[PATCH bpf-next v1 2/8] bpf: Add bpf_dynptr_trim and bpf_dynptr_advance

[Joanne Koong <joannelkoong@gmail.com>]

Add two new helper functions: bpf_dynptr_trim and bpf_dynptr_advance.

bpf_dynptr_trim decreases the size of a dynptr by the specified
number of bytes (offset remains the same). bpf_dynptr_advance advances
the offset of the dynptr by the specified number of bytes (size
decreases correspondingly).

One example where trimming / advancing the dynptr may useful is for
hashing. If the dynptr points to a larger struct, it is possible to hash
an individual field within the struct through dynptrs by using
bpf_dynptr_advance+trim.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 include/uapi/linux/bpf.h       | 16 +++++++++
 kernel/bpf/helpers.c           | 63 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 16 +++++++++
 3 files changed, 95 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index cce3356765fc..3b054553be30 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5453,6 +5453,20 @@ union bpf_attr {
  *		dynptr is invalid or if the offset and length is out of bounds
  *		or in a paged buffer for skb-type dynptrs or across fragments
  *		for xdp-type dynptrs.
+ *
+ * long bpf_dynptr_advance(struct bpf_dynptr *ptr, u32 len)
+ *	Description
+ *		Advance a dynptr by *len*.
+ *	Return
+ *		0 on success, -EINVAL if the dynptr is invalid, -ERANGE if *len*
+ *		exceeds the bounds of the dynptr.
+ *
+ * long bpf_dynptr_trim(struct bpf_dynptr *ptr, u32 len)
+ *	Description
+ *		Trim a dynptr by *len*.
+ *	Return
+ *		0 on success, -EINVAL if the dynptr is invalid, -ERANGE if
+ *		trying to trim more bytes than the size of the dynptr.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5667,6 +5681,8 @@ union bpf_attr {
 	FN(dynptr_from_skb),		\
 	FN(dynptr_from_xdp),		\
 	FN(dynptr_data_rdonly),		\
+	FN(dynptr_advance),		\
+	FN(dynptr_trim),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 30a59c9e5df3..9f356105ab49 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1423,6 +1423,13 @@ static u32 bpf_dynptr_get_size(struct bpf_dynptr_kern *ptr)
 	return ptr->size & DYNPTR_SIZE_MASK;
 }
 
+static void bpf_dynptr_set_size(struct bpf_dynptr_kern *ptr, u32 new_size)
+{
+	u32 metadata = ptr->size & ~DYNPTR_SIZE_MASK;
+
+	ptr->size = new_size | metadata;
+}
+
 int bpf_dynptr_check_size(u32 size)
 {
 	return size > DYNPTR_MAX_SIZE ? -E2BIG : 0;
@@ -1646,6 +1653,58 @@ static const struct bpf_func_proto bpf_dynptr_data_proto = {
 	.arg3_type	= ARG_CONST_ALLOC_SIZE_OR_ZERO,
 };
 
+BPF_CALL_2(bpf_dynptr_advance, struct bpf_dynptr_kern *, ptr, u32, len)
+{
+	u32 size;
+
+	if (!ptr->data)
+		return -EINVAL;
+
+	size = bpf_dynptr_get_size(ptr);
+
+	if (len > size)
+		return -ERANGE;
+
+	ptr->offset += len;
+
+	bpf_dynptr_set_size(ptr, size - len);
+
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_dynptr_advance_proto = {
+	.func		= bpf_dynptr_advance,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_DYNPTR,
+	.arg2_type	= ARG_ANYTHING,
+};
+
+BPF_CALL_2(bpf_dynptr_trim, struct bpf_dynptr_kern *, ptr, u32, len)
+{
+	u32 size;
+
+	if (!ptr->data)
+		return -EINVAL;
+
+	size = bpf_dynptr_get_size(ptr);
+
+	if (len > size)
+		return -ERANGE;
+
+	bpf_dynptr_set_size(ptr, size - len);
+
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_dynptr_trim_proto = {
+	.func		= bpf_dynptr_trim,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_DYNPTR,
+	.arg2_type	= ARG_ANYTHING,
+};
+
 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;
@@ -1718,6 +1777,10 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 		return &bpf_dynptr_data_proto;
 	case BPF_FUNC_dynptr_data_rdonly:
 		return &bpf_dynptr_data_rdonly_proto;
+	case BPF_FUNC_dynptr_advance:
+		return &bpf_dynptr_advance_proto;
+	case BPF_FUNC_dynptr_trim:
+		return &bpf_dynptr_trim_proto;
 	default:
 		break;
 	}
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index cce3356765fc..3b054553be30 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5453,6 +5453,20 @@ union bpf_attr {
  *		dynptr is invalid or if the offset and length is out of bounds
  *		or in a paged buffer for skb-type dynptrs or across fragments
  *		for xdp-type dynptrs.
+ *
+ * long bpf_dynptr_advance(struct bpf_dynptr *ptr, u32 len)
+ *	Description
+ *		Advance a dynptr by *len*.
+ *	Return
+ *		0 on success, -EINVAL if the dynptr is invalid, -ERANGE if *len*
+ *		exceeds the bounds of the dynptr.
+ *
+ * long bpf_dynptr_trim(struct bpf_dynptr *ptr, u32 len)
+ *	Description
+ *		Trim a dynptr by *len*.
+ *	Return
+ *		0 on success, -EINVAL if the dynptr is invalid, -ERANGE if
+ *		trying to trim more bytes than the size of the dynptr.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5667,6 +5681,8 @@ union bpf_attr {
 	FN(dynptr_from_skb),		\
 	FN(dynptr_from_xdp),		\
 	FN(dynptr_data_rdonly),		\
+	FN(dynptr_advance),		\
+	FN(dynptr_trim),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
-- 
2.30.2