bpf: make programs see skb->data == L2 for ingress and egress

eBPF programs attached to ingress and egress qdiscs see inconsistent skb->data.
For ingress L2 header is already pulled, whereas for egress it's present.
This is known to program writers which are currently forced to use
BPF_LL_OFF workaround.
Since programs don't change skb internal pointers it is safe to do
pull/push right around invocation of the program and earlier taps and
later pt->func() will not be affected.
Multiple taps via packet_rcv(), tpacket_rcv() are doing the same trick
around run_filter/BPF_PROG_RUN even if skb_shared.

This fix finally allows programs to use optimized LD_ABS/IND instructions
without BPF_LL_OFF for higher performance.
tc ingress + cls_bpf + samples/bpf/tcbpf1_kern.o
       w/o JIT   w/JIT
before  20.5     23.6 Mpps
after   21.8     26.6 Mpps

Old programs with BPF_LL_OFF will still work as-is.

We can now undo most of the earlier workaround commit:
a166151c ("bpf: fix bpf helpers to use skb->mac_header relative offsets")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -1238,21 +1238,6 @@ int sk_attach_bpf(u32 ufd, struct sock *sk)
 	return 0;
 }
 
-/**
- *	bpf_skb_clone_not_writable - is the header of a clone not writable
- *	@skb: buffer to check
- *	@len: length up to which to write, can be negative
- *
- *	Returns true if modifying the header part of the cloned buffer
- *	does require the data to be copied. I.e. this version works with
- *	negative lengths needed for eBPF case!
- */
-static bool bpf_skb_clone_unwritable(const struct sk_buff *skb, int len)
-{
-	return skb_header_cloned(skb) ||
-	       (int) skb_headroom(skb) + len > skb->hdr_len;
-}
-
 #define BPF_RECOMPUTE_CSUM(flags)	((flags) & 1)
 
 static u64 bpf_skb_store_bytes(u64 r1, u64 r2, u64 r3, u64 r4, u64 flags)
@@ -1275,9 +1260,8 @@ static u64 bpf_skb_store_bytes(u64 r1, u64 r2, u64 r3, u64 r4, u64 flags)
 	if (unlikely((u32) offset > 0xffff || len > sizeof(buf)))
 		return -EFAULT;
 
-	offset -= skb->data - skb_mac_header(skb);
 	if (unlikely(skb_cloned(skb) &&
-		     bpf_skb_clone_unwritable(skb, offset + len)))
+		     !skb_clone_writable(skb, offset + len)))
 		return -EFAULT;
 
 	ptr = skb_header_pointer(skb, offset, len, buf);
@@ -1321,9 +1305,8 @@ static u64 bpf_l3_csum_replace(u64 r1, u64 r2, u64 from, u64 to, u64 flags)
 	if (unlikely((u32) offset > 0xffff))
 		return -EFAULT;
 
-	offset -= skb->data - skb_mac_header(skb);
 	if (unlikely(skb_cloned(skb) &&
-		     bpf_skb_clone_unwritable(skb, offset + sizeof(sum))))
+		     !skb_clone_writable(skb, offset + sizeof(sum))))
 		return -EFAULT;
 
 	ptr = skb_header_pointer(skb, offset, sizeof(sum), &sum);
@@ -1369,9 +1352,8 @@ static u64 bpf_l4_csum_replace(u64 r1, u64 r2, u64 from, u64 to, u64 flags)
 	if (unlikely((u32) offset > 0xffff))
 		return -EFAULT;
 
-	offset -= skb->data - skb_mac_header(skb);
 	if (unlikely(skb_cloned(skb) &&
-		     bpf_skb_clone_unwritable(skb, offset + sizeof(sum))))
+		     !skb_clone_writable(skb, offset + sizeof(sum))))
 		return -EFAULT;
 
 	ptr = skb_header_pointer(skb, offset, sizeof(sum), &sum);
@@ -1425,8 +1407,6 @@ static u64 bpf_clone_redirect(u64 r1, u64 ifindex, u64 flags, u64 r4, u64 r5)
 	if (unlikely(!skb2))
 		return -ENOMEM;
 
-	skb_push(skb2, skb2->data - skb_mac_header(skb2));
-
 	if (BPF_IS_REDIRECT_INGRESS(flags))
 		return dev_forward_skb(dev, skb2);
 

net/sched/act_bpf.c

@@ -37,6 +37,7 @@ static int tcf_bpf(struct sk_buff *skb, const struct tc_action *act,
 {
 	struct tcf_bpf *prog = act->priv;
 	int action, filter_res;
+	bool at_ingress = G_TC_AT(skb->tc_verd) & AT_INGRESS;
 
 	if (unlikely(!skb_mac_header_was_set(skb)))
 		return TC_ACT_UNSPEC;
@@ -48,7 +49,13 @@ static int tcf_bpf(struct sk_buff *skb, const struct tc_action *act,
 
