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@xxxxxxxxxxx/
[...]
