[PATCH bpf-next 2/3] bpf: infer packet range for 'if pkt ==/!= pkt_end' comparisons

[Eduard Zingerman <eddyz87@gmail.com>]

Extend try_match_pkt_pointers() to handle == and != operations.
For instruction:

      .--------------- pointer to packet with some range R
      |     .--------- pointer to packet end
      v     v
  if rA == rB goto ...

It is valid to infer that R bytes are available in packet.
This change should allow verification of BPF generated for
C code like below:

  if (data + 42 != data_end) { ... }

Suggested-by: Maciej Żenczykowski <zenczykowski@gmail.com>
Link: https://lore.kernel.org/bpf/CAHo-Oow5V2u4ZYvzuR8NmJmFDPNYp0pQDJX66rZqUjFHvhx82A@mail.gmail.com/
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 kernel/bpf/verifier.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 918e6a7912e2..b229ba0ad114 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14677,6 +14677,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 				   struct bpf_verifier_state *this_branch,
 				   struct bpf_verifier_state *other_branch)
 {
+	struct bpf_verifier_state *eq_branch;
 	int opcode = BPF_OP(insn->code);
 	int dst_regno = insn->dst_reg;
 
@@ -14713,6 +14714,13 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 		find_good_pkt_pointers(other_branch, dst_reg, dst_reg->type, opcode == BPF_JLT);
 		mark_pkt_end(this_branch, dst_regno, opcode == BPF_JLE);
 		break;
+	case BPF_JEQ:
+	case BPF_JNE:
+		/* pkt_data ==/!= pkt_end, pkt_meta ==/!= pkt_data */
+		eq_branch = opcode == BPF_JEQ ? other_branch : this_branch;
+		find_good_pkt_pointers(eq_branch, dst_reg, dst_reg->type, true);
+		mark_pkt_end(eq_branch, dst_regno, false);
+		break;
 	default:
 		return false;
 	}
-- 
2.43.0