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@szeredi.hu) wrote:

quoted

On Tue, Aug 11, 2020 at 6:05 PM Linus Torvalds
[off-list ref] wrote:

quoted

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