[PATCH RFC bpf-next v6 2/6] bpf: Add support for KF_ARG_PTR_TO_TIMER

[bentiss]

From: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>

Introduce support for KF_ARG_PTR_TO_TIMER. The kfuncs will use bpf_timer
as argument and that will be recognized as timer argument by verifier.
bpf_timer_kern casting can happen inside kfunc, but using bpf_timer in
argument makes life easier for users who work with non-kern type in BPF
progs.

Fix up process_timer_func's meta argument usage (ignore if NULL) so that
we can share the same checks for helpers and kfuncs. meta argument is
only needed to ensure bpf_timer_init's timer and map arguments are
coming from the same map (map_uid logic is necessary for correct
inner-map handling).

No such concerns will be necessary for kfuncs as timer initialization
happens using helpers, hence pass NULL to process_timer_func from kfunc
argument handling code to ignore it.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
Signed-off-by: Benjamin Tissoires <bentiss@xxxxxxxxxx>

---

changes in v6:
- used Kumar's version of the patch
- reverted `+BTF_ID(struct, bpf_timer_kern)`

changes in v5:
- also check for the reg offset

changes in v4:
- enforce KF_ARG_PTR_TO_TIMER to be of type PTR_TO_MAP_VALUE

new in v3 (split from v2 02/10)
---
 kernel/bpf/verifier.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ca6cacf7b42f..ccfe9057d8dc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7568,12 +7568,16 @@ static int process_timer_func(struct bpf_verifier_env *env, int regno,
 			val + reg->off, map->record->timer_off);
 		return -EINVAL;
 	}
+	/* meta is only needed for bpf_timer_init to match timer and map */
+	if (!meta)
+		goto out;
 	if (meta->map_ptr) {
 		verbose(env, "verifier bug. Two map pointers in a timer helper\n");
 		return -EFAULT;
 	}
 	meta->map_uid = reg->map_uid;
 	meta->map_ptr = map;
+out:
 	return 0;
 }
 
@@ -10826,6 +10830,7 @@ enum {
 	KF_ARG_LIST_NODE_ID,
 	KF_ARG_RB_ROOT_ID,
 	KF_ARG_RB_NODE_ID,
+	KF_ARG_TIMER_ID,
 };
 
 BTF_ID_LIST(kf_arg_btf_ids)
@@ -10834,6 +10839,7 @@ BTF_ID(struct, bpf_list_head)
 BTF_ID(struct, bpf_list_node)
 BTF_ID(struct, bpf_rb_root)
 BTF_ID(struct, bpf_rb_node)
+BTF_ID(struct, bpf_timer_kern)
 
 static bool __is_kfunc_ptr_arg_type(const struct btf *btf,
 				    const struct btf_param *arg, int type)
@@ -10877,6 +10883,11 @@ static bool is_kfunc_arg_rbtree_node(const struct btf *btf, const struct btf_par
 	return __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_RB_NODE_ID);
 }
 
+static bool is_kfunc_arg_timer(const struct btf *btf, const struct btf_param *arg)
+{
+	return __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_TIMER_ID);
+}
+
 static bool is_kfunc_arg_callback(struct bpf_verifier_env *env, const struct btf *btf,
 				  const struct btf_param *arg)
 {
@@ -10946,6 +10957,7 @@ enum kfunc_ptr_arg_type {
 	KF_ARG_PTR_TO_NULL,
 	KF_ARG_PTR_TO_CONST_STR,
 	KF_ARG_PTR_TO_MAP,
+	KF_ARG_PTR_TO_TIMER,
 };
 
 enum special_kfunc_type {
@@ -11102,6 +11114,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	if (is_kfunc_arg_map(meta->btf, &args[argno]))
 		return KF_ARG_PTR_TO_MAP;
 
+	if (is_kfunc_arg_timer(meta->btf, &args[argno]))
+		return KF_ARG_PTR_TO_TIMER;
+
 	if ((base_type(reg->type) == PTR_TO_BTF_ID || reg2btf_ids[base_type(reg->type)])) {
 		if (!btf_type_is_struct(ref_t)) {
 			verbose(env, "kernel function %s args#%d pointer type %s %s is not supported\n",
@@ -11735,6 +11750,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		case KF_ARG_PTR_TO_CALLBACK:
 		case KF_ARG_PTR_TO_REFCOUNTED_KPTR:
 		case KF_ARG_PTR_TO_CONST_STR:
+		case KF_ARG_PTR_TO_TIMER:
 			/* Trusted by default */
 			break;
 		default:
@@ -12021,6 +12037,15 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			if (ret)
 				return ret;
 			break;
+		case KF_ARG_PTR_TO_TIMER:
+			if (reg->type != PTR_TO_MAP_VALUE) {
+				verbose(env, "arg#%d doesn't point to a map value\n", i);
+				return -EINVAL;
+			}
+			ret = process_timer_func(env, regno, NULL);
+			if (ret < 0)
+				return ret;
+			break;
 		}
 	}
 

-- 
2.44.0