linux-kernel - Re: [PATCH bpf] selftests/bpf: Add a copyright notice to lpm_trie_map_get_next

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Message-ID: <b75e09a9-1028-28a2-f85d-5c7130a201f6@huaweicloud.com>
Date: Tue, 5 Nov 2024 10:34:08 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>,
 Byeonguk Jeong <jungbu2855@...il.com>
Cc: andrii@...nel.org, daniel@...earbox.net, martin.lau@...ux.dev,
 Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf] selftests/bpf: Add a copyright notice to
 lpm_trie_map_get_next_key

Hi Ilya,

On 11/4/2024 6:07 PM, Ilya Leoshkevich wrote:
> On Mon, 2024-11-04 at 09:34 +0800, Hou Tao wrote:
>> Hi,
>>
>> On 11/3/2024 2:04 PM, Byeonguk Jeong wrote:
>>> Hi,
>>>
>>> The selftest "verifier_bits_iter/bad words" has been failed with
>>> retval 115, while I did not touched anything but a comment.
>>>
>>> Do you have any idea why it failed? I am not sure whether it
>>> indicates
>>> any bugs in the kernel.
>>>
>>> Best,
>>> Byeonguk
>> Sorry for the inconvenience. It seems the test case
>> "verifier_bits_iter/bad words" is flaky. It may fail randomly, such
>> as
>> in [1]. I think calling bpf_probe_read_kernel_common() on 3GB addr
>> under
>> s390 host may succeed and the content of the memory address will
>> decide
>> whether the test case will succeed or not. Do not know the reason why
>> reading 3GB address succeeds under s390. Hope to get some insight
>> from
>> Ilya.  I think we could fix the failure first by using NULL as the
>> address of bad words just like null_pointer test case does. Will
>> merge
>> the test in bad_words into the null_pointer case.
> Hi,
>
> s390 kernel runs in a completely separate address space, there is no
> user/kernel split at TASK_SIZE. The same address may be valid in both
> the kernel and the user address spaces, there is no way to tell by
> looking at it. The config option related to this property is
> ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.
>
> Also, unfortunately, 0 is a valid address in the s390 kernel address
> space.

Thanks for the detailed explanation. It seems both arm and x86 have
select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.
>
> I wonder if we could use -4095 as an address that cannot be
> dereferenced on all platforms?

I have tested it in both arm64 and x86-64 that reading from -4095 by
using copy_from_kernel_nofault() will return -EFAULT . For s390, I hope
the bpf CI could help to test it. Will post a fix patch later.
>
> Best regards,
> Ilya