Re: [PATCH bpf-next v4 01/15] xsk: fix xsk_umem_xdp_frame_sz()
From: Jesper Dangaard Brouer <hidden>
Date: 2020-05-20 13:18:38
Also in:
bpf
On Wed, 20 May 2020 11:47:28 +0200 Björn Töpel [off-list ref] wrote:
quoted hunk ↗ jump to hunk
From: Björn Töpel <redacted> 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 <redacted> --- 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?
#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).
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer