Re: [RFC PATCH bpf-next v3 00/37] FUSE BPF: A Stacked Filesystem Extension for FUSE

[Gao Xiang]

Hi Amir,

On 2023/5/17 23:51, Amir Goldstein wrote:
> On Wed, May 17, 2023 at 5:50 AM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
> > On 2023/5/2 17:07, Daniel Rosenberg wrote:
> > > On Mon, Apr 24, 2023 at 8:32 AM Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > > > The security model needs to be thought about and documented.  Think
> > > > about this: the fuse server now delegates operations it would itself
> > > > perform to the passthrough code in fuse.  The permissions that would
> > > > have been checked in the context of the fuse server are now checked in
> > > > the context of the task performing the operation.  The server may be
> > > > able to bypass seccomp restrictions.  Files that are open on the
> > > > backing filesystem are now hidden (e.g. lsof won't find these), which
> > > > allows the server to obfuscate accesses to backing files.  Etc.
> > > >
> > > > These are not particularly worrying if the server is privileged, but
> > > > fuse comes with the history of supporting unprivileged servers, so we
> > > > should look at supporting passthrough with unprivileged servers as
> > > > well.
> > >
> > > This is on my todo list. My current plan is to grab the creds that the
> > > daemon uses to respond to FUSE_INIT. That should keep behavior fairly
> > > similar. I'm not sure if there are cases where the fuse server is
> > > operating under multiple contexts.
> > > I don't currently have a plan for exposing open files via lsof. Every
> > > such file should relate to one that will show up though. I haven't dug
> > > into how that's set up, but I'm open to suggestions.
> > >
> > > > My other generic comment is that you should add justification for
> > > > doing this in the first place.  I guess it's mainly performance.  So
> > > > how performance can be won in real life cases?   It would also be good
> > > > to measure the contribution of individual ops to that win.   Is there
> > > > another reason for this besides performance?
> > > >
> > > > Thanks,
> > > > Miklos
> > >
> > > Our main concern with it is performance. We have some preliminary
> > > numbers looking at the pure passthrough case. We've been testing using
> > > a ramdrive on a somewhat slow machine, as that should highlight
> > > differences more. We ran fio for sequential reads, and random
> > > read/write. For sequential reads, we were seeing libfuse's
> > > passthrough_hp take about a 50% hit, with fuse-bpf not being
> > > detectably slower. For random read/write, we were seeing a roughly 90%
> > > drop in performance from passthrough_hp, while fuse-bpf has about a 7%
> > > drop in read and write speed. When we use a bpf that traces every
> > > opcode, that performance hit increases to a roughly 1% drop in
> > > sequential read performance, and a 20% drop in both read and write
> > > performance for random read/write. We plan to make more complex bpf
> > > examples, with fuse daemon equivalents to compare against.
> > >
> > > We have not looked closely at the impact of individual opcodes yet.
> > >
> > > There's also a potential ease of use for fuse-bpf. If you're
> > > implementing a fuse daemon that is largely mirroring a backing
> > > filesystem, you only need to write code for the differences in
> > > behavior. For instance, say you want to remove image metadata like
> > > location. You could give bpf information on what range of data is
> > > metadata, and zero out that section without having to handle any other
> > > operations.
> >
> > A bit out of topic (although I'm not quite look into FUSE BPF internals)
> > After roughly listening to this topic in FS track last week, I'm not
> > quite sure (at least in the long term) if it might be better if
> > ebpf-related filter/redirect stuffs could be landed in vfs or in a
> > somewhat stackable fs so that we could redirect/filter any sub-fstree
> > in principle?    It's just an open question and I have no real tendency
> > of this but do we really need a BPF-filter functionality for each
> > individual fs?
>
> I think that is a valid question, but the answer is that even if it makes sense,
> doing something like this in vfs would be a much bigger project with larger
> consequences on performance and security and whatnot, so even if
> (and a very big if) this ever happens, using FUSE-BPF as a playground for
> this sort of stuff would be a good idea.

My current observation is that the total Fuse-BPF LoC is already beyond the
whole FUSE itself.  In addition, it almost hooks all fs operations which
impacts something to me.

> This reminds me of union mounts - it made sense to have union mount
> functionality in vfs, but after a long winding road, a stacked fs (overlayfs)
> turned out to be a much more practical solution.

Yeah, I agree.  So it was just a pure hint on my side.

> > It sounds much like
> > https://learn.microsoft.com/en-us/windows-hardware/drivers/ifs/about-file-system-filter-drivers
>
> Nice reference.
> I must admit that I found it hard to understand what Windows filter drivers
> can do compared to FUSE-BPF design.
> It'd be nice to get some comparison from what is planned for FUSE-BPF.

At least some investigation/analysis first might be better in the long
term development.

> Interesting to note that there is a "legacy" Windows filter driver API,
> so Windows didn't get everything right for the first API - that is especially
> interesting to look at as repeating other people's mistakes would be a shame.

I'm not familiar with that details as well, yet I saw that they have a
filesystem filter subsystem, so I mentioned it here.

Thanks,
Gao Xiang

> Thanks,
> Amir.