netdev - Re: [PATCH bpf-next v2 3/3] selftests/bpf: Fix test for 4-byte load from dst

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Date:   Thu, 03 Mar 2022 18:12:02 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        kernel-team@...udflare.com, Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: Re: [PATCH bpf-next v2 3/3] selftests/bpf: Fix test for 4-byte load
 from dst_port on big-endian

On Mon, Feb 28, 2022 at 10:22 PM -08, Martin KaFai Lau wrote:
> On Sun, Feb 27, 2022 at 09:27:57PM +0100, Jakub Sitnicki wrote:

[...]

>> --- a/tools/testing/selftests/bpf/progs/test_sock_fields.c
>> +++ b/tools/testing/selftests/bpf/progs/test_sock_fields.c
>> @@ -256,10 +256,23 @@ int ingress_read_sock_fields(struct __sk_buff *skb)
>>  	return CG_OK;
>>  }
>>  
>> +/*
>> + * NOTE: 4-byte load from bpf_sock at dst_port offset is quirky. The
>> + * result is left shifted on little-endian architectures because the
>> + * access is converted to a 2-byte load. The quirky behavior is kept
>> + * for backward compatibility.
>> + */
>>  static __noinline bool sk_dst_port__load_word(struct bpf_sock *sk)
>>  {
>> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
>> +	const __u8 SHIFT = 16;
>> +#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
>> +	const __u8 SHIFT = 0;
>> +#else
>> +#error "Unrecognized __BYTE_ORDER__"
>> +#endif
>>  	__u32 *word = (__u32 *)&sk->dst_port;
>> -	return word[0] == bpf_htonl(0xcafe0000);
>> +	return word[0] == bpf_htonl(0xcafe << SHIFT);
> I believe it should be fine.  It is the behavior even before
> commit 4421a582718a ("bpf: Make dst_port field in struct bpf_sock 16-bit wide") ?

Yes, exactly. AFAICT there was no change in behavior in commit
4421a582718a, that is:

  1. 4-byte load behaves like it did, in its quirky way,
  2. 2-byte load at offset dst_port works the same
  3. 2-byte load at offset dst_port+2 continues to be rejected.

> btw, is it the same as testing "return word[0] == bpf_hton's'(0xcafe);"

Right. Clever observation. I got the impression from the original
problem report [1] that the users were failing when trying to do:

  bpf_htonl(sk->dst_port) == 0xcafe

Hence I the bpf_htonl() use here.

But perhaps it's better to promote this cleaner pattern in tests.

I will respin it once we hash out the details of what the access should
look like on big-endian with Ilya.

[1] https://lore.kernel.org/bpf/20220113070245.791577-1-imagedong@tencent.com/

[...]