bpf: Assign reg->id when getting referenced kptr from ctx

Assign reg->id when getting referenced kptr from read program context
to be consistent with R0 of KF_ACQUIRE kfunc. skb dynptr will track the
referenced skb in qdisc programs using a new field reg->parent_id in
a later patch.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260529014936.2811085-4-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: ameryhung

kernel/bpf/verifier.c

@@ -6208,8 +6208,6 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
 			} else {
 				mark_reg_known_zero(env, regs,
 						    value_regno);
-				if (type_may_be_null(info.reg_type))
-					regs[value_regno].id = ++env->id_gen;
 				/* A load of ctx field could have different
 				 * actual load size with the one encoded in the
 				 * insn. When the dst is PTR, it is for sure not
@@ -6219,8 +6217,11 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
 				if (base_type(info.reg_type) == PTR_TO_BTF_ID) {
 					regs[value_regno].btf = info.btf;
 					regs[value_regno].btf_id = info.btf_id;
+					regs[value_regno].id = info.ref_obj_id;
 					regs[value_regno].ref_obj_id = info.ref_obj_id;
 				}
+				if (type_may_be_null(info.reg_type) && !regs[value_regno].id)
+					regs[value_regno].id = ++env->id_gen;
 			}
 			regs[value_regno].type = info.reg_type;
 		}