Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2020-02-24 at 11:24 +0100, Miklos Szeredi wrote:
> On Fri, Feb 21, 2020 at 9:21 PM James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
[...]
> > 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.
> 
> And that makes me wonder: would a
> "/sys/class/fs/$ST_DEV/options/$OPTION" type interface be feasible
> for this?

Once it's table driven, certainly a sysfs directory becomes possible. 
The problem with ST_DEV is filesystems like btrfs and xfs that may have
multiple devices.  The current fsinfo takes a fspick'd directory fd so
the input to the query is a path, which gets messy in sysfs, although I
could see something like /sys/class/fs/mount/<path>/$OPTION working.

James