On Fri, Mar 01, 2024 at 07:56:51AM +0100, Stefano Brivio wrote:On Fri, 1 Mar 2024 10:09:39 +1100 David Gibson <david(a)gibson.dropbear.id.au> wrote:No... the loads are still from b->ip6h.saddr, b->ip6h.daddr and b->ip6h.payload_len. Just because we're computing the offset a bit differently doesn't change the load itself. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibsonOn Thu, Feb 29, 2024 at 03:15:53PM +0100, Stefano Brivio wrote:For UDP and IPv4 (from 6/9): + b->iph.check = csum_ip4_header(b->iph.tot_len, IPPROTO_UDP, + b->iph.saddr, b->iph.daddr); and for IPv6 (this patch): + b->uh.check = csum(&b->uh, ntohs(b->ip6h.payload_len), + proto_ipv6_header_psum(b->ip6h.payload_len, + IPPROTO_UDP, + b->ip6h.saddr, + b->ip6h.daddr)); these cause loads starting from 'b', which is aligned, instead of passing 'iph' or 'ip6h', unaligned, and loading from there.On Thu, 29 Feb 2024 09:56:25 +0100 Stefano Brivio <sbrivio(a)redhat.com> wrote:Ok... but my point remains, I'm not seeing that passing the address by value actually helps - it just seems to change whether we need to handle the unaligned load in the caller or the callee.On Thu, 29 Feb 2024 19:49:09 +1100 David Gibson <david(a)gibson.dropbear.id.au> wrote: > On Thu, Feb 29, 2024 at 08:05:09AM +0100, Stefano Brivio wrote: > > On Thu, 29 Feb 2024 11:38:53 +1100 > > David Gibson <david(a)gibson.dropbear.id.au> wrote: > > > > > On Wed, Feb 28, 2024 at 02:26:18PM +0100, Laurent Vivier wrote: > > > > On 2/19/24 04:08, David Gibson wrote: > > > > > On Sat, Feb 17, 2024 at 04:07:23PM +0100, Laurent Vivier wrote: > > > > > > > > > > [...] > > > > > > > > > > > +/** > > > > > > + * proto_ipv6_header_psum() - Calculates the partial checksum of an > > > > > > + * IPv6 header for UDP or TCP > > > > > > + * @payload_len: Payload length > > > > > > + * @proto: Protocol number > > > > > > + * @saddr: Source address > > > > > > + * @daddr: Destination address > > > > > > + * Returns: Partial checksum of the IPv6 header > > > > > > + */ > > > > > > +uint32_t proto_ipv6_header_psum(uint16_t payload_len, uint8_t protocol, > > > > > > + struct in6_addr saddr, struct in6_addr daddr) > > > > > > > > > > Hrm, this is passing 2 16-byte IPv6 addresses by value, which might > > > > > not be what we want. > > > > > > > > The idea here is to avoid the pointer alignment problem (&ip6h->saddr and > > > > &ip6h->daddr can be misaligned). > > > > > > Ah, right. That's a neat idea, but I'm not sure it really helps: I > > > think it will just move the misaligned access from inside the function > > > to the call site, where we try to marshal the parameter from something > > > unaligned. > > > > I haven't tested this yet, but note that this is generally okay: the > > problem is *dereferencing* an unaligned pointer. But if you load memory > > from an aligned pointer, and extract a value from this memory, it's all > > fine. > > Right, that's kind of what I'm getting at. Assuming this value starts > in an unaligned buffer, then in order to pass this by value the caller > will need to load from that unaligned pointer. AFAIK, the compiler > will base the type of loads only on the pointed to type, which isn't > changed whether we dereference in the caller or the callee. > > > > > Speaking MIPS, this is not safe on all CPU models: > > > > la $1, 1002 # s1 now contains the value 1002 > > lw $2, 0($1) # load word from memory at 1002 + 0 into s2 > > > > but this is: > > > > la $1, 1000 # s1 now contains the value 1000 > > la $2, 1004 # s3 now contains the value 1004 > > lw $3, 0($1) # load word from memory at 1000 + 0 into s3 > > lw $4, 0($3) # load word from memory at 1004 + 0 into s4 > > sll $5, $3, 16 # 16-bit shift left s3 into s5 > > srl $6, $4, 16 # 16-bit shift right s4 into s6 > > or $2, $5, $6 # OR s5 and s6 into s2 > > Right, but I don't think merely moving the dereference to the caller > will necessarily induce the compiler to generate this rather than the > former. Oh, oops, I didn't realise this was the case (I haven't reviewed the patch yet)....no, that's not the case. Dereferencing 'iph' from struct tcp[46]_l2_buf_t is fine: struct tcp4_l2_buf_t { uint8_t pad[2]; /* 0 2 */ struct tap_hdr taph; /* 2 18 */ struct iphdr iph; /* 20 20 */ [...] } __attribute__((__packed__)); struct tcp6_l2_buf_t { uint8_t pad[2]; /* 0 2 */ struct tap_hdr taph; /* 2 18 */ struct ipv6hdr ip6h; /* 20 40 */ [...] } __attribute__((__packed__)); The problematic structures are the UDP buffers: struct udp4_l2_buf_t { struct sockaddr_in s_in; /* 0 16 */ struct tap_hdr taph; /* 16 18 */ struct iphdr iph; /* 34 20 */ [...] } __attribute__((__aligned__(4))); and for UDP, this patch is dereferencing buffer pointers only, not pointers to headers.