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

[Alexander Lobakin]

From: Martin Kafai Lau <martin.lau@xxxxxxxxx>
Date: Tue, 21 Feb 2023 09:52:52 -0800

> 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.

Ah okay, I'll resend when bpf pulls merged net-next from Linus' tree.

> 
>>
>>>
>>> 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.

Thanks,
Olek