This patch implements a new helper bpf_mptcp_get_subflows() to get all the
subflows of the given mptcp_sock, it returns the number of suflows. Add
a new member subflows in struct mptcp_sock as a pointers array of all the
subflows.

Register this helper in bpf_mptcp_sched_kfunc_init() to make sure it can
be accessed from the BPF context.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
---
 net/mptcp/bpf.c      | 47 ++++++++++++++++++++++++++++++++++++++++++++
 net/mptcp/protocol.h |  7 +++++++
 2 files changed, 54 insertions(+)

diff --git a/net/mptcp/bpf.c b/net/mptcp/bpf.c
index 6c01f6b959a3..3367541b353c 100644
--- a/net/mptcp/bpf.c
+++ b/net/mptcp/bpf.c
@@ -160,6 +160,23 @@ struct bpf_struct_ops bpf_mptcp_sched_ops = {
 	.name		= "mptcp_sched_ops",
 };
 
+BTF_SET_START(bpf_mptcp_sched_kfunc_ids)
+BTF_ID(func, bpf_mptcp_get_subflows_array)
+BTF_ID(func, bpf_mptcp_put_subflows_array)
+BTF_SET_END(bpf_mptcp_sched_kfunc_ids)
+
+static const struct btf_kfunc_id_set bpf_mptcp_sched_kfunc_set = {
+	.owner		= THIS_MODULE,
+	.check_set	= &bpf_mptcp_sched_kfunc_ids,
+};
+
+static int __init bpf_mptcp_sched_kfunc_init(void)
+{
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
+					 &bpf_mptcp_sched_kfunc_set);
+}
+late_initcall(bpf_mptcp_sched_kfunc_init);
+
 struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk)
 {
 	if (sk && sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP && sk_is_mptcp(sk))
@@ -168,3 +185,33 @@ struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk)
 	return NULL;
 }
 EXPORT_SYMBOL(bpf_mptcp_sock_from_subflow);
+
+struct mptcp_subflows_array *bpf_mptcp_get_subflows_array(struct mptcp_sock *msk)
+{
+	struct mptcp_subflow_context *subflow;
+	struct mptcp_subflows_array *array;
+
+	array = kzalloc(sizeof(*array), GFP_KERNEL);
+	if (!array)
+		return array;
+
+	mptcp_for_each_subflow(msk, subflow)
+		array->subflows[array->nr++] = subflow;
+
+	return array;
+}
+EXPORT_SYMBOL(bpf_mptcp_get_subflows_array);
+
+void bpf_mptcp_put_subflows_array(struct mptcp_subflows_array *array)
+{
+	int i;
+
+	if (!array)
+		return;
+
+	for (i = 0; i < MPTCP_SUBFLOWS_MAX; i++)
+		array->subflows[i] = NULL;
+	array->nr = 0;
+	kfree(array);
+}
+EXPORT_SYMBOL(bpf_mptcp_put_subflows_array);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 006914cb78de..c42fb54298ef 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -500,6 +500,13 @@ struct mptcp_subflow_context {
 	struct	rcu_head rcu;
 };
 
+#define MPTCP_SUBFLOWS_MAX	8
+
+struct mptcp_subflows_array {
+	u8 nr;
+	struct mptcp_subflow_context *subflows[MPTCP_SUBFLOWS_MAX];
+};
+
 static inline struct mptcp_subflow_context *
 mptcp_subflow_ctx(const struct sock *sk)
 {
-- 
2.34.1

On Fri, 22 Apr 2022, Geliang Tang wrote:

