[Qemu-devel] Re: [PATCH] slirp: ensure minimum packet size

["Hervé Poussineau" <hpoussin@reactos.org>]

Hi,

qemu-devel-request@nongnu.org a écrit :
> Date: Thu, 10 Feb 2011 15:54:28 -0700
> From: "Bruce Rogers" <brogers@novell.com>
> Subject: [Qemu-devel] [PATCH] slirp: ensure minimum packet size
> To: <qemu-devel@nongnu.org>
> Message-ID: <4D540A3402000048000A9C8C@novprvoes0310.provo.novell.com>
> Content-Type: text/plain; charset=US-ASCII
>
> With recent gpxe eepro100 drivers, short packets are rejected,
> so ensure the minimum ethernet packet size.
>
> Signed-off-by: Bruce Rogers <brogers@novell.com>
> ---
>  slirp/slirp.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/slirp/slirp.c b/slirp/slirp.c
> index 332d83b..b611cf7 100644
> --- a/slirp/slirp.c
> +++ b/slirp/slirp.c
> @@ -697,7 +697,7 @@ void if_encap(Slirp *slirp, const uint8_t *ip_data, int ip_data_len)
>          return;
>      
>      if (!memcmp(slirp->client_ethaddr, zero_ethaddr, ETH_ALEN)) {
> -        uint8_t arp_req[ETH_HLEN + sizeof(struct arphdr)];
> +        uint8_t arp_req[max(ETH_HLEN + sizeof(struct arphdr), 64)];
>          struct ethhdr *reh = (struct ethhdr *)arp_req;
>          struct arphdr *rah = (struct arphdr *)(arp_req + ETH_HLEN);
>          const struct ip *iph = (const struct ip *)ip_data;
>   
Ack for this part.
Slirp must not generate frames which are not 802.3 compliant, ie less 
than 64 bytes.
A similar fix has already been done at 
http://git.qemu.org/qemu.git/commit/?id=dbf3c4b4baceb91eb64d09f787cbe92d65188813
On a side note, maybe you need to add a memset to clear the unused part 
of the buffer?

> @@ -734,7 +734,7 @@ void if_encap(Slirp *slirp, const uint8_t *ip_data, int ip_data_len)
>          memcpy(&eh->h_source[2], &slirp->vhost_addr, 4);
>          eh->h_proto = htons(ETH_P_IP);
>          memcpy(buf + sizeof(struct ethhdr), ip_data, ip_data_len);
> -        slirp_output(slirp->opaque, buf, ip_data_len + ETH_HLEN);
> +        slirp_output(slirp->opaque, buf, max(ip_data_len + ETH_HLEN, 64));
>      }
>  }
>  
>   
Nack for this part.
Here, you're limiting the Ethernet frame to at most 64 bytes. You 
probably meant min(ip_data_len + ETH_HLEN, 64).

Even with min(), I'm not sure if it is slirp responsability to increase 
buffer size to meet Ethernet standards.

Hervé