Re: [Qemu-devel] [PATCH 1/6] rtl8139: limit transmission buffer size in c+ mode

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Mon, Mar 05, 2012 at 11:08:24AM +0800, Jason Wang wrote:
> The tx buffer would be re-allocated for tx descriptor with big size
> and without LS bit set, this would make guest driver could easily let
> qemu to allocate unlimited.
> 
> In linux host, a glib failure were easy to be triggered:
> 
> GLib-ERROR **: gmem.c:176: failed to allocate 18446744071562067968 bytes
> 
> This patch fix this by adding a limit. As the spec didn't tell the maximum size
> of buffer allowed, stick it to  current CP_TX_BUFFER_SIZE (65536).
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  hw/rtl8139.c |    9 ++++-----
>  1 files changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/rtl8139.c b/hw/rtl8139.c
> index 05b8e1e..d9e742c 100644
> --- a/hw/rtl8139.c
> +++ b/hw/rtl8139.c
> @@ -2063,11 +2063,10 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
> 
>      while (s->cplus_txbuffer && s->cplus_txbuffer_offset + txsize >= s->cplus_txbuffer_len)
>      {
> -        s->cplus_txbuffer_len += CP_TX_BUFFER_SIZE;
> -        s->cplus_txbuffer = g_realloc(s->cplus_txbuffer, s->cplus_txbuffer_len);
> -
> -        DPRINTF("+++ C+ mode transmission buffer space changed to %d\n",
> -            s->cplus_txbuffer_len);
> +        /* The spec didn't tell the maximum size, stick to CP_TX_BUFFER_SIZE */
> +        txsize = s->cplus_txbuffer_len - s->cplus_txbuffer_offset;
> +        DPRINTF("+++ C+ mode transmission buffer overrun, truncated descriptor"
> +                "length to %d\n", txsize);

This makes sense to me since a 64 KB MTU is not possible - it's a safe
upper limit that will not impact guests.

Please change this while statement to an if statement and drop the
s->cplus_txbuffer check (which is always true since we allocate
immediately prior, if it is not already allocated).

Stefan