bpf: Add bpf_refcount_acquire kfunc

Currently, BPF programs can interact with the lifetime of refcounted
local kptrs in the following ways:

  bpf_obj_new  - Initialize refcount to 1 as part of new object creation
  bpf_obj_drop - Decrement refcount and free object if it's 0
  collection add - Pass ownership to the collection. No change to
                   refcount but collection is responsible for
		   bpf_obj_dropping it

In order to be able to add a refcounted local kptr to multiple
collections we need to be able to increment the refcount and acquire a
new owning reference. This patch adds a kfunc, bpf_refcount_acquire,
implementing such an operation.

bpf_refcount_acquire takes a refcounted local kptr and returns a new
owning reference to the same underlying memory as the input. The input
can be either owning or non-owning. To reinforce why this is safe,
consider the following code snippets:

  struct node *n = bpf_obj_new(typeof(*n)); // A
  struct node *m = bpf_refcount_acquire(n); // B

In the above snippet, n will be alive with refcount=1 after (A), and
since nothing changes that state before (B), it's obviously safe. If
n is instead added to some rbtree, we can still safely refcount_acquire
it:

  struct node *n = bpf_obj_new(typeof(*n));
  struct node *m;

  bpf_spin_lock(&glock);
  bpf_rbtree_add(&groot, &n->node, less);   // A
  m = bpf_refcount_acquire(n);              // B
  bpf_spin_unlock(&glock);

In the above snippet, after (A) n is a non-owning reference, and after
(B) m is an owning reference pointing to the same memory as n. Although
n has no ownership of that memory's lifetime, it's guaranteed to be
alive until the end of the critical section, and n would be clobbered if
we were past the end of the critical section, so it's safe to bump
refcount.

Implementation details:

* From verifier's perspective, bpf_refcount_acquire handling is similar
  to bpf_obj_new and bpf_obj_drop. Like the former, it returns a new
  owning reference matching input type, although like the latter, type
  can be inferred from concrete kptr input. Verifier changes in
  {check,fixup}_kfunc_call and check_kfunc_args are largely copied from
  aforementioned functions' verifier changes.

* An exception to the above is the new KF_ARG_PTR_TO_REFCOUNTED_KPTR
  arg, indicated by new "__refcounted_kptr" kfunc arg suffix. This is
  necessary in order to handle both owning and non-owning input without
  adding special-casing to "__alloc" arg handling. Also a convenient
  place to confirm that input type has bpf_refcount field.

* The implemented kfunc is actually bpf_refcount_acquire_impl, with
  'hidden' second arg that the verifier sets to the type's struct_meta
  in fixup_kfunc_call.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230415201811.343116-5-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -1917,6 +1917,20 @@ __bpf_kfunc void bpf_obj_drop_impl(void *p__alloc, void *meta__ign)
 	__bpf_obj_drop_impl(p, meta ? meta->record : NULL);
 }
 
+__bpf_kfunc void *bpf_refcount_acquire_impl(void *p__refcounted_kptr, void *meta__ign)
+{
+	struct btf_struct_meta *meta = meta__ign;
+	struct bpf_refcount *ref;
+
+	/* Could just cast directly to refcount_t *, but need some code using
+	 * bpf_refcount type so that it is emitted in vmlinux BTF
+	 */
+	ref = (struct bpf_refcount *)p__refcounted_kptr + meta->record->refcount_off;
+
+	refcount_inc((refcount_t *)ref);
+	return (void *)p__refcounted_kptr;
+}
+
 static void __bpf_list_add(struct bpf_list_node *node, struct bpf_list_head *head, bool tail)
 {
 	struct list_head *n = (void *)node, *h = (void *)head;
@@ -2276,6 +2290,7 @@ BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)
 #endif
 BTF_ID_FLAGS(func, bpf_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_obj_drop_impl, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_refcount_acquire_impl, KF_ACQUIRE)
 BTF_ID_FLAGS(func, bpf_list_push_front)
 BTF_ID_FLAGS(func, bpf_list_push_back)
 BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)

kernel/bpf/verifier.c

@@ -273,6 +273,11 @@ struct bpf_call_arg_meta {
 	struct btf_field *kptr_field;
 };
 
+struct btf_and_id {
+	struct btf *btf;
+	u32 btf_id;
+};
+
 struct bpf_kfunc_call_arg_meta {
 	/* In parameters */
 	struct btf *btf;
@@ -291,10 +296,10 @@ struct bpf_kfunc_call_arg_meta {
 		u64 value;
 		bool found;
 	} arg_constant;
-	struct {
+	union {
-		struct btf *btf;
+		struct btf_and_id arg_obj_drop;
-		u32 btf_id;
+		struct btf_and_id arg_refcount_acquire;
-	} arg_obj_drop;
+	};
 	struct {
 		struct btf_field *field;
 	} arg_list_head;
@@ -9403,6 +9408,11 @@ static bool is_kfunc_arg_uninit(const struct btf *btf, const struct btf_param *a
 	return __kfunc_param_match_suffix(btf, arg, "__uninit");
 }
 
