Re: [Qemu-devel] [PATCH] [slirp] Make ARP replies at least 64 bytes long

[Edgar E. Iglesias]

On Mon, Sep 13, 2010 at 11:02:42PM +0200, Hervé Poussineau wrote:
> IEEE 802.3 standard requires Ethernet frames to be at least 64 bytes long.
> If it is not the case, they will be considered as runt frames, and may be 
> ignored by netcard and/or OS
> 
> Signed-off-by: Hervé Poussineau <address@hidden>
> ---
>  slirp/slirp.c |   12 ++++++++----
>  1 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/slirp/slirp.c b/slirp/slirp.c
> index 82fd9b4..2e8c017 100644
> --- a/slirp/slirp.c
> +++ b/slirp/slirp.c
> @@ -599,9 +599,12 @@ static void arp_input(Slirp *slirp, const uint8_t *pkt, 
> int pkt_len)
>  {
>      struct ethhdr *eh = (struct ethhdr *)pkt;
>      struct arphdr *ah = (struct arphdr *)(pkt + ETH_HLEN);
> -    uint8_t arp_reply[ETH_HLEN + sizeof(struct arphdr)];
> -    struct ethhdr *reh = (struct ethhdr *)arp_reply;
> -    struct arphdr *rah = (struct arphdr *)(arp_reply + ETH_HLEN);
> +    union {
> +        uint8_t data[ETH_HLEN + sizeof(struct arphdr)];
> +        uint8_t payload[64]; /* Minimum Ethernet frame size */
> +    } arp_reply;
> +    struct ethhdr *reh = (struct ethhdr *)arp_reply.data;
> +    struct arphdr *rah = (struct arphdr *)(arp_reply.data + ETH_HLEN);


Hi,

Would you mind explaning the point of the union here? Why not just
do something like:

-    uint8_t arp_reply[ETH_HLEN + sizeof(struct arphdr)];
+    uint8_t arp_reply[MAX(ETH_HLEN + sizeof(struct arphdr), 64)];

Cheers


>      int ar_op;
>      struct ex_list *ex_ptr;
>  
> @@ -619,6 +622,7 @@ static void arp_input(Slirp *slirp, const uint8_t *pkt, 
> int pkt_len)
>              }
>              return;
>          arp_ok:
> +            memset(&arp_reply, 0, sizeof(arp_reply));
>              /* XXX: make an ARP request to have the client address */
>              memcpy(slirp->client_ethaddr, eh->h_source, ETH_ALEN);
>  
> @@ -637,7 +641,7 @@ static void arp_input(Slirp *slirp, const uint8_t *pkt, 
> int pkt_len)
>              rah->ar_sip = ah->ar_tip;
>              memcpy(rah->ar_tha, ah->ar_sha, ETH_ALEN);
>              rah->ar_tip = ah->ar_sip;
> -            slirp_output(slirp->opaque, arp_reply, sizeof(arp_reply));
> +            slirp_output(slirp->opaque, (const uint8_t *)&arp_reply, 
> sizeof(arp_reply));
>          }
>          break;
>      case ARPOP_REPLY:
> -- 
> 1.7.1.GIT
> 
>