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

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Message-ID: <mb61p1q09d2eb.fsf@kernel.org>
Date: Mon, 21 Oct 2024 13:14:04 +0000
From: Puranjay Mohan <puranjay@...nel.org>
To: Helge Deller <deller@....de>, 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

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

Thanks,
Puranjay

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