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

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Message-ID: <b48b96ca-2387-ee01-f45f-2bf0ffc82a7c@intel.com>
Date:   Wed, 15 Feb 2023 16:01:28 +0100
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        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: Toke Høiland-Jørgensen <toke@...hat.com>
Date: Tue, 14 Feb 2023 22:05:26 +0100

> Alexander Lobakin <alexandr.lobakin@...el.com> writes:
> 
>> 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.
> 
> The 32/64 bit split should be straight-forward to handle for the head;
> an xdp_buff is 6*sizeof(void)+8 bytes long, and xdp_page_head is just
> two of those after this patch. The skb_shared_info size is a bit harder;
> do we have the alignment / size macros available to userspace somewhere?
> 
> Hmm, the selftests generate a vmlinux.h file which would have the
> structure definitions; maybe something could be generated from that? Not
> straight-forward to include it in a userspace application, though.
> 
> Otherwise, does anyone run the selftests on architectures that don't
> have a 64-byte cache-line size? Or even on 32-bit arches? We don't
> handle larger page sizes either...

I believe nobody does that :D Everyone just use x86_64 and ARM64 with 4k
pages.

I think for this particular patch we can just change 3368 to 3408
without trying to assert it in the kernel code. And later I'll
brainstorm this :D

> 
> -Toke
> 
Thanks,
Olek