[PATCH 00/18] VFS: Filesystem information [ver #21]

[Miklos Szeredi]

On Tue, Aug 4, 2020 at 5:40 PM James Bottomley
<James.Bottomley at hansenpartnership.com> wrote:
>
> On Mon, 2020-08-03 at 14:36 +0100, David Howells wrote:
> > Here's a set of patches that adds a system call, fsinfo(), that
> > allows information about the VFS, mount topology, superblock and
> > files to be retrieved.
> >
> > The patchset is based on top of the notifications patchset and allows
> > event counters implemented in the latter to be retrieved to allow
> > overruns to be efficiently managed.
>
> Could I repeat the question I asked about six months back that never
> got answered:
>
> https://lore.kernel.org/linux-api/1582316494.3376.45.camel@HansenPartnership.com/
>
> It sort of petered out into a long winding thread about why not use
> sysfs instead, which really doesn't look like a good idea to me.
>
> I'll repeat the information for those who want to quote it easily on
> reply without having to use a web interface:
>
> ---
> Could I make a suggestion about how this should be done in a way that
> doesn't actually require the fsinfo syscall at all: it could just be
> done with fsconfig.  The idea is based on something I've wanted to do
> for configfd but couldn't because otherwise it wouldn't substitute for
> fsconfig, but Christian made me think it was actually essential to the
> ability of the seccomp and other verifier tools in the critique of
> configfd and I belive the same critique applies here.
>
> Instead of making fsconfig functionally configure ... as in you pass
> the attribute name, type and parameters down into the fs specific
> handler and the handler does a string match and then verifies the
> parameters and then acts on them, make it table configured, so what
> each fstype does is register a table of attributes which can be got and
> optionally set (with each attribute having a get and optional set
> function).  We'd have multiple tables per fstype, so the generic VFS
> can register a table of attributes it understands for every fstype
> (things like name, uuid and the like) and then each fs type would
> register a table of fs specific attributes following the same pattern.
> The system would examine the fs specific table before the generic one,
> allowing overrides.  fsconfig would have the ability to both get and
> set attributes, permitting retrieval as well as setting (which is how I
> get rid of the fsinfo syscall), we'd have a global parameter, which
> would retrieve the entire table by name and type so the whole thing is
> introspectable because the upper layer knows a-priori all the
> attributes which can be set for a given fs type and what type they are
> (so we can make more of the parsing generic).  Any attribute which
> doesn't have a set routine would be read only and all attributes would
> have to have a get routine meaning everything is queryable.

fsconfig(2) takes an fd referring to an fs_context, that in turn
refers to a super_block.

So using fsconfig() for retrieving super_block attributes would be
fine (modulo value being const, and lack of buffer size).

But what about mount attributes?

I don't buy the argument that an API needs to be designed around the
requirements of seccomp and the like.  It should be the other way
round.  In that, I think your configfd idea was fine, and would answer
the above question.

Thanks,
Miklos