Re: [PATCH bpf-next v9 1/5] bpf: Introduce cgroup iter

[Michal Koutný]

Hello.

On Tue, Aug 23, 2022 at 08:00:27PM -0700, Hao Luo <haoluo@xxxxxxxxxx> wrote:
> +static int bpf_iter_attach_cgroup(struct bpf_prog *prog,
> +				  union bpf_iter_link_info *linfo,
> +				  struct bpf_iter_aux_info *aux)
> +{
> +	int fd = linfo->cgroup.cgroup_fd;
> +	u64 id = linfo->cgroup.cgroup_id;
> +	int order = linfo->cgroup.order;
> +	struct cgroup *cgrp;
> +
> +	if (order != BPF_ITER_DESCENDANTS_PRE &&
> +	    order != BPF_ITER_DESCENDANTS_POST &&
> +	    order != BPF_ITER_ANCESTORS_UP &&
> +	    order != BPF_ITER_SELF_ONLY)
> +		return -EINVAL;
> +
> +	if (fd && id)
> +		return -EINVAL;
> +
> +	if (fd)
> +		cgrp = cgroup_get_from_fd(fd);
> +	else if (id)
> +		cgrp = cgroup_get_from_id(id);
> +	else /* walk the entire hierarchy by default. */
> +		cgrp = cgroup_get_from_path("/");
> +
> +	if (IS_ERR(cgrp))
> +		return PTR_ERR(cgrp);

This section caught my eye.

Perhaps the simpler way for the default hierachy fallback would be

		cgrp = &cgrp_dfl_root.cgrp;
		cgroup_get(cgroup)

But maybe it's not what is the intention if cgroup NS should be taken
into account and cgroup_get_from_path() is buggy in this regard.

Would it make sense to prepend the patch below to your series?

Also, that makes me think about iter initialization with ID. In contrast
with FD passing (that's subject to some permissions and NS checks), the
retrieval via ID is not equipped with that, ids are not unguessable and
I'd consider cgroup IDs an implementation detail.

So, is the ID initialization that much useful? (I have no idea about
permissions model of BPF here, so it might be just fine but still it'd
be good to take cgroup NS into account. Likely for BPF_ITER_ANCESTORS_UP
too.)

HTH,
Michal

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