[PATCH bpf-next v2 2/3] bpf: Remove 'may_goto 0' instruction in opt_remove_nops()

[Yonghong Song <yonghong.song@linux.dev>]

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>
---
 kernel/bpf/verifier.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 963dfda81c06..784547aa40a8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -20187,20 +20187,25 @@ static const struct bpf_insn NOP = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
 
 static int opt_remove_nops(struct bpf_verifier_env *env)
 {
+	const struct bpf_insn may_goto_0 = BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0, 0);
 	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], &ja, sizeof(ja));
+
+		if (!is_may_goto_0 && !is_ja)
 			continue;
 
 		err = verifier_remove_insns(env, i, 1);
 		if (err)
 			return err;
 		insn_cnt--;
-		i--;
+		i -= (is_may_goto_0 && i > 0) ? 2 : 1;
 	}
 
 	return 0;
-- 
2.43.5