Re: [PATCH v2] Fix AF_PACKET ABI breakage in 4.2

[Greg Kurz <gkurz-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

On Wed, 23 Sep 2015 19:45:08 +0100
David Woodhouse <dwmw2-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org> wrote:

> Commit 7d82410950aa ("virtio: add explicit big-endian support to memory
> accessors") accidentally changed the virtio_net header used by
> AF_PACKET with PACKET_VNET_HDR from host-endian to big-endian.
> 

Hi David,

Oops my bad... I obviously overlooked this one when adding cross-endian
support.

> Since virtio_legacy_is_little_endian() is a very long identifier,
> define a VIO_LE macro and use that throughout the code instead of the

VIO usually refers to virtual IO adapters for the PowerPC pSeries platform.

> hard-coded 'false' for little-endian.
> 

What about introducing dedicated accessors as it is done in many other
locations where we do virtio byteswap ? Something like:

static inline bool packet_is_little_endian(void)
{
	return virtio_legacy_is_little_endian();
}

static inline u16 packet16_to_cpu(__virtio16 val)
{
	return __virtio16_to_cpu(packet_is_little_endian(), val);
}

static inline __virtio16 cpu_to_packet16(u16 val)
{
	return __cpu_to_virtio16(packet_is_little_endian(), val);
}

It results in prettier code IMHO. Have a look at drivers/net/tun.c or
drivers/vhost/vhost.c.

> This restores the ABI to match 4.1 and earlier kernels, and makes my
> test program work again.
> 

BTW, there is still work to do if we want to support cross-endian legacy or
virtio 1 on a big endian arch...

Cheers.

--
Greg

> Signed-off-by: David Woodhouse <David.Woodhouse-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
> On Wed, 2015-09-23 at 11:09 -0700, David Miller wrote:
> > > +#define VIO_LE virtio_legacy_is_little_endian()
> > 
> > When you define a shorthand macro, the defines to a function call,
> > make the macro have parenthesis too.
> 
> In which case I suppose it also wants to be lower-case. Although
> "function call" is a bit strong since it's effectively just a constant.
> I'm still wondering if it'd be nicer just to use (__force u16) instead.
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 7b8e39a..aa4b15c 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -230,6 +230,8 @@ struct packet_skb_cb {
>  	} sa;
>  };
>  
> +#define vio_le() virtio_legacy_is_little_endian()
> +
>  #define PACKET_SKB_CB(__skb)	((struct packet_skb_cb *)((__skb)->cb))
>  
>  #define GET_PBDQC_FROM_RB(x)	((struct tpacket_kbdq_core *)(&(x)->prb_bdqc))
> @@ -2680,15 +2682,15 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>  			goto out_unlock;
>  
>  		if ((vnet_hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
> -		    (__virtio16_to_cpu(false, vnet_hdr.csum_start) +
> -		     __virtio16_to_cpu(false, vnet_hdr.csum_offset) + 2 >
> -		      __virtio16_to_cpu(false, vnet_hdr.hdr_len)))
> -			vnet_hdr.hdr_len = __cpu_to_virtio16(false,
> -				 __virtio16_to_cpu(false, vnet_hdr.csum_start) +
> -				__virtio16_to_cpu(false, vnet_hdr.csum_offset) + 2);
> +		    (__virtio16_to_cpu(vio_le(), vnet_hdr.csum_start) +
> +		     __virtio16_to_cpu(vio_le(), vnet_hdr.csum_offset) + 2 >
> +		      __virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len)))
> +			vnet_hdr.hdr_len = __cpu_to_virtio16(vio_le(),
> +				 __virtio16_to_cpu(vio_le(), vnet_hdr.csum_start) +
> +				__virtio16_to_cpu(vio_le(), vnet_hdr.csum_offset) + 2);
>  
>  		err = -EINVAL;
> -		if (__virtio16_to_cpu(false, vnet_hdr.hdr_len) > len)
> +		if (__virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len) > len)
>  			goto out_unlock;
>  
>  		if (vnet_hdr.gso_type != VIRTIO_NET_HDR_GSO_NONE) {
> @@ -2731,7 +2733,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>  	hlen = LL_RESERVED_SPACE(dev);
>  	tlen = dev->needed_tailroom;
>  	skb = packet_alloc_skb(sk, hlen + tlen, hlen, len,
> -			       __virtio16_to_cpu(false, vnet_hdr.hdr_len),
> +			       __virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len),
>  			       msg->msg_flags & MSG_DONTWAIT, &err);
>  	if (skb == NULL)
>  		goto out_unlock;
> @@ -2778,8 +2780,8 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>  
>  	if (po->has_vnet_hdr) {
>  		if (vnet_hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> -			u16 s = __virtio16_to_cpu(false, vnet_hdr.csum_start);
> -			u16 o = __virtio16_to_cpu(false, vnet_hdr.csum_offset);
> +			u16 s = __virtio16_to_cpu(vio_le(), vnet_hdr.csum_start);
> +			u16 o = __virtio16_to_cpu(vio_le(), vnet_hdr.csum_offset);
>  			if (!skb_partial_csum_set(skb, s, o)) {
>  				err = -EINVAL;
>  				goto out_free;
> @@ -2787,7 +2789,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>  		}
>  
>  		skb_shinfo(skb)->gso_size =
> -			__virtio16_to_cpu(false, vnet_hdr.gso_size);
> +			__virtio16_to_cpu(vio_le(), vnet_hdr.gso_size);
>  		skb_shinfo(skb)->gso_type = gso_type;
>  
>  		/* Header must be checked, and gso_segs computed. */
> @@ -3161,9 +3163,9 @@ static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
>  
>  			/* This is a hint as to how much should be linear. */
>  			vnet_hdr.hdr_len =
> -				__cpu_to_virtio16(false, skb_headlen(skb));
> +				__cpu_to_virtio16(vio_le(), skb_headlen(skb));
>  			vnet_hdr.gso_size =
> -				__cpu_to_virtio16(false, sinfo->gso_size);
> +				__cpu_to_virtio16(vio_le(), sinfo->gso_size);
>  			if (sinfo->gso_type & SKB_GSO_TCPV4)
>  				vnet_hdr.gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
>  			else if (sinfo->gso_type & SKB_GSO_TCPV6)
> @@ -3181,9 +3183,9 @@ static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
>  
>  		if (skb->ip_summed == CHECKSUM_PARTIAL) {
>  			vnet_hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
> -			vnet_hdr.csum_start = __cpu_to_virtio16(false,
> +			vnet_hdr.csum_start = __cpu_to_virtio16(vio_le(),
>  					  skb_checksum_start_offset(skb));
> -			vnet_hdr.csum_offset = __cpu_to_virtio16(false,
> +			vnet_hdr.csum_offset = __cpu_to_virtio16(vio_le(),
>  							 skb->csum_offset);
>  		} else if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
>  			vnet_hdr.flags = VIRTIO_NET_HDR_F_DATA_VALID;
>