netdev - [RFCv2 06/16] bpf: verifier: recognize rN ^ rN as load of 0

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Date:   Fri, 26 Aug 2016 19:06:05 +0100
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     netdev@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net,
        dinan.gunawardena@...ronome.com, jiri@...nulli.us,
        john.fastabend@...il.com, kubakici@...pl,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Subject: [RFCv2 06/16] bpf: verifier: recognize rN ^ rN as load of 0

Teach the verifier to recognize that xoring a register with
itself makes it a constant (0).

Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
---
 kernel/bpf/verifier.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index db68a0e5db1e..0f4494c194f9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1550,6 +1550,12 @@ static int check_alu_op(struct verifier_env *env, struct bpf_insn *insn)
 		verbose("invalid BPF_ALU opcode %x\n", opcode);
 		return -EINVAL;
 
+	} else if (opcode == BPF_XOR && BPF_SRC(insn->code) == BPF_X &&
+		   insn->src_reg == insn->dst_reg) {
+
+		regs[insn->dst_reg].type = CONST_IMM;
+		regs[insn->dst_reg].imm = 0;
+
 	} else {	/* all other ALU ops: and, sub, xor, add, ... */
 
 		if (BPF_SRC(insn->code) == BPF_X) {
-- 
1.9.1