netdev - [PATCH bpf-next 2/5] bpf: Simplify insn rewrite on BPF_READ __sk

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Date:   Wed, 9 Mar 2022 01:04:56 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     <bpf@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        <netdev@...r.kernel.org>
Subject: [PATCH bpf-next 2/5] bpf: Simplify insn rewrite on BPF_READ __sk_buff->tstamp

The skb->tc_at_ingress and skb->mono_delivery_time are at the same
byte offset.  Thus, only one BPF_LDX_MEM(BPF_B) is needed
and both bits can be tested together.

/* BPF_READ: a = __sk_buff->tstamp */
if (skb->tc_at_ingress && skb->mono_delivery_time)
	a = 0;
else
	a = skb->tstamp;

Signed-off-by: Martin KaFai Lau <kafai@...com>
---
 net/core/filter.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 738a294a3c82..2c83d1f38704 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8956,21 +8956,22 @@ static struct bpf_insn *bpf_convert_tstamp_read(const struct bpf_prog *prog,
 	__u8 skb_reg = si->src_reg;
 
 #ifdef CONFIG_NET_CLS_ACT
+	/* If the delivery_time_type is read,
+	 * the bpf prog is aware the tstamp could have delivery time.
+	 * Thus, read skb->tstamp as is if delivery_time_access is true.
+	 */
 	if (!prog->delivery_time_access) {
+		/* AX is needed because src_reg and dst_reg could be the same */
 		__u8 tmp_reg = BPF_REG_AX;
 
 		*insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg, PKT_VLAN_PRESENT_OFFSET);
-		*insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg, TC_AT_INGRESS_MASK);
-		*insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg, 0, 5);
-		/* @ingress, read __sk_buff->tstamp as the (rcv) timestamp,
-		 * so check the skb->mono_delivery_time.
-		 */
-		*insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg,
-				      PKT_VLAN_PRESENT_OFFSET);
 		*insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg,
-					SKB_MONO_DELIVERY_TIME_MASK);
-		*insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg, 0, 2);
-		/* skb->mono_delivery_time is set, read 0 as the (rcv) timestamp. */
+					TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK);
+		*insn++ = BPF_JMP32_IMM(BPF_JNE, tmp_reg,
+					TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK, 2);
+		/* skb->tc_at_ingress && skb->mono_delivery_time,
+		 * read 0 as the (rcv) timestamp.
+		 */
 		*insn++ = BPF_MOV64_IMM(value_reg, 0);
 		*insn++ = BPF_JMP_A(1);
 	}
-- 
2.30.2