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

[Alexander Lobakin]

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

> 
> 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)

Thanks,
Olek