linux-kernel - Re: [PATCH v6 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Message-ID: <c5d8a333-aff5-49e1-a7ee-29a023266d2d@roeck-us.net>
Date: Mon, 12 Feb 2024 10:34:10 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Charlie Jenkins <charlie@...osinc.com>,
 David Laight <David.Laight@...lab.com>, Palmer Dabbelt <palmer@...belt.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/2] lib: checksum: Use aligned accesses for
 ip_fast_csum and csum_ipv6_magic tests

On 2/12/24 10:12, Al Viro wrote:
> On Mon, Feb 12, 2024 at 09:18:14AM -0800, Guenter Roeck wrote:
> 
>> Almost. Turns out the csum parameter of csum_ipv6_magic() needs to be in
>> network byte order, and the length parameter needs to be in host byte order.
>> So instead of
>> 	data.len = data_ptr->len;
>> 	data.csum = (__force __wsum)htonl((__force u32)data_ptr->csum);
>> it needs to be something like
>> 	data.len = ntohl(data_ptr->len);
>> 	data.csum = data_ptr->csum;
>>
>> Also, as you mentioned, either the returned checksum or the expected
>> checksum needs to be converted for the comparison because one is in
>> network byte order and the other in host byte order.
> 
>          for (int i = 0; i < NUM_IPv6_TESTS; i++) {
> 		struct args {
> 			struct in6_addr saddr;
> 			struct in6_addr daddr;
> 			__be32 len;
> 			__wsum csum;
> 			unsigned char proto;
> 		} __packed data = (struct args *)(random_buf + i);
>                  CHECK_EQ(cpu_to_le16(expected_csum_ipv6_magic[i]),
>                           csum_ipv6_magic(data.saddr, data.daddr, ntohl(data.len),
> 					 data.proto, data.sum));
>          }
> and to hell with field-by-field copying.  __packed here will tell the compiler
> that alignment of the entire thing is 1 - the total size of fields is 41 bytes,
> so "no padding" translates into "can't even assume that address is even".
> 

Except that the data pointer needs to be aligned because otherwise architectures
not supporting unaligned accesses will bail out (as observed on mps2-an385).
But then I am no longer sure if that is correct - maybe csum_ipv6_magic()
is supposed to be able to handle unaligned accesses. If so, maybe it would be
more appropriate to skip this test on the affected architectures.

Guenter