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@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220309090456.3711530-1-kafai@fb.com

diff --git a/net/core/filter.c b/net/core/filter.c
index 738a294..2c83d1f 100644 (file)
--- 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);
        }