Subject: [PATCH bpf-next v3 01/11] bpf: adding bpf_xdp_adjust_tail helper

From: "Nikita V. Shirokov" <[email-masked]>
Date: Tue, 17 Apr 2018 21:42:13 -0700

Adding new bpf helper which would allow us to manipulate
xdp's data_end pointer, and allow us to reduce packet's size
indended use case: to generate ICMP messages from XDP context,
where such message would contain truncated original packet.

Signed-off-by: Nikita V. Shirokov <[email-masked]>
---
 include/uapi/linux/bpf.h | 10 +++++++++-
 net/core/filter.c        | 29 ++++++++++++++++++++++++++++-
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index c5ec89732a8d..9a2d1a04eb24 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -755,6 +755,13 @@ union bpf_attr {
  *     @addr: pointer to struct sockaddr to bind socket to
  *     @addr_len: length of sockaddr structure
  *     Return: 0 on success or negative error code
+ *
+ * int bpf_xdp_adjust_tail(xdp_md, delta)
+ *     Adjust the xdp_md.data_end by delta. Only shrinking of packet's
+ *     size is supported.
+ *     @xdp_md: pointer to xdp_md
+ *     @delta: A negative integer to be added to xdp_md.data_end
+ *     Return: 0 on success or negative on error
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -821,7 +828,8 @@ union bpf_attr {
 	FN(msg_apply_bytes),		\
 	FN(msg_cork_bytes),		\
 	FN(msg_pull_data),		\
-	FN(bind),
+	FN(bind),			\
+	FN(xdp_adjust_tail),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
diff --git a/net/core/filter.c b/net/core/filter.c
index a374b8560bc4..29318598fd60 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2725,6 +2725,30 @@ static const struct bpf_func_proto bpf_xdp_adjust_head_proto = {
 	.arg2_type	= ARG_ANYTHING,
 };
 
+BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
+{
+	void *data_end = xdp->data_end + offset;
+
+	/* only shrinking is allowed for now. */
+	if (unlikely(offset >= 0))
+		return -EINVAL;
+
+	if (unlikely(data_end < xdp->data + ETH_HLEN))
+		return -EINVAL;
+
+	xdp->data_end = data_end;
+
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_xdp_adjust_tail_proto = {
+	.func		= bpf_xdp_adjust_tail,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_ANYTHING,
+};
+
 BPF_CALL_2(bpf_xdp_adjust_meta, struct xdp_buff *, xdp, int, offset)
 {
 	void *meta = xdp->data_meta + offset;
@@ -3074,7 +3098,8 @@ bool bpf_helper_changes_pkt_data(void *func)
 	    func == bpf_l4_csum_replace ||
 	    func == bpf_xdp_adjust_head ||
 	    func == bpf_xdp_adjust_meta ||
-	    func == bpf_msg_pull_data)
+	    func == bpf_msg_pull_data ||
+	    func == bpf_xdp_adjust_tail)
 		return true;
 
 	return false;
@@ -3888,6 +3913,8 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_xdp_redirect_proto;
 	case BPF_FUNC_redirect_map:
 		return &bpf_xdp_redirect_map_proto;
+	case BPF_FUNC_xdp_adjust_tail:
+		return &bpf_xdp_adjust_tail_proto;
 	default:
 		return bpf_base_func_proto(func_id);
 	}
-- 
2.15.1