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

[Alexei Starovoitov]

On Tue, Apr 17, 2018 at 09:42:13PM -0700, Nikita V. Shirokov wrote:
> 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 

Acked-by: Alexei Starovoitov 

patches look great. thanks!

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

[Nikita V. Shirokov]

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 
---
 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 _xdp_redirect_proto;
case BPF_FUNC_redirect_map:
return _xdp_redirect_map_proto;
+   case BPF_FUNC_xdp_adjust_tail:
+   return _xdp_adjust_tail_proto;
default:
return bpf_base_func_proto(func_id);
}
-- 
2.15.1