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

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Date:   Tue, 14 Feb 2023 17:04:30 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        "Daniel Borkmann" <daniel@...earbox.net>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        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

From: Alexander Lobakin <alexandr.lobakin@...el.com>
Date: Tue, 14 Feb 2023 16:39:25 +0100

> From: Daniel Borkmann <daniel@...earbox.net>
> Date: Tue, 14 Feb 2023 16:24:10 +0100
> 
>> On 2/13/23 3:27 PM, Alexander Lobakin wrote:

[...]

>>> 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
> Ah I see. xdp_do_redirect.c test defines:
> 
> /* The maximum permissible size is: PAGE_SIZE -
>  * sizeof(struct xdp_page_head) - sizeof(struct skb_shared_info) -
>  * XDP_PACKET_HEADROOM = 3368 bytes
>  */
> #define MAX_PKT_SIZE 3368
> 
> This needs to be updated as it now became bigger. The test checks that
> this size passes and size + 1 fails, but now it doesn't.
> Will send v3 in a couple minutes.

Problem :s

This 3368/3408 assumes %L1_CACHE_BYTES is 64 and we're running on a
64-bit arch. For 32 bits the value will be bigger, also for cachelines
bigger than 64 it will be smaller (skb_shared_info has to be aligned).
Given that selftests are generic / arch-independent, how to approach
this? I added a static_assert() to test_run.c to make sure this value
is in sync to not run into the same problem in future, but then realized
it will fail on a number of architectures.

My first thought was to hardcode the worst-case value (64 bit, cacheline
is 128) in test_run.c for every architecture, but there might be more
elegant ways.

> 
> Thanks,
> Olek
Thanks,
Olek