+static bool is_kfunc_arg_refcounted_kptr(const struct btf *btf, const struct btf_param *arg)
+{
+	return __kfunc_param_match_suffix(btf, arg, "__refcounted_kptr");
+}
+
 static bool is_kfunc_arg_scalar_with_name(const struct btf *btf,
 					  const struct btf_param *arg,
 					  const char *name)
@@ -9542,15 +9552,16 @@ static u32 *reg2btf_ids[__BPF_REG_TYPE_MAX] = {
 
 enum kfunc_ptr_arg_type {
 	KF_ARG_PTR_TO_CTX,
-	KF_ARG_PTR_TO_ALLOC_BTF_ID,  /* Allocated object */
+	KF_ARG_PTR_TO_ALLOC_BTF_ID,    /* Allocated object */
-	KF_ARG_PTR_TO_KPTR,	     /* PTR_TO_KPTR but type specific */
+	KF_ARG_PTR_TO_REFCOUNTED_KPTR, /* Refcounted local kptr */
+	KF_ARG_PTR_TO_KPTR,	       /* PTR_TO_KPTR but type specific */
 	KF_ARG_PTR_TO_DYNPTR,
 	KF_ARG_PTR_TO_ITER,
 	KF_ARG_PTR_TO_LIST_HEAD,
 	KF_ARG_PTR_TO_LIST_NODE,
-	KF_ARG_PTR_TO_BTF_ID,	     /* Also covers reg2btf_ids conversions */
+	KF_ARG_PTR_TO_BTF_ID,	       /* Also covers reg2btf_ids conversions */
 	KF_ARG_PTR_TO_MEM,
-	KF_ARG_PTR_TO_MEM_SIZE,	     /* Size derived from next argument, skip it */
+	KF_ARG_PTR_TO_MEM_SIZE,	       /* Size derived from next argument, skip it */
 	KF_ARG_PTR_TO_CALLBACK,
 	KF_ARG_PTR_TO_RB_ROOT,
 	KF_ARG_PTR_TO_RB_NODE,
@@ -9559,6 +9570,7 @@ enum kfunc_ptr_arg_type {
 enum special_kfunc_type {
 	KF_bpf_obj_new_impl,
 	KF_bpf_obj_drop_impl,
+	KF_bpf_refcount_acquire_impl,
 	KF_bpf_list_push_front,
 	KF_bpf_list_push_back,
 	KF_bpf_list_pop_front,
@@ -9579,6 +9591,7 @@ enum special_kfunc_type {
 BTF_SET_START(special_kfunc_set)
 BTF_ID(func, bpf_obj_new_impl)
 BTF_ID(func, bpf_obj_drop_impl)
+BTF_ID(func, bpf_refcount_acquire_impl)
 BTF_ID(func, bpf_list_push_front)
 BTF_ID(func, bpf_list_push_back)
 BTF_ID(func, bpf_list_pop_front)
@@ -9597,6 +9610,7 @@ BTF_SET_END(special_kfunc_set)
 BTF_ID_LIST(special_kfunc_list)
 BTF_ID(func, bpf_obj_new_impl)
 BTF_ID(func, bpf_obj_drop_impl)
+BTF_ID(func, bpf_refcount_acquire_impl)
 BTF_ID(func, bpf_list_push_front)
 BTF_ID(func, bpf_list_push_back)
 BTF_ID(func, bpf_list_pop_front)
@@ -9649,6 +9663,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	if (is_kfunc_arg_alloc_obj(meta->btf, &args[argno]))
 		return KF_ARG_PTR_TO_ALLOC_BTF_ID;
 
+	if (is_kfunc_arg_refcounted_kptr(meta->btf, &args[argno]))
+		return KF_ARG_PTR_TO_REFCOUNTED_KPTR;
+
 	if (is_kfunc_arg_kptr_get(meta, argno)) {
 		if (!btf_type_is_ptr(ref_t)) {
 			verbose(env, "arg#0 BTF type must be a double pointer for kptr_get kfunc\n");
@@ -9952,7 +9969,8 @@ static bool is_bpf_rbtree_api_kfunc(u32 btf_id)
 
 static bool is_bpf_graph_api_kfunc(u32 btf_id)
 {
-	return is_bpf_list_api_kfunc(btf_id) || is_bpf_rbtree_api_kfunc(btf_id);
+	return is_bpf_list_api_kfunc(btf_id) || is_bpf_rbtree_api_kfunc(btf_id) ||
+	       btf_id == special_kfunc_list[KF_bpf_refcount_acquire_impl];
 }
 
 static bool is_callback_calling_kfunc(u32 btf_id)
@@ -10171,6 +10189,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 	const char *func_name = meta->func_name, *ref_tname;
 	const struct btf *btf = meta->btf;
 	const struct btf_param *args;
+	struct btf_record *rec;
 	u32 i, nargs;
 	int ret;
 
