[PATCH bpf-next v5 04/12] bpf: verifier: remove unconditional branches by 0

[Jakub Kicinski]

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.kicin...@netronome.com>
Reviewed-by: Jiong Wang <jiong.w...@netronome.com>
Acked-by: Yonghong Song <y...@fb.com>
---
 kernel/bpf/verifier.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 099b2541f87f..f39bca188a5c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6660,6 +6660,27 @@ 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
@@ -7452,6 +7473,8 @@ 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