
> devices, parse the superblock, and if everything was ok, request a mount
> through the socket.  The mount helper would then open /dev/fuse and
> mount the filesystem, and if successful, pass the /dev/fuse fd through
> the socket to the fuse2fs server.  At that point the fuse2fs server
> would attach to the /dev/fuse device and handle the usual events.
> 
> Finally we'd have to train people/daemons to run "mount -t safe.ext4
> /dev/sda1 /mnt" to get the contained version of ext4.
> 
> (Yeah, #f is all Neal. ;))
> 
> g. fuse2fs doesn't support the ext4 journal.  Urk.
> 
> I'll work on these in July/August, but for now here's an unmergeable RFC
> to start some discussion.
> 
> --Darrick
> 

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