Re: [PATCH RFC bpf-next v2 5/5] selftests/bpf: test for pinning for cgroup_view link

[Hao Luo]

On Sat, Feb 5, 2022 at 8:29 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Fri, Feb 4, 2022 at 10:27 AM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> > >
> > > > In our use case, we can't ask the users who create cgroups to do the
> > > > pinning. Pinning requires root privilege. In our use case, we have
> > > > non-root users who can create cgroup directories and still want to
> > > > read bpf stats. They can't do pinning by themselves. This is why
> > > > inheritance is a requirement for us. With inheritance, they only need
> > > > to mkdir in cgroupfs and bpffs (unprivileged operations), no pinning
> > > > operation is required. Patch 1-4 are needed to implement inheritance.
> > > >
> > > > It's also not a good idea in our use case to add a userspace
> > > > privileged process to monitor cgroupfs operations and perform the
> > > > pinning. It's more complex and has a higher maintenance cost and
> > > > runtime overhead, compared to the solution of asking whoever makes
> > > > cgroups to mkdir in bpffs. The other problem is: if there are nodes in
> > > > the data center that don't have the userspace process deployed, the
> > > > stats will be unavailable, which is a no-no for some of our users.
> > >
> > > The commit log says that there will be a daemon that does that
> > > monitoring of cgroupfs. And that daemon needs to mkdir
> > > directories in bpffs when a new cgroup is created, no?
> > > The kernel is only doing inheritance of bpf progs into
> > > new dirs. I think that daemon can pin as well.
> > >
> > > The cgroup creation is typically managed by an agent like systemd.
> > > Sounds like you have your own agent that creates cgroups?
> > > If so it has to be privileged and it can mkdir in bpffs and pin too ?
> >
> > Ah, yes, we have our own daemon to manage cgroups. That daemon creates
> > the top-level cgroup for each job to run inside. However, the job can
> > create its own cgroups inside the top-level cgroup, for fine grained
> > resource control. This doesn't go through the daemon. The job-created
> > cgroups don't have the pinned objects and this is a no-no for our
> > users.
>
> We can whitelist certain tracepoints to be sleepable and extend
> tp_btf prog type to include everything from prog_type_syscall.
> Such prog would attach to cgroup_mkdir and cgroup_release
> and would call bpf_sys_bpf() helper to pin progs in new bpffs dirs.
> We can allow prog_type_syscall to do mkdir in bpffs as well.
>
> This feature could be useful for similar monitoring/introspection tasks.
> We can write a program that would monitor bpf prog load/unload
> and would pin an iterator prog that would show debug info about a prog.
> Like cat /sys/fs/bpf/progs.debug shows a list of loaded progs.
> With this feature we can implement:
> ls /sys/fs/bpf/all_progs.debug/
> and each loaded prog would have a corresponding file.
> The file name would be a program name, for example.
> cat /sys/fs/bpf/all_progs.debug/my_prog
> would pretty print info about 'my_prog' bpf program.
>
> This way the kernfs/cgroupfs specific logic from patches 1-4
> will not be necessary.
>
> wdyt?

Thanks Alexei. I gave it more thought in the last couple of days.
Actually I think it's a good idea, more flexible. It gets rid of the
need of a user space daemon for monitoring cgroup creation and
destruction. We could monitor task creations and exits as well, so
that we can export per-task information (e.g. task_vma_iter) more
efficiently.

A couple of thoughts when thinking about the details:

- Regarding parameterized pinning, I don't think we can have one
single bpf_iter_link object, but with different parameters. Because
parameters are part of the bpf_iter_link (bpf_iter_aux_info). So every
time we pin, we have to attach iter in order to get a new link object
first. So we need to add attach and detach in bpf_sys_bpf().
- We also need to add those syscalls for cleanup: (1) unlink for
removing pinned obj and (2) rmdir for removing the directory in
prog_type_syscall.

With these extensions, we can shift some of the bpf operations
currently performed in system daemons into the kernel. IMHO it's a
great thing, making system monitoring more flexible.