 	/* Needed here for accessing maps. */
 	rcu_read_lock();
+	if (at_ingress) {
+		__skb_push(skb, skb->mac_len);
 		filter_res = BPF_PROG_RUN(prog->filter, skb);
+		__skb_pull(skb, skb->mac_len);
+	} else {
+		filter_res = BPF_PROG_RUN(prog->filter, skb);
+	}
 	rcu_read_unlock();
 
 	/* A BPF program may overwrite the default action opcode.

net/sched/cls_bpf.c

@@ -64,6 +64,11 @@ static int cls_bpf_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 {
 	struct cls_bpf_head *head = rcu_dereference_bh(tp->root);
 	struct cls_bpf_prog *prog;
+#ifdef CONFIG_NET_CLS_ACT
+	bool at_ingress = G_TC_AT(skb->tc_verd) & AT_INGRESS;
+#else
+	bool at_ingress = false;
+#endif
 	int ret = -1;
 
 	if (unlikely(!skb_mac_header_was_set(skb)))
@@ -72,7 +77,16 @@ static int cls_bpf_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 	/* Needed here for accessing maps. */
 	rcu_read_lock();
 	list_for_each_entry_rcu(prog, &head->plist, link) {
-		int filter_res = BPF_PROG_RUN(prog->filter, skb);
+		int filter_res;
+
+		if (at_ingress) {
+			/* It is safe to push/pull even if skb_shared() */
+			__skb_push(skb, skb->mac_len);
+			filter_res = BPF_PROG_RUN(prog->filter, skb);
+			__skb_pull(skb, skb->mac_len);
+		} else {
+			filter_res = BPF_PROG_RUN(prog->filter, skb);
+		}
 
 		if (filter_res == 0)
 			continue;

samples/bpf/tcbpf1_kern.c

@@ -21,7 +21,7 @@ static inline void set_dst_mac(struct __sk_buff *skb, char *mac)
 
 static inline void set_ip_tos(struct __sk_buff *skb, __u8 new_tos)
 {
-	__u8 old_tos = load_byte(skb, BPF_LL_OFF + TOS_OFF);
+	__u8 old_tos = load_byte(skb, TOS_OFF);
 
 	bpf_l3_csum_replace(skb, IP_CSUM_OFF, htons(old_tos), htons(new_tos), 2);
 	bpf_skb_store_bytes(skb, TOS_OFF, &new_tos, sizeof(new_tos), 0);
@@ -34,7 +34,7 @@ static inline void set_ip_tos(struct __sk_buff *skb, __u8 new_tos)
 
 static inline void set_tcp_ip_src(struct __sk_buff *skb, __u32 new_ip)
 {
-	__u32 old_ip = _htonl(load_word(skb, BPF_LL_OFF + IP_SRC_OFF));
+	__u32 old_ip = _htonl(load_word(skb, IP_SRC_OFF));
 
 	bpf_l4_csum_replace(skb, TCP_CSUM_OFF, old_ip, new_ip, IS_PSEUDO | sizeof(new_ip));
 	bpf_l3_csum_replace(skb, IP_CSUM_OFF, old_ip, new_ip, sizeof(new_ip));
@@ -44,7 +44,7 @@ static inline void set_tcp_ip_src(struct __sk_buff *skb, __u32 new_ip)
 #define TCP_DPORT_OFF (ETH_HLEN + sizeof(struct iphdr) + offsetof(struct tcphdr, dest))
 static inline void set_tcp_dest_port(struct __sk_buff *skb, __u16 new_port)
 {
-	__u16 old_port = htons(load_half(skb, BPF_LL_OFF + TCP_DPORT_OFF));
+	__u16 old_port = htons(load_half(skb, TCP_DPORT_OFF));
 
 	bpf_l4_csum_replace(skb, TCP_CSUM_OFF, old_port, new_port, sizeof(new_port));
 	bpf_skb_store_bytes(skb, TCP_DPORT_OFF, &new_port, sizeof(new_port), 0);
@@ -53,7 +53,7 @@ static inline void set_tcp_dest_port(struct __sk_buff *skb, __u16 new_port)
 SEC("classifier")
 int bpf_prog1(struct __sk_buff *skb)
 {
-	__u8 proto = load_byte(skb, BPF_LL_OFF + ETH_HLEN + offsetof(struct iphdr, protocol));
+	__u8 proto = load_byte(skb, ETH_HLEN + offsetof(struct iphdr, protocol));
 	long *value;
 
 	if (proto == IPPROTO_TCP) {