linux-kernel - Re: [PATCH v2 bpf] bpf, test_run: fix &xdp_frame misplacement for LIVE

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Date:   Tue, 14 Feb 2023 16:24:10 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 bpf] bpf, test_run: fix &xdp_frame misplacement for
 LIVE_FRAMES

On 2/13/23 3:27 PM, Alexander Lobakin wrote:
> &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.
> 
> 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)
> 
> Fixes: b530e9e1063e ("bpf: Add "live packet" mode for XDP in BPF_PROG_RUN")
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>

Could you double check BPF CI? Looks like a number of XDP related tests
are failing on your patch which I'm not seeing on other patches where runs
are green, for example test_progs on several archs report the below:

https://github.com/kernel-patches/bpf/actions/runs/4164593416/jobs/7207290499

   [...]
   test_xdp_do_redirect:PASS:prog_run 0 nsec
   test_xdp_do_redirect:PASS:pkt_count_xdp 0 nsec
   test_xdp_do_redirect:PASS:pkt_count_zero 0 nsec
   test_xdp_do_redirect:PASS:pkt_count_tc 0 nsec
   test_max_pkt_size:PASS:prog_run_max_size 0 nsec
   test_max_pkt_size:FAIL:prog_run_too_big unexpected prog_run_too_big: actual -28 != expected -22
   close_netns:PASS:setns 0 nsec
   #275     xdp_do_redirect:FAIL
   Summary: 273/1581 PASSED, 21 SKIPPED, 2 FAILED