[PATCH v2 bpf-next 1/6] bpf: Add bpf_dynptr_trim and bpf_dynptr_advance

From: Joanne Koong <joannelkoong@gmail.com>
To: bpf@vger.kernel.org
Cc: andrii@kernel.org, kernel-team@meta.com, ast@kernel.org,
	daniel@iogearbox.net, martin.lau@linux.dev, song@kernel.org,
	Joanne Koong <joannelkoong@gmail.com>
Subject: [PATCH v2 bpf-next 1/6] bpf: Add bpf_dynptr_trim and bpf_dynptr_advance
Date: Wed,  7 Dec 2022 12:55:32 -0800	[thread overview]
Message-ID: <20221207205537.860248-2-joannelkoong@gmail.com> (raw)
In-Reply-To: <20221207205537.860248-1-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       | 18 +++++++++
 kernel/bpf/helpers.c           | 67 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 18 +++++++++
 3 files changed, 103 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index a9bb98365031..c2d915601484 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5537,6 +5537,22 @@ union bpf_attr {
  *		*flags* is currently unused, it must be 0 for now.
  *	Return
  *		0 on success, -EINVAL if flags is not 0.
+ *
+ * long bpf_dynptr_advance(struct bpf_dynptr *ptr, u32 len)
+ *	Description
+ *		Advance a dynptr's internal offset by *len* bytes.
+ *	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 the size of memory pointed to by the dynptr by *len* bytes.
+ *
+ *		The offset is unmodified.
+ *	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, ctx...)			\
 	FN(unspec, 0, ##ctx)				\
@@ -5753,6 +5769,8 @@ union bpf_attr {
 	FN(cgrp_storage_delete, 211, ##ctx)		\
 	FN(dynptr_from_skb, 212, ##ctx)			\
 	FN(dynptr_from_xdp, 213, ##ctx)			\
+	FN(dynptr_advance, 214, ##ctx)			\
+	FN(dynptr_trim, 215, ##ctx)			\
 	/* */
 
 /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 3a9c8814aaf6..fa3989047ff6 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1429,6 +1429,13 @@ 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;
@@ -1640,6 +1647,62 @@ static const struct bpf_func_proto bpf_dynptr_data_proto = {
 	.arg3_type	= ARG_CONST_ALLOC_SIZE_OR_ZERO,
 };
 
+/* For dynptrs, the offset may only be advanced and the size may only be decremented */
+static int bpf_dynptr_adjust(struct bpf_dynptr_kern *ptr, u32 off_inc, u32 sz_dec)
+{
+	u32 size;
+
+	if (!ptr->data)
+		return -EINVAL;
+
+	size = bpf_dynptr_get_size(ptr);
+
+	if (sz_dec > size)
+		return -ERANGE;
+
+	if (off_inc) {
+		u32 new_off;
+
+		if (off_inc > size)
+			return -ERANGE;
+
+		if (check_add_overflow(ptr->offset, off_inc, &new_off))
+			return -ERANGE;
+
+		ptr->offset = new_off;
+	}
+
+	bpf_dynptr_set_size(ptr, size - sz_dec);
+
+	return 0;
+}
+
+BPF_CALL_2(bpf_dynptr_advance, struct bpf_dynptr_kern *, ptr, u32, len)
+{
+	return bpf_dynptr_adjust(ptr, len, len);
+}
+
+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)
+{
+	return bpf_dynptr_adjust(ptr, 0, len);
+}
+
+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;
@@ -1744,6 +1807,10 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 		return &bpf_dynptr_write_proto;
 	case BPF_FUNC_dynptr_data:
 		return &bpf_dynptr_data_proto;
+	case BPF_FUNC_dynptr_advance:
+		return &bpf_dynptr_advance_proto;
+	case BPF_FUNC_dynptr_trim:
+		return &bpf_dynptr_trim_proto;
 #ifdef CONFIG_CGROUPS
 	case BPF_FUNC_cgrp_storage_get:
 		return &bpf_cgrp_storage_get_proto;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index a9bb98365031..c2d915601484 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5537,6 +5537,22 @@ union bpf_attr {
  *		*flags* is currently unused, it must be 0 for now.
  *	Return
  *		0 on success, -EINVAL if flags is not 0.
+ *
+ * long bpf_dynptr_advance(struct bpf_dynptr *ptr, u32 len)
+ *	Description
+ *		Advance a dynptr's internal offset by *len* bytes.
+ *	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 the size of memory pointed to by the dynptr by *len* bytes.
+ *
+ *		The offset is unmodified.
+ *	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, ctx...)			\
 	FN(unspec, 0, ##ctx)				\
@@ -5753,6 +5769,8 @@ union bpf_attr {
 	FN(cgrp_storage_delete, 211, ##ctx)		\
 	FN(dynptr_from_skb, 212, ##ctx)			\
 	FN(dynptr_from_xdp, 213, ##ctx)			\
+	FN(dynptr_advance, 214, ##ctx)			\
+	FN(dynptr_trim, 215, ##ctx)			\
 	/* */
 
 /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
-- 
2.30.2

