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

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Date:   Tue, 21 Feb 2023 09:52:52 -0800
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Song Liu <song@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Martin KaFai Lau <martin.lau@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH bpf-next v5] bpf, test_run: fix &xdp_frame misplacement
 for LIVE_FRAMES

On 2/21/23 4:35 AM, Alexander Lobakin wrote:
> From: Alexander Lobakin <aleksander.lobakin@...el.com>
> 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@...hat.com>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
>> Link: https://lore.kernel.org/r/20230215185440.4126672-1-aleksander.lobakin@intel.com
>> Signed-off-by: Martin KaFai Lau <martin.lau@...nel.org>
> (>_< 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.