> This patch implements a new helper bpf_mptcp_get_subflows() to get all the
> subflows of the given mptcp_sock, it returns the number of suflows. Add
> a new member subflows in struct mptcp_sock as a pointers array of all the
> subflows.
>
> Register this helper in bpf_mptcp_sched_kfunc_init() to make sure it can
> be accessed from the BPF context.
>
> Signed-off-by: Geliang Tang <geliang.tang@suse.com>
> ---
> net/mptcp/bpf.c      | 47 ++++++++++++++++++++++++++++++++++++++++++++
> net/mptcp/protocol.h |  7 +++++++
> 2 files changed, 54 insertions(+)
>
> diff --git a/net/mptcp/bpf.c b/net/mptcp/bpf.c
> index 6c01f6b959a3..3367541b353c 100644
> --- a/net/mptcp/bpf.c
> +++ b/net/mptcp/bpf.c
> @@ -160,6 +160,23 @@ struct bpf_struct_ops bpf_mptcp_sched_ops = {
> 	.name		= "mptcp_sched_ops",
> };
>
> +BTF_SET_START(bpf_mptcp_sched_kfunc_ids)
> +BTF_ID(func, bpf_mptcp_get_subflows_array)
> +BTF_ID(func, bpf_mptcp_put_subflows_array)
> +BTF_SET_END(bpf_mptcp_sched_kfunc_ids)
> +
> +static const struct btf_kfunc_id_set bpf_mptcp_sched_kfunc_set = {
> +	.owner		= THIS_MODULE,
> +	.check_set	= &bpf_mptcp_sched_kfunc_ids,
> +};
> +
> +static int __init bpf_mptcp_sched_kfunc_init(void)
> +{
> +	return register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
> +					 &bpf_mptcp_sched_kfunc_set);
> +}
> +late_initcall(bpf_mptcp_sched_kfunc_init);
> +
> struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk)
> {
> 	if (sk && sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP && sk_is_mptcp(sk))
> @@ -168,3 +185,33 @@ struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk)
> 	return NULL;
> }
> EXPORT_SYMBOL(bpf_mptcp_sock_from_subflow);
> +
> +struct mptcp_subflows_array *bpf_mptcp_get_subflows_array(struct mptcp_sock *msk)
> +{
> +	struct mptcp_subflow_context *subflow;
> +	struct mptcp_subflows_array *array;
> +
> +	array = kzalloc(sizeof(*array), GFP_KERNEL);
> +	if (!array)
> +		return array;
> +
> +	mptcp_for_each_subflow(msk, subflow)
> +		array->subflows[array->nr++] = subflow;
> +
> +	return array;
> +}
> +EXPORT_SYMBOL(bpf_mptcp_get_subflows_array);
> +
> +void bpf_mptcp_put_subflows_array(struct mptcp_subflows_array *array)
> +{
> +	int i;
> +
> +	if (!array)
> +		return;
> +
> +	for (i = 0; i < MPTCP_SUBFLOWS_MAX; i++)
> +		array->subflows[i] = NULL;
> +	array->nr = 0;
> +	kfree(array);

We can't trust the caller to always call this function, since we don't 
want to allow userspace to leak this array memory either accidentally or 
intentionally.

Can BPF code call a helper with a pointer to a struct on the BPF stack?

> +}
> +EXPORT_SYMBOL(bpf_mptcp_put_subflows_array);
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index 006914cb78de..c42fb54298ef 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -500,6 +500,13 @@ struct mptcp_subflow_context {
> 	struct	rcu_head rcu;
> };
>
> +#define MPTCP_SUBFLOWS_MAX	8

I noticed that this value is separately defined here and in the next 
commit (tools/testing/selftests/bpf/bpf_mptcp_helpers.h). That seems 
fragile to me, there's no way for bpf_mptcp_sched_btf_struct_access() to 
enforce access limits correctly.

Do you have some other ideas for how to share this data between kernel and 
BPF contexts? Maybe mptcp_sched_data could be expanded to pass the subflow 
list/array in to the scheduler function.

> +
> +struct mptcp_subflows_array {
> +	u8 nr;
> +	struct mptcp_subflow_context *subflows[MPTCP_SUBFLOWS_MAX];
> +};
> +
> static inline struct mptcp_subflow_context *
> mptcp_subflow_ctx(const struct sock *sk)
> {
> -- 
> 2.34.1
>
>
>

--
Mat Martineau
Intel