bpf: Add bpf_dynptr_from_mem for local dynptrs

This patch adds a new api bpf_dynptr_from_mem:

long bpf_dynptr_from_mem(void *data, u32 size, u64 flags, struct bpf_dynptr *ptr);

which initializes a dynptr to point to a bpf program's local memory. For now
only local memory that is of reg type PTR_TO_MAP_VALUE is supported.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220523210712.3641569-3-joannelkoong@gmail.com

include/uapi/linux/bpf.h

@@ -5178,6 +5178,17 @@ union bpf_attr {
  *		Dynamically cast a *sk* pointer to a *mptcp_sock* pointer.
  *	Return
  *		*sk* if casting is valid, or **NULL** otherwise.
+ *
+ * long bpf_dynptr_from_mem(void *data, u32 size, u64 flags, struct bpf_dynptr *ptr)
+ *	Description
+ *		Get a dynptr to local memory *data*.
+ *
+ *		*data* must be a ptr to a map value.
+ *		The maximum *size* supported is DYNPTR_MAX_SIZE.
+ *		*flags* is currently unused.
+ *	Return
+ *		0 on success, -E2BIG if the size exceeds DYNPTR_MAX_SIZE,
+ *		-EINVAL if flags is not 0.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5377,6 +5388,7 @@ union bpf_attr {
 	FN(kptr_xchg),			\
 	FN(map_lookup_percpu_elem),     \
 	FN(skc_to_mptcp_sock),		\
+	FN(dynptr_from_mem),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

kernel/bpf/helpers.c

@@ -1412,6 +1412,69 @@ const struct bpf_func_proto bpf_kptr_xchg_proto = {
 	.arg2_btf_id  = BPF_PTR_POISON,
 };
 
+/* Since the upper 8 bits of dynptr->size is reserved, the
+ * maximum supported size is 2^24 - 1.
+ */
+#define DYNPTR_MAX_SIZE	((1UL << 24) - 1)
+#define DYNPTR_TYPE_SHIFT	28
+
+static void bpf_dynptr_set_type(struct bpf_dynptr_kern *ptr, enum bpf_dynptr_type type)
+{
+	ptr->size |= type << DYNPTR_TYPE_SHIFT;
+}
+
+static int bpf_dynptr_check_size(u32 size)
+{
+	return size > DYNPTR_MAX_SIZE ? -E2BIG : 0;
+}
+
+static void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
+			    enum bpf_dynptr_type type, u32 offset, u32 size)
+{
+	ptr->data = data;
+	ptr->offset = offset;
+	ptr->size = size;
+	bpf_dynptr_set_type(ptr, type);
+}
+
+static void bpf_dynptr_set_null(struct bpf_dynptr_kern *ptr)
+{
+	memset(ptr, 0, sizeof(*ptr));
+}
+
+BPF_CALL_4(bpf_dynptr_from_mem, void *, data, u32, size, u64, flags, struct bpf_dynptr_kern *, ptr)
+{
+	int err;
+
+	err = bpf_dynptr_check_size(size);
+	if (err)
+		goto error;
+
+	/* flags is currently unsupported */
+	if (flags) {
+		err = -EINVAL;
+		goto error;
+	}
+
+	bpf_dynptr_init(ptr, data, BPF_DYNPTR_TYPE_LOCAL, 0, size);
+
+	return 0;
+
+error:
+	bpf_dynptr_set_null(ptr);
+	return err;
+}
+
+const struct bpf_func_proto bpf_dynptr_from_mem_proto = {
+	.func		= bpf_dynptr_from_mem,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_UNINIT_MEM,
+	.arg2_type	= ARG_CONST_SIZE_OR_ZERO,
+	.arg3_type	= ARG_ANYTHING,
+	.arg4_type	= ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_LOCAL | MEM_UNINIT,
+};
+
 const struct bpf_func_proto bpf_get_current_task_proto __weak;
 const struct bpf_func_proto bpf_get_current_task_btf_proto __weak;
 const struct bpf_func_proto bpf_probe_read_user_proto __weak;
@@ -1466,6 +1529,8 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 		return &bpf_loop_proto;
 	case BPF_FUNC_strncmp:
 		return &bpf_strncmp_proto;
+	case BPF_FUNC_dynptr_from_mem:
+		return &bpf_dynptr_from_mem_proto;
 	default:
 		break;
 	}

kernel/bpf/verifier.c

@@ -7204,6 +7204,12 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
 					set_loop_callback_state);
 		break;
+	case BPF_FUNC_dynptr_from_mem:
+		if (regs[BPF_REG_1].type != PTR_TO_MAP_VALUE) {
+			verbose(env, "Unsupported reg type %s for bpf_dynptr_from_mem data\n",
+				reg_type_str(env, regs[BPF_REG_1].type));
+			return -EACCES;
+		}
 	}
 
 	if (err)

tools/include/uapi/linux/bpf.h

@@ -5178,6 +5178,17 @@ union bpf_attr {
  *		Dynamically cast a *sk* pointer to a *mptcp_sock* pointer.
  *	Return
  *		*sk* if casting is valid, or **NULL** otherwise.
+ *
+ * long bpf_dynptr_from_mem(void *data, u32 size, u64 flags, struct bpf_dynptr *ptr)
+ *	Description
+ *		Get a dynptr to local memory *data*.
+ *
+ *		*data* must be a ptr to a map value.
+ *		The maximum *size* supported is DYNPTR_MAX_SIZE.
+ *		*flags* is currently unused.
+ *	Return
+ *		0 on success, -E2BIG if the size exceeds DYNPTR_MAX_SIZE,
+ *		-EINVAL if flags is not 0.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5377,6 +5388,7 @@ union bpf_attr {
 	FN(kptr_xchg),			\
 	FN(map_lookup_percpu_elem),     \
 	FN(skc_to_mptcp_sock),		\
+	FN(dynptr_from_mem),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper