bpf: Compute data_end dynamically with JIT code

Currently, we compute ->data_end with a compile-time constant
offset of skb. But as Jakub pointed out, we can actually compute
it in eBPF JIT code at run-time, so that we can competely get
rid of ->data_end. This is similar to skb_shinfo(skb) computation
in bpf_convert_shinfo_access().

Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210223184934.6054-4-xiyou.wangcong@gmail.com

include/net/tcp.h

@@ -886,18 +886,12 @@ struct tcp_skb_cb {
 		struct {
 			__u32 flags;
 			struct sock *sk_redir;
-			void *data_end;
 		} bpf;
 	};
 };
 
 #define TCP_SKB_CB(__skb)	((struct tcp_skb_cb *)&((__skb)->cb[0]))
 
-static inline void bpf_compute_data_end_sk_skb(struct sk_buff *skb)
-{
-	TCP_SKB_CB(skb)->bpf.data_end = skb->data + skb_headlen(skb);
-}
-
 static inline bool tcp_skb_bpf_ingress(const struct sk_buff *skb)
 {
 	return TCP_SKB_CB(skb)->bpf.flags & BPF_F_INGRESS;

net/core/filter.c

@@ -1863,10 +1863,7 @@ static const struct bpf_func_proto bpf_sk_fullsock_proto = {
 static inline int sk_skb_try_make_writable(struct sk_buff *skb,
 					   unsigned int write_len)
 {
-	int err = __bpf_try_make_writable(skb, write_len);
-
-	bpf_compute_data_end_sk_skb(skb);
-	return err;
+	return __bpf_try_make_writable(skb, write_len);
 }
 
 BPF_CALL_2(sk_skb_pull_data, struct sk_buff *, skb, u32, len)
@@ -3577,7 +3574,6 @@ BPF_CALL_4(sk_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 			return -ENOMEM;
 		__skb_pull(skb, len_diff_abs);
 	}
-	bpf_compute_data_end_sk_skb(skb);
 	if (tls_sw_has_ctx_rx(skb->sk)) {
 		struct strp_msg *rxm = strp_msg(skb);
 
@@ -3742,10 +3738,7 @@ static const struct bpf_func_proto bpf_skb_change_tail_proto = {
 BPF_CALL_3(sk_skb_change_tail, struct sk_buff *, skb, u32, new_len,
 	   u64, flags)
 {
-	int ret = __bpf_skb_change_tail(skb, new_len, flags);
-
-	bpf_compute_data_end_sk_skb(skb);
-	return ret;
+	return __bpf_skb_change_tail(skb, new_len, flags);
 }
 
 static const struct bpf_func_proto sk_skb_change_tail_proto = {
@@ -3808,10 +3801,7 @@ static const struct bpf_func_proto bpf_skb_change_head_proto = {
 BPF_CALL_3(sk_skb_change_head, struct sk_buff *, skb, u32, head_room,
 	   u64, flags)
 {
-	int ret = __bpf_skb_change_head(skb, head_room, flags);
-
-	bpf_compute_data_end_sk_skb(skb);
-	return ret;
+	return __bpf_skb_change_head(skb, head_room, flags);
 }
 
 static const struct bpf_func_proto sk_skb_change_head_proto = {
@@ -9655,22 +9645,40 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
 	return insn - insn_buf;
 }
 
+/* data_end = skb->data + skb_headlen() */
+static struct bpf_insn *bpf_convert_data_end_access(const struct bpf_insn *si,
+						    struct bpf_insn *insn)
+{
+	/* si->dst_reg = skb->data */
+	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data),
+			      si->dst_reg, si->src_reg,
+			      offsetof(struct sk_buff, data));
+	/* AX = skb->len */
+	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, len),
+			      BPF_REG_AX, si->src_reg,
+			      offsetof(struct sk_buff, len));
+	/* si->dst_reg = skb->data + skb->len */
+	*insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
+	/* AX = skb->data_len */
+	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data_len),
+			      BPF_REG_AX, si->src_reg,
+			      offsetof(struct sk_buff, data_len));
+	/* si->dst_reg = skb->data + skb->len - skb->data_len */
+	*insn++ = BPF_ALU64_REG(BPF_SUB, si->dst_reg, BPF_REG_AX);
+
+	return insn;
+}
+
 static u32 sk_skb_convert_ctx_access(enum bpf_access_type type,
 				     const struct bpf_insn *si,
 				     struct bpf_insn *insn_buf,
 				     struct bpf_prog *prog, u32 *target_size)
 {
 	struct bpf_insn *insn = insn_buf;
-	int off;
 
 	switch (si->off) {
 	case offsetof(struct __sk_buff, data_end):
-		off  = si->off;
-		off -= offsetof(struct __sk_buff, data_end);
-		off += offsetof(struct sk_buff, cb);
-		off += offsetof(struct tcp_skb_cb, bpf.data_end);
-		*insn++ = BPF_LDX_MEM(BPF_SIZEOF(void *), si->dst_reg,
-				      si->src_reg, off);
+		insn = bpf_convert_data_end_access(si, insn);
 		break;
 	default:
 		return bpf_convert_ctx_access(type, si, insn_buf, prog,

net/core/skmsg.c

@@ -746,7 +746,6 @@ EXPORT_SYMBOL_GPL(sk_psock_msg_verdict);
 static int sk_psock_bpf_run(struct sk_psock *psock, struct bpf_prog *prog,
 			    struct sk_buff *skb)
 {
-	bpf_compute_data_end_sk_skb(skb);
 	return bpf_prog_run_pin_on_cpu(prog, skb);
 }