Commit eb18b49e authored by Martin KaFai Lau's avatar Martin KaFai Lau Committed by Alexei Starovoitov
Browse files

bpf: tcp: Allow bpf-tcp-cc to call bpf_(get|set)sockopt 



This patch allows the bpf-tcp-cc to call bpf_setsockopt.  One use
case is to allow a bpf-tcp-cc switching to another cc during init().
For example, when the tcp flow is not ecn ready, the bpf_dctcp
can switch to another cc by calling setsockopt(TCP_CONGESTION).

During setsockopt(TCP_CONGESTION), the new tcp-cc's init() will be
called and this could cause a recursion but it is stopped by the
current trampoline's logic (in the prog->active counter).

While retiring a bpf-tcp-cc (e.g. in tcp_v[46]_destroy_sock()),
the tcp stack calls bpf-tcp-cc's release().  To avoid the retiring
bpf-tcp-cc making further changes to the sk, bpf_setsockopt is not
available to the bpf-tcp-cc's release().  This will avoid release()
making setsockopt() call that will potentially allocate new resources.

Although the bpf-tcp-cc already has a more powerful way to read tcp_sock
from the PTR_TO_BTF_ID, it is usually expected that bpf_getsockopt and
bpf_setsockopt are available together.  Thus, bpf_getsockopt() is also
added to all tcp_congestion_ops except release().

When the old bpf-tcp-cc is calling setsockopt(TCP_CONGESTION)
to switch to a new cc, the old bpf-tcp-cc will be released by
bpf_struct_ops_put().  Thus, this patch also puts the bpf_struct_ops_map
after a rcu grace period because the trampoline's image cannot be freed
while the old bpf-tcp-cc is still running.

bpf-tcp-cc can only access icsk_ca_priv as SCALAR.  All kernel's
tcp-cc is also accessing the icsk_ca_priv as SCALAR.   The size
of icsk_ca_priv has already been raised a few times to avoid
extra kmalloc and memory referencing.  The only exception is the
kernel's tcp_cdg.c that stores a kmalloc()-ed pointer in icsk_ca_priv.
To avoid the old bpf-tcp-cc accidentally overriding this tcp_cdg's pointer
value stored in icsk_ca_priv after switching and without over-complicating
the bpf's verifier for this one exception in tcp_cdg, this patch does not
allow switching to tcp_cdg.  If there is a need, bpf_tcp_cdg can be
implemented and then use the bpf_sk_storage as the extended storage.

bpf_sk_setsockopt proto has only been recently added and used
in bpf-sockopt and bpf-iter-tcp, so impose the tcp_cdg limitation in the
same proto instead of adding a new proto specifically for bpf-tcp-cc.

Signed-off-by: default avatarMartin KaFai Lau <kafai@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210824173007.3976921-1-kafai@fb.com
parent 7d789bd0

kernel/bpf/bpf_struct_ops.c

+21 −1
Original line number Diff line number Diff line
@@ -28,6 +28,7 @@ struct bpf_struct_ops_value { 


struct bpf_struct_ops_map {

	struct bpf_map map;

	struct rcu_head rcu;

	const struct bpf_struct_ops *st_ops;

	/* protect map_update */

	struct mutex lock;
@@ -622,6 +623,14 @@ bool bpf_struct_ops_get(const void *kdata) 
	return refcount_inc_not_zero(&kvalue->refcnt);

}



static void bpf_struct_ops_put_rcu(struct rcu_head *head)

{

	struct bpf_struct_ops_map *st_map;



	st_map = container_of(head, struct bpf_struct_ops_map, rcu);

	bpf_map_put(&st_map->map);

}



void bpf_struct_ops_put(const void *kdata)

{

	struct bpf_struct_ops_value *kvalue;
@@ -632,6 +641,17 @@ void bpf_struct_ops_put(const void *kdata) 


		st_map = container_of(kvalue, struct bpf_struct_ops_map,

				      kvalue);

		bpf_map_put(&st_map->map);

		/* The struct_ops's function may switch to another struct_ops.

