Re: [PATCH v6 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests

From: Al Viro
Date: Mon Feb 12 2024 - 13:12:37 EST

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);
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.