netdev - [RFC bpf-next v3 04/12] bpf: verifier: remove unconditional branches by 0

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Date:   Fri, 28 Dec 2018 19:09:15 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     alexei.starovoitov@...il.com, daniel@...earbox.net
Cc:     yhs@...com, netdev@...r.kernel.org, oss-drivers@...ronome.com,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Subject: [RFC bpf-next v3 04/12] bpf: verifier: remove unconditional branches by 0

Unconditional branches by 0 instructions are basically noops
but they can result from earlier optimizations, e.g. a conditional
jumps which would never be taken or a conditional jump around
dead code.

Remove those branches.

v0.2:
 - s/opt_remove_dead_branches/opt_remove_nops/ (Jiong).

Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
Reviewed-by: Jiong Wang <jiong.wang@...ronome.com>
---
 kernel/bpf/verifier.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a3880c6dac97..598ef51e299d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6482,6 +6482,27 @@ static int opt_remove_dead_code(struct bpf_verifier_env *env)
 	return 0;
 }
 
+static int opt_remove_nops(struct bpf_verifier_env *env)
+{
+	const struct bpf_insn ja = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
+	struct bpf_insn *insn = env->prog->insnsi;
+	int insn_cnt = env->prog->len;
+	int i, err;
+
+	for (i = 0; i < insn_cnt; i++) {
+		if (memcmp(&insn[i], &ja, sizeof(ja)))
+			continue;
+
+		err = verifier_remove_insns(env, i, 1);
+		if (err)
+			return err;
+		insn_cnt--;
+		i--;
+	}
+
+	return 0;
+}
+
 /* convert load instructions that access fields of a context type into a
  * sequence of instructions that access fields of the underlying structure:
  *     struct __sk_buff    -> struct sk_buff
@@ -7223,6 +7244,8 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
 			opt_hard_wire_dead_code_branches(env);
 		if (ret == 0)
 			ret = opt_remove_dead_code(env);
+		if (ret == 0)
+			ret = opt_remove_nops(env);
 	} else {
 		if (ret == 0)
 			sanitize_dead_code(env);
-- 
2.19.2