@@ -10306,6 +10325,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		case KF_ARG_PTR_TO_MEM:
 		case KF_ARG_PTR_TO_MEM_SIZE:
 		case KF_ARG_PTR_TO_CALLBACK:
+		case KF_ARG_PTR_TO_REFCOUNTED_KPTR:
 			/* Trusted by default */
 			break;
 		default:
@@ -10523,6 +10543,26 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		case KF_ARG_PTR_TO_CALLBACK:
 			meta->subprogno = reg->subprogno;
 			break;
+		case KF_ARG_PTR_TO_REFCOUNTED_KPTR:
+			if (!type_is_ptr_alloc_obj(reg->type) && !type_is_non_owning_ref(reg->type)) {
+				verbose(env, "arg#%d is neither owning or non-owning ref\n", i);
+				return -EINVAL;
+			}
+
+			rec = reg_btf_record(reg);
+			if (!rec) {
+				verbose(env, "verifier internal error: Couldn't find btf_record\n");
+				return -EFAULT;
+			}
+
+			if (rec->refcount_off < 0) {
+				verbose(env, "arg#%d doesn't point to a type with bpf_refcount field\n", i);
+				return -EINVAL;
+			}
+
+			meta->arg_refcount_acquire.btf = reg->btf;
+			meta->arg_refcount_acquire.btf_id = reg->btf_id;
+			break;
 		}
 	}
 
@@ -10699,7 +10739,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 
 	if (is_kfunc_acquire(&meta) && !btf_type_is_struct_ptr(meta.btf, t)) {
 		/* Only exception is bpf_obj_new_impl */
-		if (meta.btf != btf_vmlinux || meta.func_id != special_kfunc_list[KF_bpf_obj_new_impl]) {
+		if (meta.btf != btf_vmlinux ||
+		    (meta.func_id != special_kfunc_list[KF_bpf_obj_new_impl] &&
+		     meta.func_id != special_kfunc_list[KF_bpf_refcount_acquire_impl])) {
 			verbose(env, "acquire kernel function does not return PTR_TO_BTF_ID\n");
 			return -EINVAL;
 		}
@@ -10747,6 +10789,15 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 				insn_aux->obj_new_size = ret_t->size;
 				insn_aux->kptr_struct_meta =
 					btf_find_struct_meta(ret_btf, ret_btf_id);
+			} else if (meta.func_id == special_kfunc_list[KF_bpf_refcount_acquire_impl]) {
+				mark_reg_known_zero(env, regs, BPF_REG_0);
+				regs[BPF_REG_0].type = PTR_TO_BTF_ID | MEM_ALLOC;
+				regs[BPF_REG_0].btf = meta.arg_refcount_acquire.btf;
+				regs[BPF_REG_0].btf_id = meta.arg_refcount_acquire.btf_id;
+
+				insn_aux->kptr_struct_meta =
+					btf_find_struct_meta(meta.arg_refcount_acquire.btf,
+							     meta.arg_refcount_acquire.btf_id);
 			} else if (meta.func_id == special_kfunc_list[KF_bpf_list_pop_front] ||
 				   meta.func_id == special_kfunc_list[KF_bpf_list_pop_back]) {
 				struct btf_field *field = meta.arg_list_head.field;
@@ -17393,7 +17444,8 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		insn_buf[2] = addr[1];
 		insn_buf[3] = *insn;
 		*cnt = 4;
-	} else if (desc->func_id == special_kfunc_list[KF_bpf_obj_drop_impl]) {
+	} else if (desc->func_id == special_kfunc_list[KF_bpf_obj_drop_impl] ||
+		   desc->func_id == special_kfunc_list[KF_bpf_refcount_acquire_impl]) {
 		struct btf_struct_meta *kptr_struct_meta = env->insn_aux_data[insn_idx].kptr_struct_meta;
 		struct bpf_insn addr[2] = { BPF_LD_IMM64(BPF_REG_2, (long)kptr_struct_meta) };
 

tools/testing/selftests/bpf/bpf_experimental.h

@@ -37,6 +37,19 @@ extern void bpf_obj_drop_impl(void *kptr, void *meta) __ksym;
 /* Convenience macro to wrap over bpf_obj_drop_impl */
 #define bpf_obj_drop(kptr) bpf_obj_drop_impl(kptr, NULL)
 
+/* Description
+ *	Increment the refcount on a refcounted local kptr, turning the
+ *	non-owning reference input into an owning reference in the process.
+ *
+ *	The 'meta' parameter is a hidden argument that is ignored.
+ * Returns
+ *	An owning reference to the object pointed to by 'kptr'
+ */
+extern void *bpf_refcount_acquire_impl(void *kptr, void *meta) __ksym;
+
+/* Convenience macro to wrap over bpf_refcount_acquire_impl */
+#define bpf_refcount_acquire(kptr) bpf_refcount_acquire_impl(kptr, NULL)
+
 /* Description
  *	Add a new entry to the beginning of the BPF linked list.
  * Returns