bpf: frame_insn_idx() utility function

A function to return IP for a given frame in a call stack of a state.
Will be used by a next patch.

The `state->insn_idx = env->insn_idx;` assignment in the do_check()
allows to use frame_insn_idx with env->cur_state.
At the moment bpf_verifier_state->insn_idx is set when new cached
state is added in is_state_visited() and accessed only in the contexts
when the state is already in the cache. Hence this assignment does not
change verifier behaviour.

Signed-off-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: eddyz87

kernel/bpf/verifier.c

@@ -1964,6 +1964,14 @@ static void update_loop_entry(struct bpf_verifier_env *env,
 	}
 }
 
+/* Return IP for a given frame in a call stack */
+static u32 frame_insn_idx(struct bpf_verifier_state *st, u32 frame)
+{
+	return frame == st->curframe
+	       ? st->insn_idx
+	       : st->frame[frame + 1]->callsite;
+}
+
 static void update_branch_counts(struct bpf_verifier_env *env, struct bpf_verifier_state *st)
 {
 	struct bpf_verifier_state_list *sl = NULL, *parent_sl;
@@ -18790,9 +18798,7 @@ static bool states_equal(struct bpf_verifier_env *env,
 	 * and all frame states need to be equivalent
 	 */
 	for (i = 0; i <= old->curframe; i++) {
-		insn_idx = i == old->curframe
-			   ? env->insn_idx
-			   : old->frame[i + 1]->callsite;
+		insn_idx = frame_insn_idx(old, i);
 		if (old->frame[i]->callsite != cur->frame[i]->callsite)
 			return false;
 		if (!func_states_equal(env, old->frame[i], cur->frame[i], insn_idx, exact))
@@ -19687,6 +19693,7 @@ static int do_check(struct bpf_verifier_env *env)
 		}
 
 		state->last_insn_idx = env->prev_insn_idx;
+		state->insn_idx = env->insn_idx;
 
 		if (is_prune_point(env, env->insn_idx)) {
 			err = is_state_visited(env, env->insn_idx);