[PATCH bpf-next 2/3] bpf: Remove 'may_goto 0' instruction

[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 | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index edf3cc42a220..72b474bfba2d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -20133,6 +20133,40 @@ static int opt_remove_nops(struct bpf_verifier_env *env)
 	return 0;
 }
 
+static int opt_remove_useless_may_gotos(struct bpf_verifier_env *env)
+{
+	struct bpf_insn *insn = env->prog->insnsi;
+	int i, j, err, last_may_goto, removed_cnt;
+	int insn_cnt = env->prog->len;
+
+	for (i = 0; i < insn_cnt; i++) {
+		if (!is_may_goto_insn(&insn[i]))
+			continue;
+
+		for (j = i + 1; j < insn_cnt; j++) {
+			if (!is_may_goto_insn(&insn[j]))
+				break;
+		}
+
+		last_may_goto = --j;
+		removed_cnt = 0;
+		while (j >= i) {
+			if (insn[j].off == 0) {
+				err = verifier_remove_insns(env, j, 1);
+				if (err)
+					return err;
+				removed_cnt++;
+			}
+			j--;
+		}
+
+		insn_cnt -= removed_cnt;
+		i = last_may_goto - removed_cnt;
+	}
+
+	return 0;
+}
+
 static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
 					 const union bpf_attr *attr)
 {
@@ -23089,6 +23123,8 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr, __u3
 			ret = opt_remove_dead_code(env);
 		if (ret == 0)
 			ret = opt_remove_nops(env);
+		if (ret == 0)
+			ret = opt_remove_useless_may_gotos(env);
 	} else {
 		if (ret == 0)
 			sanitize_dead_code(env);
-- 
2.43.5