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

[Christian Brauner <brauner@kernel.org>]

On Mon, May 12, 2025 at 12:06:19PM +0200, Miklos Szeredi wrote:
> On Mon, 12 May 2025 at 11:23, Amir Goldstein <amir73il@gmail.com> wrote:
> 
> > The way I see it is, generic vs. specialized have pros and cons
> > There is no clear winner.
> > Therefore, investing time on the getxattr() direction without consensus
> > with vfs maintainer is not wise IMO.
> 
> AFAIU Christian is hung up about getting a properly sized buffer for the result.

No, the xattr interface is ugly as hell and I don't want it used as a
generic information transportation information interface. And I don't
want a single thing that sets a precedent in that direction.

> But if the data is inherently variable sized, adding specialized
> interface is not going to magically solve that.
> 
> Instead we can concentrate on solving the buffer sizing problem
> generally, so that all may benefit.

The xattr system call as far as I'm concerned is not going to be pimped
to support stuff like that.

> 
> > The problem I see with this scheme is that it is not generic enough.
> > If lsof is to support displaying fuse backing files, then it needs to
> > know specifically about those magic xattrs.
> 
> Yeah, I didn't think that through.  Need some *standard* names.
> 
> > Because lsof only displays information about open files, I think
> > it would be better to come up with a standard tag in fdinfo for lsof
> > to recognize, for example:
> >
> > hidden_file: /path/to/hidden/file
> > hidden_files_list: /path/to/connections/N/backing_files
> 
> Ugh.
> 
> > Making an interface more hierarchic than hidden_files_list:
> > is useless because lsof traverses all fds anyway to filter by
> > name pattern and I am very sceptical of anyone trying to
> > push for an API get_open_fds_by_name_pattern()...
> 
> The problem is that hidden files are hidden, lsof can't traverse them
> normally.  It would be good to unhide them in some ways, and for me
> that would at least mean that you can
> 
>  1) query the path (proc/PID/fd/N link)
>  2) query fdinfo
>  3) query hidden files

Then by all means we can come up with a scheme in procfs that displays
this hierarchically if we have to.