Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Mar 28, 2024 at 12:46:03PM -0700, Alexei Starovoitov wrote:
> To use kernel_file_open() it would need path, inode, cred.

Yeah, it's more involved and potentially more controversial. That said, just
to push the argument a bit further, at least for path, it'd be nice to have
a helper anyway which can return cgroup path. I don't know whether we'd need
direct inode access. For cred, just share some root cred?

> None of that is available now.
> Allocating all these structures just to wrap a cgroup pointer
> feels like overkill.
> Of course, it would solve the need to introduce other
> cgroup apis that are already available via text based cgroupfs
> read/write. So there are pros and cons in both approaches.
> Maybe the 3rd option would be to expose:
> cgroup_lock() as a special blend of acquire plus lock.

Oh, let's not expose that. That has been a source of problem for some use
cases and we may have to change how cgroups are locked.

> Then there will be no need for bpf_task_freeze_cgroup() with task
> argument. Instead cgroup_freeze() will be such kfunc that
> takes cgroup argument and the verifier will check that
> cgroup was acquired/locked.
> Sort-of what we check to access bpf_rb_root.

There's also cgroup.kill which would be useful for similar use cases. We can
add interface for both but idk. Let's say we have something like the
following (pardon the bad naming):

  bpf_cgroup_knob_write(struct cgroup *cgrp, char *filename, char *buf)

Would that work? I'm not necessarily in love with the idea or against adding
separate helpers but the duplication still bothers me a bit.

Thanks.

-- 
tejun