Re: [PATCH v3 2/3] fs: fuse: add backing_files control file

[Miklos Szeredi <miklos@szeredi.hu>]

On Sun, 11 May 2025 at 11:56, Chen Linxuan <chenlinxuan@uniontech.com> wrote:

> I noticed that the current extended attribute names already use the
> namespace.value format.
> Perhaps we could reuse this naming scheme and extend it to support
> features like nested namespaces.

Right.  Here's a link to an old and long thread about this:

   https://lore.kernel.org/all/YnEeuw6fd1A8usjj@miu.piliscsaba.redhat.com/#r

>
> For instance, in a situation like this:
>
> A fixed file 0 in an io_uring is a FUSE fd.
> This FUSE fd belongs to FUSE connection 64.
> This FUSE fd has a backing file.
> This backing file is actually provided by mnt_id=36.
>
> Running getfattr -m '-' /proc/path/to/the/io_uring/fd could return
> something like:
>
> io_uring.fixed_files.0.fuse.conn="64"
> io_uring.fixed_files.0.fuse.backing_file.mnt_id="36"
> io_uring.fixed_files.0.fuse.backing_file.path="/path/to/real/file"

Yeah, except listxattr wouldn't be able to properly work in such
cases: it lacks support for hierarchy.

The proposed solution was something like making getxattr on the
"directory" return a listing of child object names.

I.e. getxattr("/proc/123/fd/12", "io_uring.fixed_files.") would return
the list of instantiated fixed file slots, etc...

Thanks,
Miklos