Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests

[Guenter Roeck <linux@roeck-us.net>]

On 2/26/24 03:34, Christophe Leroy wrote:
> 
> 
> Le 23/02/2024 à 23:11, Charlie Jenkins a écrit :
>> The test cases for ip_fast_csum and csum_ipv6_magic were not properly
>> aligning the IP header, which were causing failures on architectures
>> that do not support misaligned accesses like some ARM platforms. To
>> solve this, align the data along (14 + NET_IP_ALIGN) bytes which is the
>> standard alignment of an IP header and must be supported by the
>> architecture.
> 
> I'm still wondering what we are really trying to fix here.
> 
> All other tests are explicitely testing that it works with any alignment.
> 
> Shouldn't ip_fast_csum() and csum_ipv6_magic() work for any alignment as
> well ? I would expect it, I see no comment in arm code which explicits
> that assumption around those functions.
> 
> Isn't the problem only the following line, because csum_offset is
> unaligned ?
> 
> csum = *(__wsum *)(random_buf + i + csum_offset);
> 
> Otherwise, if there really is an alignment issue for the IPv6 source or
> destination address, isn't it enough to perform a 32 bits alignment ?
> 

It isn't just arm.

Question should be what alignments the functions are supposed to be able
to handle, not what they are optimized for. If byte and/or half word alignments
are expected to be supported, there is still architecture code which would
have to be fixed. Unaligned accesses are known to fail on hppa64/parisc64
and on sh4, for example. If unaligned accesses are expected to be handled,
it would probably make sense to add a separate test case, though, to clarify
that the test fails due to alignment issues, not due to input parameters.

Thanks,
Guenter