bpf: signal error if old liveness is more conservative than new

Unlike the new algorithm, register chain based liveness tracking is
fully path sensitive, and thus should be strictly more accurate.
Validate the new algorithm by signaling an error whenever it considers
a stack slot dead while the old algorithm considers it alive.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-8-c3cd27bacc60@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

include/linux/bpf_verifier.h

@@ -852,6 +852,7 @@ struct bpf_verifier_env {
 	/* array of pointers to bpf_scc_info indexed by SCC id */
 	struct bpf_scc_info **scc_info;
 	u32 scc_cnt;
+	bool internal_error;
 };
 
 static inline struct bpf_func_info_aux *subprog_aux(struct bpf_verifier_env *env, int subprog)

kernel/bpf/verifier.c

@@ -18576,6 +18576,11 @@ static void clean_func_state(struct bpf_verifier_env *env,
 
 	for (i = 0; i < st->allocated_stack / BPF_REG_SIZE; i++) {
 		if (!bpf_stack_slot_alive(env, st->frameno, i)) {
+			if (st->stack[i].spilled_ptr.live & REG_LIVE_READ) {
+				verifier_bug(env, "incorrect live marks #1 for insn %d frameno %d spi %d\n",
+					     env->insn_idx, st->frameno, i);
+				env->internal_error = true;
+			}
 			__mark_reg_not_init(env, &st->stack[i].spilled_ptr);
 			for (j = 0; j < BPF_REG_SIZE; j++)
 				st->stack[i].slot_type[j] = STACK_INVALID;
@@ -19546,6 +19551,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 		loop = incomplete_read_marks(env, &sl->state);
 		if (states_equal(env, &sl->state, cur, loop ? RANGE_WITHIN : NOT_EXACT)) {
 hit:
+			if (env->internal_error)
+				return -EFAULT;
 			sl->hit_cnt++;
 			/* reached equivalent register/stack state,
 			 * prune the search.
@@ -19660,6 +19667,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 			return 1;
 		}
 miss:
+		if (env->internal_error)
+			return -EFAULT;
 		/* when new state is not going to be added do not increase miss count.
 		 * Otherwise several loop iterations will remove the state
 		 * recorded earlier. The goal of these heuristics is to have