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

From: Puranjay Mohan <puranjay@kernel.org>
To: Helge Deller <deller@gmx.de>, Albert Ou <aou@eecs.berkeley.edu>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	bpf@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
	"David S. Miller" <davem@davemloft.net>,
	Eduard Zingerman <eddyz87@gmail.com>,
	Eric Dumazet <edumazet@google.com>, Hao Luo <haoluo@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	"James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
	Jiri Olsa <jolsa@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	linux-kernel@vger.kernel.org, linux-parisc@vger.kernel.org,
	linux-riscv@lists.infradead.org,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Mykola Lysenko <mykolal@fb.com>,
	netdev@vger.kernel.org, Palmer Dabbelt <palmer@dabbelt.com>,
	Paolo Abeni <pabeni@redhat.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Shuah Khan <shuah@kernel.org>, Song Liu <song@kernel.org>,
	Stanislav Fomichev <sdf@fomichev.me>,
	Yonghong Song <yonghong.song@linux.dev>
Subject: Re: [PATCH bpf-next 3/5] selftests/bpf: don't mask result of bpf_csum_diff() in test_verifier
Date: Mon, 21 Oct 2024 13:14:04 +0000	[thread overview]
Message-ID: <mb61p1q09d2eb.fsf@kernel.org> (raw)
In-Reply-To: <31b8ea3b-f765-43c0-9cee-49bc13064f04@gmx.de>

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Helge Deller <deller@gmx.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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