linux-kernel - Re: Upcoming: Notifications, FS notifications and fsinfo()

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Message-ID: <20200403151223.GB34800@gardel-login>
Date:   Fri, 3 Apr 2020 17:12:23 +0200
From:   Lennart Poettering <mzxreary@...inter.de>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Ian Kent <raven@...maw.net>, David Howells <dhowells@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, dray@...hat.com,
        Karel Zak <kzak@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Jeff Layton <jlayton@...hat.com>, andres@...razel.de,
        keyrings@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()

On Fr, 03.04.20 13:38, Miklos Szeredi (miklos@...redi.hu) wrote:

> On Fri, Apr 3, 2020 at 1:11 PM Lennart Poettering <mzxreary@...inter.de> wrote:
> >
> > On Fr, 03.04.20 09:44, Ian Kent (raven@...maw.net) wrote:
> >
> > > > Currently the only way to find the mount id from a path is by parsing
> > > > /proc/self/fdinfo/$fd.  It is trivial, however, to extend statx(2) to
> > > > return it directly from a path.   Also the mount notification queue
> > > > that David implemented contains the mount ID of the changed mount.
> >
> > I would love to have the mount ID exposed via statx().
>
> Here's a patch.

Oh, this is excellent. I love it, thanks!

BTW, while we are at it: one more thing I'd love to see exposed by
statx() is a simple flag whether the inode is a mount point. There's
plenty code that implements a test like this all over the place, and
it usually isn't very safe. There's one implementation in util-linux
for example (in the /usr/bin/mountpoint binary), and another one in
systemd. Would be awesome to just have a statx() return flag for that,
that would make things *so* much easier and more robust. because in
fact most code isn't very good that implements this, as much of it
just compares st_dev of the specified file and its parent. Better code
compares the mount ID, but as mentioned that's not as pretty as it
could be so far...

Lennart

--
Lennart Poettering, Berlin