Subject: Re: [PATCH v2 bpf-next 2/9] bpf: Add bpf helper bpf_tcp_enter_cwr

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 25 Feb 2019 19:32:25 -0800

On 02/26, Martin Lau wrote:
> On Mon, Feb 25, 2019 at 03:14:38PM -0800, Stanislav Fomichev wrote:
> [ ... ]
> 
> > > 
> > > To ensure it is only called from BPF_CGROUP_INET_EGRESS, the
> > > attr->expected_attach_type must be specified as BPF_CGROUP_INET_EGRESS
> > > during load time if the prog uses this new helper.
> > > The newly added prog->enforce_expected_attach_type bit will also be set
> > > if this new helper is used.  This bit is for backward compatibility reason
> > > because currently prog->expected_attach_type has been ignored in
> > > BPF_PROG_TYPE_CGROUP_SKB.  During attach time,
> > > prog->expected_attach_type is only enforced if the
> > > prog->enforce_expected_attach_type bit is set.
> > > i.e. prog->expected_attach_type is only enforced if this new helper
> > > is used by the prog.
> > > 
> [ ... ]
> 
> > > @@ -1725,6 +1733,10 @@ static int bpf_prog_attach_check_attach_type(const struct bpf_prog *prog,
> > >  	case BPF_PROG_TYPE_CGROUP_SOCK:
> > >  	case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
> > >  		return attach_type == prog->expected_attach_type ? 0 : -EINVAL;
> > > +	case BPF_PROG_TYPE_CGROUP_SKB:
> > > +		return prog->enforce_expected_attach_type &&
> > > +			prog->expected_attach_type != attach_type ?
> > > +			-EINVAL : 0;
> > >  	default:
> > >  		return 0;
> > >  	}
> [ ... ]
> 
> > > diff --git a/net/core/filter.c b/net/core/filter.c
> > > index 97916eedfe69..ca57ef25279c 100644
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -5426,6 +5426,24 @@ static const struct bpf_func_proto bpf_tcp_sock_proto = {
> > >  	.arg1_type	= ARG_PTR_TO_SOCK_COMMON,
> > >  };
> > >  
> > > +BPF_CALL_1(bpf_tcp_enter_cwr, struct tcp_sock *, tp)
> > > +{
> > > +	struct sock *sk = (struct sock *)tp;
> > > +
> > > +	if (sk->sk_state == TCP_ESTABLISHED) {
> > > +		tcp_enter_cwr(sk);
> > > +		return 0;
> > > +	}
> > > +
> > > +	return -EINVAL;
> > > +}
> > > +
> > > +static const struct bpf_func_proto bpf_tcp_enter_cwr_proto = {
> > > +	.func        = bpf_tcp_enter_cwr,
> > > +	.gpl_only    = false,
> > > +	.ret_type    = RET_INTEGER,
> > > +	.arg1_type    = ARG_PTR_TO_TCP_SOCK,
> > > +};
> > >  #endif /* CONFIG_INET */
> > >  
> > >  bool bpf_helper_changes_pkt_data(void *func)
> > > @@ -5585,6 +5603,13 @@ cg_skb_func_proto(enum bpf_func_id func_id, struct bpf_prog *prog)
> > >  #ifdef CONFIG_INET
> > >  	case BPF_FUNC_tcp_sock:
> > >  		return &bpf_tcp_sock_proto;
> > 
> > [...]
> > > +	case BPF_FUNC_tcp_enter_cwr:
> > > +		if (prog->expected_attach_type == BPF_CGROUP_INET_EGRESS) {
> > > +			prog->enforce_expected_attach_type = 1;
> > > +			return &bpf_tcp_enter_cwr_proto;
> > Instead of this back and forth with enforce_expected_attach_type, can we
> > just do here:
> > 
> > if (prog->expected_attach_type == BPF_CGROUP_INET_EGRESS)
> > 	return &bpf_tcp_enter_cwr_proto;
> > else
> > 	return null;
> > 
> > Wouldn't it have the same effect?
> The attr->expected_attach_type is currently ignored (i.e. not checked)
> during the bpf load time.
But nothing stops you form checking prog->expected_attach_type in
the cg_skb_func_proto, right? That is done at the time of loading.
So depending on prog->expected_attach_type just return null or non-null
and the verifier will take care of the rest. Then, at attach time just
make sure we are attaching it to the expected_attach_type.

We also should not have any existing use cases for
BPF_FUNC_tcp_enter_cwr I suppose.

In other words, why something like below won't work? Am I missing something?

---

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index b155cd17c1bd..86dc7cd00f34 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1678,6 +1678,10 @@ static int bpf_prog_attach_check_attach_type(const struct bpf_prog *prog,
 	case BPF_PROG_TYPE_CGROUP_SOCK:
 	case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
 		return attach_type == prog->expected_attach_type ? 0 : -EINVAL;
+	case BPF_PROG_TYPE_CGROUP_SKB:
+		if (prog->expected_attach_type)
+			return attach_type == prog->expected_attach_type ? 0 : -EINVAL;
+		return 0;
 	default:
 		return 0;
 	}
diff --git a/net/core/filter.c b/net/core/filter.c
index 7559d6835ecb..56f70468fc7a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5396,6 +5396,11 @@ cg_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	switch (func_id) {
 	case BPF_FUNC_get_local_storage:
 		return &bpf_get_local_storage_proto;
+	case BPF_FUNC_tcp_enter_cwr:
+		if (prog->expected_attach_type == BPF_CGROUP_INET_EGRESS)
+			return &bpf_tcp_enter_cwr_proto;
+		else
+			return NULL;
 	default:
 		return sk_filter_func_proto(func_id, prog);
 	}

> 
> How to avoid breaking backward compatibility without selectively
> enforcing prog->expected_attach_type during attach time?