Subject: Re: [PATCH bpf-next v4 01/15] xsk: fix xsk_umem_xdp_frame_sz()

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 20 May 2020 15:18:19 +0200

On Wed, 20 May 2020 11:47:28 +0200
Björn Töpel <[email-masked]> wrote:

> From: Björn Töpel <[email-masked]>
> 
> 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 <[email-masked]>
> ---
>  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