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 06:26:14 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Guenter Roeck <linux@...ck-us.net>,
	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 Wed, Feb 07, 2024 at 04:22:51PM -0800, Charlie Jenkins wrote:
> +	struct csum_ipv6_magic_data {
> +		const struct in6_addr saddr;
> +		const struct in6_addr daddr;
> +		unsigned int len;
> +		__wsum csum;
> +		unsigned char proto;
> +	} data, *data_ptr;

Huh?

> +	int num_tests = MAX_LEN / WORD_ALIGNMENT - sizeof(struct csum_ipv6_magic_data);
> +
> +	for (int i = 0; i < num_tests; i++) {
> +		data_ptr = (struct csum_ipv6_magic_data *)(random_buf + (i * WORD_ALIGNMENT));
> +
> +		cpu_to_be32_array((__be32 *)&data.saddr, (const u32 *)&data_ptr->saddr,
> +				  sizeof_field(struct csum_ipv6_magic_data, saddr) / 4);
> +		cpu_to_be32_array((__be32 *)&data.daddr, (const u32 *)&data_ptr->daddr,
> +				  sizeof_field(struct csum_ipv6_magic_data, daddr) / 4);
> +		data.len = data_ptr->len;
> +		data.csum = (__force __wsum)htonl((__force u32)data_ptr->csum);

What are those cpu_to_be32() about?  Checksum calculations *DO* *NOT* involve
any endianness conversions.  At any point.

Replace those assignments with memcpy() and be done with that - that will take
care of unaligned accesses.

Result will have host-independent memory representation.  The only place where you
might want to play with byteswaps (only 16-bit ones) is if you initialized the
array of expected results with u16 constants.  That will have opposite memory
representations on l-e and b-e, so you'll need to byteswap to compare with
what you get from function.  Alternatively, make it an array of bytes and
do
	sum16 = csum_ipv6_magic(saddr, daddr, len, proto, csum);
	if (memcmp(sum16, expected_csum_ipv6_magic + i * 2, 2))
		complain

That's it.