linux-kernel - Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

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Date:   Tue, 11 Aug 2020 21:31:05 +0200
From:   Lennart Poettering <mzxreary@...inter.de>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, Karel Zak <kzak@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

On Di, 11.08.20 20:49, Miklos Szeredi (miklos@...redi.hu) wrote:

> On Tue, Aug 11, 2020 at 6:05 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>
> > and then people do "$(srctree)/". If you haven't seen that kind of
> > pattern where the pathname has two (or sometimes more!) slashes in the
> > middle, you've led a very sheltered life.
>
> Oh, I have.   That's why I opted for triple slashes, since that should
> work most of the time even in those concatenated cases.  And yes, I
> know, most is not always, and this might just be hiding bugs, etc...
> I think the pragmatic approach would be to try this and see how many
> triple slash hits a normal workload gets and if it's reasonably low,
> then hopefully that together with warnings for O_ALT would be enough.

There's no point. Userspace relies on the current meaning of triple
slashes. It really does.

I know many places in systemd where we might end up with a triple
slash. Here's a real-life example: some code wants to access the
cgroup attribute 'cgroup.controllers' of the root cgroup. It thus
generates the right path in the fs for it, which is the concatenation of
"/sys/fs/cgroup/" (because that's where cgroupfs is mounted), of "/"
(i.e. for the root cgroup) and of "/cgroup.controllers" (as that's the
file the attribute is exposed under).

And there you go:

   "/sys/fs/cgroup/" + "/" + "/cgroup.controllers" → "/sys/fs/cgroup///cgroup.controllers"

This is a real-life thing. Don't break this please.

Lennart

--
Lennart Poettering, Berlin