bpf: verifier: centralize const dynptr check in unmark_stack_slots_dynptr()

Move the const dynptr check into unmark_stack_slots_dynptr() so callers
don’t have to duplicate it. This puts the validation next to the code
that manipulates dynptr stack slots and allows upcoming changes to reuse
it directly.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>

Author: mykyta5

kernel/bpf/verifier.c

@@ -828,6 +828,15 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re
 	struct bpf_func_state *state = func(env, reg);
 	int spi, ref_obj_id, i;
 
+	/*
+	 * This can only be set for PTR_TO_STACK, as CONST_PTR_TO_DYNPTR cannot
+	 * be released by any dynptr helper. Hence, unmark_stack_slots_dynptr
+	 * is safe to do directly.
+	 */
+	if (reg->type == CONST_PTR_TO_DYNPTR) {
+		verifier_bug(env, "CONST_PTR_TO_DYNPTR cannot be released");
+		return -EFAULT;
+	}
 	spi = dynptr_get_spi(env, reg);
 	if (spi < 0)
 		return spi;
@@ -11514,15 +11523,7 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 
 	if (meta.release_regno) {
 		err = -EINVAL;
-		/* This can only be set for PTR_TO_STACK, as CONST_PTR_TO_DYNPTR cannot
-		 * be released by any dynptr helper. Hence, unmark_stack_slots_dynptr
-		 * is safe to do directly.
-		 */
 		if (arg_type_is_dynptr(fn->arg_type[meta.release_regno - BPF_REG_1])) {
-			if (regs[meta.release_regno].type == CONST_PTR_TO_DYNPTR) {
-				verifier_bug(env, "CONST_PTR_TO_DYNPTR cannot be released");
-				return -EFAULT;
-			}
 			err = unmark_stack_slots_dynptr(env, &regs[meta.release_regno]);
 		} else if (func_id == BPF_FUNC_kptr_xchg && meta.ref_obj_id) {
 			u32 ref_obj_id = meta.ref_obj_id;