[PATCH 00/14] VFS: Filesystem information [ver #18]

David Howells dhowells at redhat.com
Mon Mar 9 22:52:53 UTC 2020

Miklos Szeredi <miklos at szeredi.hu> wrote:

> >  (1) It can be targetted.  It makes it easy to query directly by path or
> >      fd, but can also query by mount ID or fscontext fd.  procfs and sysfs
> >      cannot do three of these things easily.
> See above: with the addition of open(path, O_PATH) it can do all of these.

That's a horrible interface.  To query a file by path, you have to do:

	fd = open(path, O_PATH);
	sprintf(procpath, "/proc/self/fdmount/%u/<attr>");
	fd2 = open(procpath, O_RDONLY);
	read(fd2, ...);

See point (3) about efficiency also.  You're having to open *two* files.

> >  (2) Easier to provide LSM oversight.  Is the accessing process allowed to
> >      query information pertinent to a particular file?
> Not quite sure why this would be easier for a new ad-hoc interface than for
> the well established filesystem API.

You're right.  That's why fsinfo() uses standard pathwalk where possible,

	fsinfo(AT_FDCWD, "/path/to/file", ...);

or a fairly standard fd-querying interface:

	fsinfo(fd, "", { resolve_flags = RESOLVE_EMPTY_PATH },  ...);

to query an open file descriptor.  These are well-established filesystem APIs.

Where I vary from this is allowing direct specification of a mount ID also,
with a special flag to say that's what I'm doing:

	fsinfo(AT_FDCWD, "23", { flags = FSINFO_QUERY_FLAGS_MOUNT },  ...);

> >  (7) Don't have to create/delete a bunch of sysfs/procfs nodes each time a
> >      mount happens or is removed - and since systemd makes much use of
> >      mount namespaces and mount propagation, this will create a lot of
> >      nodes.
> This patch creates a single struct mountfs_entry per mount, which is 48bytes.

fsinfo() doesn't create any.  Furthermore, it seems that mounts get multiplied
8-10 times by systemd - though, as you say, it's not necessarily a great deal
of memory.

> Now onto the advantages of a filesystem based API:
>  - immediately usable from all programming languages, including scripts

This is not true.  You can't open O_PATH from shell scripts, so you can't
query things by path that you can't or shouldn't open (dev file paths, for
example; symlinks).

I imagine you're thinking of something like:

		id=`cat /proc/self/fdmount/5/parent_mount`
	} 5</my/path/to/my/file

but what if /my/path/to/my/file is actually /dev/foobar?

I've had a grep through the bash sources, but can't seem to find anywhere that
uses O_PATH.

>  - same goes for future extensions: no need to update libc, utils, language
>    bindings, strace, etc...

Applications and libraries using these attributes would have to change anyway
to make use of additional information.

But it's not a good argument since you now have to have text parsers that
change over time.


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