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

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi Amir,

On 2023/5/17 23:51, Amir Goldstein wrote:
> On Wed, May 17, 2023 at 5:50 AM Gao Xiang <hsiangkao@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2023/5/2 17:07, Daniel Rosenberg wrote:
>>> On Mon, Apr 24, 2023 at 8:32 AM Miklos Szeredi <miklos@szeredi.hu> 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.