linux-kernel - Re: [PATCH bpf-next 3/5] selftests/bpf: don't mask result of bpf_csum_diff() in test

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Message-ID: <aef97e2b-f845-4b2e-bb35-cf89e4a7af6c@gmx.de>
Date: Mon, 21 Oct 2024 16:04:22 +0200
From: Helge Deller <deller@....de>
To: Puranjay Mohan <puranjay@...nel.org>, Albert Ou <aou@...s.berkeley.edu>,
 Alexei Starovoitov <ast@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
 Daniel Borkmann <daniel@...earbox.net>, "David S. Miller"
 <davem@...emloft.net>, Eduard Zingerman <eddyz87@...il.com>,
 Eric Dumazet <edumazet@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
 Jiri Olsa <jolsa@...nel.org>, John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>, linux-kernel@...r.kernel.org,
 linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
 Martin KaFai Lau <martin.lau@...ux.dev>, Mykola Lysenko <mykolal@...com>,
 netdev@...r.kernel.org, Palmer Dabbelt <palmer@...belt.com>,
 Paolo Abeni <pabeni@...hat.com>, Paul Walmsley <paul.walmsley@...ive.com>,
 Shuah Khan <shuah@...nel.org>, Song Liu <song@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH bpf-next 3/5] selftests/bpf: don't mask result of
 bpf_csum_diff() in test_verifier

On 10/21/24 15:14, Puranjay Mohan wrote:
> Helge Deller <deller@....de> writes:
>
>> On 10/21/24 14:21, Puranjay Mohan wrote:
>>> The bpf_csum_diff() helper has been fixed to return a 16-bit value for
>>> all archs, so now we don't need to mask the result.
>>>
>>> ...
>>> --- a/tools/testing/selftests/bpf/progs/verifier_array_access.c
>>> +++ b/tools/testing/selftests/bpf/progs/verifier_array_access.c
>>> @@ -368,8 +368,7 @@ __naked void a_read_only_array_2_1(void)
>>>    	r4 = 0;						\
>>>    	r5 = 0;						\
>>>    	call %[bpf_csum_diff];				\
>>> -l0_%=:	r0 &= 0xffff;					\
>>> -	exit;						\
>>> +l0_%=:	exit;						\
>>
>> Instead of dropping the masking, would it make sense to
>> check here if (r0 >> 16) == 0 ?
>
> We define the expected value in R0 to be 65507(0xffe3) in the line at the top:
> __success __retval(65507)
>
> So, we should just not do anything to R0 and it should contain this value
> after returning from bpf_csum_diff()
>
> This masking hack was added in:
>
> 6185266c5a853 ("selftests/bpf: Mask bpf_csum_diff() return value to 16 bits in test_verifier")
>
> because without the fix in patch 2 bpf_csum_diff() would return the
> following for this test:
>
> x86                    :    -29 : 0xffffffe3
> generic (arm64, riscv) :  65507 : 0x0000ffe3

You're right.
Thanks for explaining.

Helge