Re: [PATCH] bpf: selftests: test_verifier: mask bpf_csum_diff() return value to 16 bits

From: Daniel Borkmann <daniel@iogearbox.net>
To: Yonghong Song <yhs@fb.com>,
	Yauheni Kaliuta <yauheni.kaliuta@redhat.com>,
	bpf@vger.kernel.org
Cc: toke@redhat.com
Subject: Re: [PATCH] bpf: selftests: test_verifier: mask bpf_csum_diff() return value to 16 bits
Date: Tue, 2 Mar 2021 12:14:38 +0100	[thread overview]
Message-ID: <db5684bf-3757-3fdf-2581-002191f7a899@iogearbox.net> (raw)
In-Reply-To: <acf00517-6129-869b-cd2a-03715de5fc61@fb.com>

On 3/1/21 6:18 AM, Yonghong Song wrote:
> On 2/28/21 2:30 AM, Yauheni Kaliuta wrote:
>> The verifier test labelled "valid read map access into a read-only array
>> 2" calls the bpf_csum_diff() helper and checks its return value.
>> However, architecture implementations of csum_partial() (which is what
>> the helper uses) differ in whether they fold the return value to 16 bit
>> or not. For example, x86 version has:
>>
>>     if (unlikely(odd)) {
>>         result = from32to16(result);
>>         result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
>>     }
>>
>> while generic lib/checksum.c does:
>>
>>     result = from32to16(result);
>>     if (odd)
>>         result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
>>
>> This makes the helper return different values on different
>> architectures, breaking the test on non-x86. To fix this, add an
> 
> I remember there is a previous discussion for this issue, csum_diff()
> returns different results for different architecture? Daniel?
> Any conclusion how to deal with this?

I took in the test case fix for now. After getting cascading work for !x86-64,
we can always revert this one again. Plan of action to resume this work was
to at least update the other 64-bit archs successively to a no-fold variant as
well, so their arch specific implementations can mimic what x86-64 is doing.

>> additional instruction to always mask the return value to 16 bits, and
>> update the expected return value accordingly.
>>
>> Fixes: fb2abb73e575 ("bpf, selftest: test {rd, wr}only flags and direct value access")
>> Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
>> ---
>>   tools/testing/selftests/bpf/verifier/array_access.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/bpf/verifier/array_access.c b/tools/testing/selftests/bpf/verifier/array_access.c
>> index bed53b561e04..1b138cd2b187 100644
>> --- a/tools/testing/selftests/bpf/verifier/array_access.c
>> +++ b/tools/testing/selftests/bpf/verifier/array_access.c
>> @@ -250,12 +250,13 @@
>>       BPF_MOV64_IMM(BPF_REG_5, 0),
>>       BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
>>                BPF_FUNC_csum_diff),
>> +    BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xffff),
>>       BPF_EXIT_INSN(),
>>       },
>>       .prog_type = BPF_PROG_TYPE_SCHED_CLS,
>>       .fixup_map_array_ro = { 3 },
>>       .result = ACCEPT,
>> -    .retval = -29,
>> +    .retval = 65507,
>>   },
>>   {
>>       "invalid write map access into a read-only array 1",
>>