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

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Date: Mon, 12 Feb 2024 18:12:17 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Guenter Roeck <linux@...ck-us.net>
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 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".

And I'd probably turn that expect_csum....[] array into an array of bytes,
with *(__sum16*)(expected_csum_ipv6_magic + 2 * i) in your CHECK_EQ instead
of arch-dependent byteswaps.