Re: [PATCH v3 7/9] checksum: introduce functions to compute the header part checksum for TCP/UDP

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 29 Feb 2024 09:56:25 +0100
Stefano Brivio <sbrivio@redhat.com> wrote:

> On Thu, 29 Feb 2024 19:49:09 +1100
> David Gibson <david@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@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.

Inconsistent if you want, but it's quite simple and it works, plus if you
had half a mind (at least for UDP) to split buffers into header and
payloads iovec parts... this doesn't need to be exceedingly elegant.

-- 
Stefano