bpf: Remove 'may_goto 0' instruction in opt_remove_nops()

Since 'may_goto 0' insns are actually no-op, let us remove them.
Otherwise, verifier will generate code like
   /* r10 - 8 stores the implicit loop count */
   r11 = *(u64 *)(r10 -8)
   if r11 == 0x0 goto pc+2
   r11 -= 1
   *(u64 *)(r10 -8) = r11

which is the pure overhead.

The following code patterns (from the previous commit) are also
handled:
   may_goto 2
   may_goto 1
   may_goto 0

With this commit, the above three 'may_goto' insns are all
eliminated.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250118192029.2124584-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Yonghong Song

kernel/bpf/verifier.c

@@ -20184,23 +20184,28 @@ static int opt_remove_dead_code(struct bpf_verifier_env *env)
 }
 
 static const struct bpf_insn NOP = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
+static const struct bpf_insn MAY_GOTO_0 = BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0, 0);
 
 static int opt_remove_nops(struct bpf_verifier_env *env)
 {
-	const struct bpf_insn ja = NOP;
 	struct bpf_insn *insn = env->prog->insnsi;
 	int insn_cnt = env->prog->len;
+	bool is_may_goto_0, is_ja;
 	int i, err;
 
 	for (i = 0; i < insn_cnt; i++) {
-		if (memcmp(&insn[i], &ja, sizeof(ja)))
+		is_may_goto_0 = !memcmp(&insn[i], &MAY_GOTO_0, sizeof(MAY_GOTO_0));
+		is_ja = !memcmp(&insn[i], &NOP, sizeof(NOP));
+
+		if (!is_may_goto_0 && !is_ja)
 			continue;
 
 		err = verifier_remove_insns(env, i, 1);
 		if (err)
 			return err;
 		insn_cnt--;
-		i--;
+		/* Go back one insn to catch may_goto +1; may_goto +0 sequence */
+		i -= (is_may_goto_0 && i > 0) ? 2 : 1;
 	}
 
 	return 0;