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

[Martin KaFai Lau]

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.