Re: [PATCH bpf-next v5] bpf, test_run: fix &xdp_frame misplacement for LIVE_FRAMES

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On 2/21/23 4:35 AM, Alexander Lobakin wrote:
From: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
Date: Mon, 20 Feb 2023 16:46:27 +0100

&xdp_buff and &xdp_frame are bound in a way that

xdp_buff->data_hard_start == xdp_frame

It's always the case and e.g. xdp_convert_buff_to_frame() relies on
this.
IOW, the following:

	for (u32 i = 0; i < 0xdead; i++) {
		xdpf = xdp_convert_buff_to_frame(&xdp);
		xdp_convert_frame_to_buff(xdpf, &xdp);
	}

shouldn't ever modify @xdpf's contents or the pointer itself.
However, "live packet" code wrongly treats &xdp_frame as part of its
context placed *before* the data_hard_start. With such flow,
data_hard_start is sizeof(*xdpf) off to the right and no longer points
to the XDP frame.

Instead of replacing `sizeof(ctx)` with `offsetof(ctx, xdpf)` in several
places and praying that there are no more miscalcs left somewhere in the
code, unionize ::frm with ::data in a flex array, so that both starts
pointing to the actual data_hard_start and the XDP frame actually starts
being a part of it, i.e. a part of the headroom, not the context.
A nice side effect is that the maximum frame size for this mode gets
increased by 40 bytes, as xdp_buff::frame_sz includes everything from
data_hard_start (-> includes xdpf already) to the end of XDP/skb shared
info.
Also update %MAX_PKT_SIZE accordingly in the selftests code. Leave it
hardcoded for 64 bit && 4k pages, it can be made more flexible later on.

Minor: align `&head->data` with how `head->frm` is assigned for
consistency.
Minor #2: rename 'frm' to 'frame' in &xdp_page_head while at it for
clarity.

(was found while testing XDP traffic generator on ice, which calls
  xdp_convert_frame_to_buff() for each XDP frame)

Sorry, maybe this could be taken directly to net-next while it's still
open? It was tested and then reverted from bpf-next only due to not 100%
compile-time assertion, which I removed in this version. No more
changes. I doubt there'll be a second PR from bpf and would like this to
hit mainline before RC1 :s

I think this could go to bpf soon instead of bpf-next. The change is specific to the bpf selftest. It is better to go through bpf to get bpf CI coverage.



Fixes: b530e9e1063e ("bpf: Add "live packet" mode for XDP in BPF_PROG_RUN")
Acked-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
Link: https://lore.kernel.org/r/20230215185440.4126672-1-aleksander.lobakin@xxxxxxxxx
Signed-off-by: Martin KaFai Lau <martin.lau@xxxxxxxxxx>
(>_< those two last tags are incorrect, lemme know if I should resubmit
  it without them or you could do it if ok with taking it now)

Please respin when it can be landed to the bpf tree on top of the s390 changes.



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