bpf: Simplify insn rewrite on BPF_WRITE __sk_buff->tstamp

BPF_JMP32_IMM(BPF_JSET) is used to save a BPF_ALU32_IMM(BPF_AND).

The skb->tc_at_ingress and skb->mono_delivery_time are at the same
offset, so only one BPF_LDX_MEM(BPF_B) is needed.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220309090502.3711982-1-kafai@fb.com

diff --git a/net/core/filter.c b/net/core/filter.c
index 2c83d1f..f914e4b 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8990,25 +8990,27 @@ static struct bpf_insn *bpf_convert_tstamp_write(const struct bpf_prog *prog,
        __u8 skb_reg = si->dst_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, write skb->tstamp as is if delivery_time_access is true.
+        * Otherwise, writing at ingress will have to clear the
+        * mono_delivery_time bit also.
+        */
        if (!prog->delivery_time_access) {
                __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, 3);
-               /* Writing __sk_buff->tstamp at ingress as the (rcv) timestamp.
-                * Clear 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_STX_MEM(BPF_B, skb_reg, tmp_reg,
-                                     PKT_VLAN_PRESENT_OFFSET);
+               /* Writing __sk_buff->tstamp as ingress, goto <clear> */
+               *insn++ = BPF_JMP32_IMM(BPF_JSET, tmp_reg, TC_AT_INGRESS_MASK, 1);
+               /* goto <store> */
+               *insn++ = BPF_JMP_A(2);
+               /* <clear>: mono_delivery_time */
+               *insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg, ~SKB_MONO_DELIVERY_TIME_MASK);
+               *insn++ = BPF_STX_MEM(BPF_B, skb_reg, tmp_reg, PKT_VLAN_PRESENT_OFFSET);
        }
 #endif
 
-       /* skb->tstamp = tstamp */
+       /* <store>: skb->tstamp = tstamp */
        *insn++ = BPF_STX_MEM(BPF_DW, skb_reg, value_reg,
                              offsetof(struct sk_buff, tstamp));
        return insn;