Re: [PATCH v6] fuse: Add support for passthrough read/write

[Alessio Balsini <balsini@android.com>]

Thanks all for the comments.

I have a patchset ready that hopefully wraps together the extendability
suggested by Nikolaus, that I agree is a good idea.
The way I tried to make it more flexible is first of all transitioning to a
ioctl(), as suggested by both Jann and Miklos, and by using a data
structure with flexible array member.

Thanks Amir for the fuse2 pointer. I didn't notice that project before, but
I really enjoyed going through its code.
I'm curious if it's intended to deprecate the current fuse implementation
or is what the current fuse will converge to. I noticed that some good
ideas that were in fuse2 have been also added to fuse, so I tried to take
fuse2 as a reference for my passthrough ioctl().

Miklos, can you please give us a glimpse of what's the future of fuse2?

Thanks a lot again for the feedback, I'll send the new patch in a few
minutes.

Cheers,
Alessio


On Mon, Aug 24, 2020 at 02:48:01PM +0200, Miklos Szeredi wrote:
> On Wed, Aug 19, 2020 at 11:25 AM Amir Goldstein <amir73il@gmail.com> wrote:
> 
> > > What I have in mind is things like not coupling the setup of the
> > > passthrough fds to open(), but having a separate notification message for
> > > this (like what we use for invalidation of cache), and adding not just
> > > an "fd" field but also "offset" and "length" fields (which would
> > > currently be required to be both zero to get the "full file" semantics).
> > >
> >
> > You mean like this?
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/commit/?h=fuse2
> 
> Look specifically at fuse_file_map_iter():
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/tree/fs/fuse2/file.c?h=fuse2#n582
> 
> and fudev_map_ioctl():
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/tree/fs/fuse2/fudev.c?h=fuse2#n601
> 
> This avoids the security issue Jann mentioned as well as allowing
> arbitrary mapping of file ranges.  E.g. it could also  be used by a
> block based filesystem to map I/O directly into the block device.
> 
> What the implementation lacks is any kind of caching.  Since your
> usecase involves just one map extent per file, special casing that
> would be trivial.  We can revisit general caching later.
> 
> Thanks,
> Miklos