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Commit 34f79502 authored by John Fastabend's avatar John Fastabend Committed by David S. Miller
Browse files

bpf: avoid preempt enable/disable in sockmap using tcp_skb_cb region 



SK_SKB BPF programs are run from the socket/tcp context but early in
the stack before much of the TCP metadata is needed in tcp_skb_cb. So
we can use some unused fields to place BPF metadata needed for SK_SKB
programs when implementing the redirect function.

This allows us to drop the preempt disable logic. It does however
require an API change so sk_redirect_map() has been updated to
additionally provide ctx_ptr to skb. Note, we do however continue to
disable/enable preemption around actual BPF program running to account
for map updates.

Signed-off-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 435bf0d3

include/linux/filter.h

+1 −1
Original line number Diff line number Diff line
@@ -728,7 +728,7 @@ void xdp_do_flush_map(void); 
void bpf_warn_invalid_xdp_action(u32 act);

void bpf_warn_invalid_xdp_redirect(u32 ifindex);



struct sock *do_sk_redirect_map(void);

struct sock *do_sk_redirect_map(struct sk_buff *skb);



#ifdef CONFIG_BPF_JIT

extern int bpf_jit_enable;

include/net/tcp.h

+5 −0
Original line number Diff line number Diff line
@@ -840,6 +840,11 @@ struct tcp_skb_cb { 
			struct inet6_skb_parm	h6;

#endif

		} header;	/* For incoming skbs */

		struct {

			__u32 key;

			__u32 flags;

			struct bpf_map *map;

		} bpf;

	};

};

kernel/bpf/sockmap.c

+9 −10
Original line number Diff line number Diff line
@@ -39,6 +39,7 @@ 
#include <linux/workqueue.h>

#include <linux/list.h>

#include <net/strparser.h>

#include <net/tcp.h>



struct bpf_stab {

	struct bpf_map map;
@@ -101,9 +102,16 @@ static int smap_verdict_func(struct smap_psock *psock, struct sk_buff *skb) 
		return SK_DROP;



	skb_orphan(skb);

	/* We need to ensure that BPF metadata for maps is also cleared

	 * when we orphan the skb so that we don't have the possibility

	 * to reference a stale map.

	 */

	TCP_SKB_CB(skb)->bpf.map = NULL;

	skb->sk = psock->sock;

	bpf_compute_data_end(skb);

	preempt_disable();

	rc = (*prog->bpf_func)(skb, prog->insnsi);

	preempt_enable();

	skb->sk = NULL;



	return rc;
@@ -114,17 +122,10 @@ static void smap_do_verdict(struct smap_psock *psock, struct sk_buff *skb) 
	struct sock *sk;

	int rc;



	/* Because we use per cpu values to feed input from sock redirect

	 * in BPF program to do_sk_redirect_map() call we need to ensure we

	 * are not preempted. RCU read lock is not sufficient in this case

	 * with CONFIG_PREEMPT_RCU enabled so we must be explicit here.

	 */

	preempt_disable();

	rc = smap_verdict_func(psock, skb);

	switch (rc) {

	case SK_REDIRECT:

		sk = do_sk_redirect_map();

		preempt_enable();

		sk = do_sk_redirect_map(skb);

		if (likely(sk)) {

			struct smap_psock *peer = smap_psock_sk(sk);
@@ -141,8 +142,6 @@ static void smap_do_verdict(struct smap_psock *psock, struct sk_buff *skb) 
	/* Fall through and free skb otherwise */

	case SK_DROP:

	default:

		if (rc != SK_REDIRECT)

			preempt_enable();

		kfree_skb(skb);

	}

}

net/core/filter.c

+15 −14
Original line number Diff line number Diff line
@@ -1839,31 +1839,31 @@ static const struct bpf_func_proto bpf_redirect_proto = { 
	.arg2_type      = ARG_ANYTHING,

};



BPF_CALL_3(bpf_sk_redirect_map, struct bpf_map *, map, u32, key, u64, flags)

BPF_CALL_4(bpf_sk_redirect_map, struct sk_buff *, skb,

	   struct bpf_map *, map, u32, key, u64, flags)

{

	struct redirect_info *ri = this_cpu_ptr(&redirect_info);

	struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);



	if (unlikely(flags))

		return SK_ABORTED;



	ri->ifindex = key;

	ri->flags = flags;

	ri->map = map;

	tcb->bpf.key = key;

	tcb->bpf.flags = flags;

	tcb->bpf.map = map;



	return SK_REDIRECT;

}



struct sock *do_sk_redirect_map(void)

struct sock *do_sk_redirect_map(struct sk_buff *skb)

{

	struct redirect_info *ri = this_cpu_ptr(&redirect_info);

	struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);

	struct sock *sk = NULL;



	if (ri->map) {

		sk = __sock_map_lookup_elem(ri->map, ri->ifindex);

	if (tcb->bpf.map) {

		sk = __sock_map_lookup_elem(tcb->bpf.map, tcb->bpf.key);



		ri->ifindex = 0;

		ri->map = NULL;

		/* we do not clear flags for future lookup */

		tcb->bpf.key = 0;

		tcb->bpf.map = NULL;

	}



	return sk;
@@ -1873,9 +1873,10 @@ static const struct bpf_func_proto bpf_sk_redirect_map_proto = { 
	.func           = bpf_sk_redirect_map,

	.gpl_only       = false,

	.ret_type       = RET_INTEGER,

	.arg1_type      = ARG_CONST_MAP_PTR,

	.arg2_type      = ARG_ANYTHING,

	.arg1_type	= ARG_PTR_TO_CTX,

	.arg2_type      = ARG_CONST_MAP_PTR,

	.arg3_type      = ARG_ANYTHING,

	.arg4_type      = ARG_ANYTHING,

};



BPF_CALL_1(bpf_get_cgroup_classid, const struct sk_buff *, skb)

samples/sockmap/sockmap_kern.c

+1 −1
Original line number Diff line number Diff line
@@ -62,7 +62,7 @@ int bpf_prog2(struct __sk_buff *skb) 
		ret = 1;



	bpf_printk("sockmap: %d -> %d @ %d\n", lport, bpf_ntohl(rport), ret);

	return bpf_sk_redirect_map(&sock_map, ret, 0);

	return bpf_sk_redirect_map(skb, &sock_map, ret, 0);

}



SEC("sockops")