Re: [PATCH bpf-next v4 01/15] xsk: fix xsk_umem_xdp_frame_sz()
From: Jesper Dangaard Brouer <hidden>
Date: 2020-05-21 04:30:08
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bpf
On Wed, 20 May 2020 16:34:05 +0200 Björn Töpel [off-list ref] wrote:
On 2020-05-20 15:18, Jesper Dangaard Brouer wrote:quoted
On Wed, 20 May 2020 11:47:28 +0200 Björn Töpel [off-list ref] wrote:quoted
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?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.
Thanks for the nice ascii art description. I can now see that you are right. We should add this kind of documentation, perhaps as a comment in the code?
quoted
#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.
Yes, we agree. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer