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: <ZcpmWINCFWd4331h@ghost>
Date: Mon, 12 Feb 2024 13:41:28 -0500
From: Charlie Jenkins <charlie@...osinc.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Al Viro <viro@...iv.linux.org.uk>,
	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 Mon, Feb 12, 2024 at 10:34:10AM -0800, Guenter Roeck wrote:
> 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
> 

We can align this against IP_ALIGNMENT since it should always be aligned
against the IP header + NET_IP_ALIGN.

- Charlie