linux-kernel - Re: [PATCH 00/14] VFS: Filesystem information [ver #18]

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Date:   Tue, 10 Mar 2020 10:18:08 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Theodore Ts'o" <tytso@....edu>,
        Stefan Metzmacher <metze@...ba.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Karel Zak <kzak@...hat.com>, Jeff Layton <jlayton@...hat.com>,
        linux-fsdevel@...r.kernel.org,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/14] VFS: Filesystem information [ver #18]

On Mon, Mar 9, 2020 at 11:53 PM David Howells <dhowells@...hat.com> wrote:
>
> Miklos Szeredi <miklos@...redi.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, ...);
>         close(fd2);
>         close(fd);
>
> See point (3) about efficiency also.  You're having to open *two* files.

I completely agree, opening two files is surely going to kill
performance of application needing to retrieve a billion mount
attributes per second.</sarcasm>

> > >  (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,
> e.g.:
>
>         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.

Yes.  The problem is with the "..." part where you pass random
structures to a function.   That's useful sometimes, but at the very
least it breaks type safety, and not what I would call a "clean" API.

> > 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).

Yes.  However, you just wrote the core of a utility that could do this
(in 6 lines, no less).  Now try that feat with fsinfo(2)!

Thanks,
Miklos