Subject: Re: [PATCH bpf-next v2 2/3] bpf, netns: Keep attached programs in bpf_prog_array

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 24 Jun 2020 19:18:42 +0200

On Tue, Jun 23, 2020 at 12:34 PM CEST, Jakub Sitnicki wrote:
> Prepare for having multi-prog attachments for new netns attach types by
> storing programs to run in a bpf_prog_array, which is well suited for
> iterating over programs and running them in sequence.
>
> Because bpf_prog_array is dynamically resized, after this change a
> potentially blocking memory allocation in bpf(PROG_QUERY) callback can
> happen, in order to collect program IDs before copying the values to
> user-space supplied buffer. This forces us to adapt how we protect access
> to the attached program in the callback. As bpf_prog_array_copy_to_user()
> helper can sleep, we switch from an RCU read lock to holding a mutex that
> serializes updaters.
>
> To handle bpf(PROG_ATTACH) scenario when we are replacing an already
> attached program, we introduce a new bpf_prog_array helper called
> bpf_prog_array_replace_item that will exchange the old program with a new
> one. bpf-cgroup does away with such helper by computing an index into the
> array from a program position in an external list of attached
> programs/links. Such approach fails when a dummy prog is left in the array
> after a memory allocation failure on link release, but is necessary in
> bpf-cgroup case because the same BPF program can be present in the array
> multiple times due to inheritance, and attachment cannot be reliably
> identified by bpf_prog pointer comparison.
>
> No functional changes intended.
>
> Acked-by: Andrii Nakryiko <[email-masked]>
> Signed-off-by: Jakub Sitnicki <[email-masked]>
> ---
>  include/linux/bpf.h        |   3 +
>  include/net/netns/bpf.h    |   5 +-
>  kernel/bpf/core.c          |  20 ++++--
>  kernel/bpf/net_namespace.c | 137 +++++++++++++++++++++++++++----------
>  net/core/flow_dissector.c  |  21 +++---
>  5 files changed, 132 insertions(+), 54 deletions(-)
>

[...]

> diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
> index b951dab2687f..593523a22168 100644
> --- a/kernel/bpf/net_namespace.c
> +++ b/kernel/bpf/net_namespace.c

[...]

> @@ -93,8 +108,16 @@ static int bpf_netns_link_update_prog(struct bpf_link *link,
>  		goto out_unlock;
>  	}
>
> +	run_array = rcu_dereference_protected(net->bpf.run_array[type],
> +					      lockdep_is_held(&netns_bpf_mutex));
> +	if (run_array)
> +		ret = bpf_prog_array_replace_item(run_array, link->prog, new_prog);

Thinking about this some more, link update should fail with -EINVAL if
new_prog already exists in run_array. Same as PROG_ATTACH fails with
-EINVAL when trying to attach the same prog for the second time.

Otherwise, LINK_UPDATE can lead to having same BPF prog present multiple
times in the prog_array, once attaching more than one prog gets enabled.

Then we would we end up with the same challenge as bpf-cgroup, that is
how to find the program index into the prog_array in presence of
dummy_prog's.

> +	else
> +		ret = -ENOENT;
> +	if (ret)
> +		goto out_unlock;
> +
>  	old_prog = xchg(&link->prog, new_prog);
> -	rcu_assign_pointer(net->bpf.progs[type], new_prog);
>  	bpf_prog_put(old_prog);
>
>  out_unlock:

[...]

> @@ -217,14 +249,25 @@ int netns_bpf_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>  	if (ret)
>  		goto out_unlock;
>
> -	attached = rcu_dereference_protected(net->bpf.progs[type],
> -					     lockdep_is_held(&netns_bpf_mutex));
> +	attached = net->bpf.progs[type];
>  	if (attached == prog) {
>  		/* The same program cannot be attached twice */
>  		ret = -EINVAL;
>  		goto out_unlock;
>  	}
> -	rcu_assign_pointer(net->bpf.progs[type], prog);
> +
> +	run_array = rcu_dereference_protected(net->bpf.run_array[type],
> +					      lockdep_is_held(&netns_bpf_mutex));
> +	if (run_array) {
> +		ret = bpf_prog_array_replace_item(run_array, attached, prog);

I didn't consider here that there can be a run_array with a dummy_prog
from a link release that failed to allocate memory.

In such case bpf_prog_array_replace_item will fail, while we actually
want to replace the dummy_prog.

The right thing to do is to replace the first item in prog array:

	if (run_array) {
		WRITE_ONCE(run_array->items[0].prog, prog);
	} else {
                /* allocate a bpf_prog_array */
        }

This leaves just one user of bpf_prog_array_replace_item(), so I think
I'm just going to fold it into its only caller, that is the update_prog
callback.

> +	} else {
> +		ret = bpf_prog_array_copy(NULL, NULL, prog, &run_array);
> +		rcu_assign_pointer(net->bpf.run_array[type], run_array);
> +	}
> +	if (ret)
> +		goto out_unlock;
> +
> +	net->bpf.progs[type] = prog;
>  	if (attached)
>  		bpf_prog_put(attached);
>

[...]