		 *

		 * For example, bpf_tcp_cc_x->init() may switch to

		 * another tcp_cc_y by calling

		 * setsockopt(TCP_CONGESTION, "tcp_cc_y").

		 * During the switch,  bpf_struct_ops_put(tcp_cc_x) is called

		 * and its map->refcnt may reach 0 which then free its

		 * trampoline image while tcp_cc_x is still running.

		 *

		 * Thus, a rcu grace period is needed here.

		 */

		call_rcu(&st_map->rcu, bpf_struct_ops_put_rcu);

	}

}

net/core/filter.c

+6 −0
Original line number Diff line number Diff line
@@ -5051,6 +5051,12 @@ static int _bpf_getsockopt(struct sock *sk, int level, int optname, 
BPF_CALL_5(bpf_sk_setsockopt, struct sock *, sk, int, level,

	   int, optname, char *, optval, int, optlen)

{

	if (level == SOL_TCP && optname == TCP_CONGESTION) {

		if (optlen >= sizeof("cdg") - 1 &&

		    !strncmp("cdg", optval, optlen))

			return -ENOTSUPP;

	}



	return _bpf_setsockopt(sk, level, optname, optval, optlen);

}

net/ipv4/bpf_tcp_ca.c

+38 −3
Original line number Diff line number Diff line
@@ -10,6 +10,9 @@ 
#include <net/tcp.h>

#include <net/bpf_sk_storage.h>



/* "extern" is to avoid sparse warning.  It is only used in bpf_struct_ops.c. */

extern struct bpf_struct_ops bpf_tcp_congestion_ops;



static u32 optional_ops[] = {

	offsetof(struct tcp_congestion_ops, init),

	offsetof(struct tcp_congestion_ops, release),
@@ -163,6 +166,19 @@ static const struct bpf_func_proto bpf_tcp_send_ack_proto = { 
	.arg2_type	= ARG_ANYTHING,

};



static u32 prog_ops_moff(const struct bpf_prog *prog)

{

	const struct btf_member *m;

	const struct btf_type *t;

	u32 midx;



	midx = prog->expected_attach_type;

	t = bpf_tcp_congestion_ops.type;

	m = &btf_type_member(t)[midx];



	return btf_member_bit_offset(t, m) / 8;

}



static const struct bpf_func_proto *

bpf_tcp_ca_get_func_proto(enum bpf_func_id func_id,

			  const struct bpf_prog *prog)
@@ -174,6 +190,28 @@ bpf_tcp_ca_get_func_proto(enum bpf_func_id func_id, 
		return &bpf_sk_storage_get_proto;

	case BPF_FUNC_sk_storage_delete:

		return &bpf_sk_storage_delete_proto;

	case BPF_FUNC_setsockopt:

		/* Does not allow release() to call setsockopt.

		 * release() is called when the current bpf-tcp-cc

		 * is retiring.  It is not allowed to call

		 * setsockopt() to make further changes which

		 * may potentially allocate new resources.

		 */

		if (prog_ops_moff(prog) !=

		    offsetof(struct tcp_congestion_ops, release))

			return &bpf_sk_setsockopt_proto;

		return NULL;

	case BPF_FUNC_getsockopt:

		/* Since get/setsockopt is usually expected to

		 * be available together, disable getsockopt for

		 * release also to avoid usage surprise.

		 * The bpf-tcp-cc already has a more powerful way

		 * to read tcp_sock from the PTR_TO_BTF_ID.

		 */

		if (prog_ops_moff(prog) !=

		    offsetof(struct tcp_congestion_ops, release))

			return &bpf_sk_getsockopt_proto;

		return NULL;

	default:

		return bpf_base_func_proto(func_id);

	}
@@ -286,9 +324,6 @@ static void bpf_tcp_ca_unreg(void *kdata) 
	tcp_unregister_congestion_control(kdata);

}



/* Avoid sparse warning.  It is only used in bpf_struct_ops.c. */

extern struct bpf_struct_ops bpf_tcp_congestion_ops;



struct bpf_struct_ops bpf_tcp_congestion_ops = {

	.verifier_ops = &bpf_tcp_ca_verifier_ops,

	.reg = bpf_tcp_ca_reg,