On 2020-05-20 15:18, Jesper Dangaard Brouer wrote:
On Wed, 20 May 2020 11:47:28 +0200
Björn Töpel <bjorn.topel@xxxxxxxxx> wrote:
From: Björn Töpel <bjorn.topel@xxxxxxxxx>
Calculating the "data_hard_end" for an XDP buffer coming from AF_XDP
zero-copy mode, the return value of xsk_umem_xdp_frame_sz() is added
to "data_hard_start".
Currently, the chunk size of the UMEM is returned by
xsk_umem_xdp_frame_sz(). This is not correct, if the fixed UMEM
headroom is non-zero. Fix this by returning the chunk_size without the
UMEM headroom.
Fixes: 2a637c5b1aaf ("xdp: For Intel AF_XDP drivers add XDP frame_sz")
Signed-off-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
---
include/net/xdp_sock.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index abd72de25fa4..6b1137ce1692 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -239,7 +239,7 @@ static inline u64 xsk_umem_adjust_offset(struct xdp_umem *umem, u64 address,
static inline u32 xsk_umem_xdp_frame_sz(struct xdp_umem *umem)
{
- return umem->chunk_size_nohr + umem->headroom;
+ return umem->chunk_size_nohr;
Hmm, is this correct?
As you write "xdp_data_hard_end" is calculated as an offset from
xdp->data_hard_start pointer based on the frame_sz. Will your
xdp->data_hard_start + frame_sz point to packet end?
Yes, I believe this is correct.
Say that a user uses a chunk size of 2k, and a umem headroom of, say,
64. This means that the kernel should (at least) leave 64B which the
kernel shouldn't touch.
umem->headroom | XDP_PACKET_HEADROOM | packet | |
^ ^ ^ ^ ^
a b c d e
a: data_hard_start
b: data
c: data_end
d: data_hard_end, (e - 320)
e: hardlimit of chunk, a + umem->chunk_size_nohr
Prior this fix the umem->headroom was *included* in frame_sz.
#define xdp_data_hard_end(xdp) \
((xdp)->data_hard_start + (xdp)->frame_sz - \
SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
Note the macro reserves the last 320 bytes (for skb_shared_info), but
for AF_XDP zero-copy mode, it will never create an SKB that use this
area. Thus, in principle we can allow XDP-progs to extend/grow tail
into this area, but I don't think there is any use-case for this, as
it's much easier to access packet-data in userspace application.
(Thus, it might not be worth the complexity to give AF_XDP
bpf_xdp_adjust_tail access to this area, by e.g. "lying" via adding 320
bytes to frame_sz).
I agree, and in the picture (well...) above that would be "d". IOW
data_hard_end is 320 "off" the real end.
Björn
