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

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Message-ID: <20200811195029.cnowfkikr2xajhgo@wittgenstein>
Date:   Tue, 11 Aug 2020 21:50:29 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Lennart Poettering <mzxreary@...inter.de>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        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 Tue, Aug 11, 2020 at 09:31:05PM +0200, Lennart Poettering wrote:
> 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.

Taken from a log from a container:

lxc f4 20200810105815.742 TRACE    cgfsng - cgroups/cgfsng.c:cg_legacy_handle_cpuset_hierarchy:552 - "cgroup.clone_children" was already set to "1"
lxc f4 20200810105815.742 WARN     cgfsng - cgroups/cgfsng.c:mkdir_eexist_on_last:1152 - File exists - Failed to create directory "/sys/fs/cgroup/cpuset///lxc.monitor.f4"
lxc f4 20200810105815.743 INFO     cgfsng - cgroups/cgfsng.c:cgfsng_monitor_create:1366 - The monitor process uses "lxc.monitor.f4" as cgroup
lxc f4 20200810105815.743 DEBUG    storage - storage/storage.c:get_storage_by_name:211 - Detected rootfs type "dir"
lxc f4 20200810105815.743 TRACE    cgfsng - cgroups/cgfsng.c:cg_legacy_handle_cpuset_hierarchy:552 - "cgroup.clone_children" was already set to "1"
lxc f4 20200810105815.743 WARN     cgfsng - cgroups/cgfsng.c:mkdir_eexist_on_last:1152 - File exists - Failed to create directory "/sys/fs/cgroup/cpuset///lxc.payload.f4"
lxc f4 20200810105815.743 INFO     cgfsng - cgroups/cgfsng.c:cgfsng_payload_create:1469 - The container process uses "lxc.payload.f4" as cgroup
lxc f4 20200810105815.744 TRACE    start - start.c:lxc_spawn:1731 - Spawned container directly into target cgroup via cgroup2 fd 17 

Christian