Re: [RFC PATCH 1/3] net: Pad short frames to minimum size (60 bytes)

[Philippe Mathieu-Daudé]

On 2/26/21 11:03 AM, Bin Meng wrote:
> From: Bin Meng <bin.meng@windriver.com>
> 
> The minimum Ethernet frame length is 60 bytes, and we should pad
> frames whose length is smaller to the minimum size.
> 
> This commit fixes the issue as seen with various ethernet models,
> that ARP requests get dropped, preventing the guest from becoming
> visible on the network.

Is it also used in commit 18995b9808d ("Send a RARP packet after
migration.")?

> The following 2 commits that attempted to workaround this issue
> in e1000 and vmxenet3 before, should be reverted.
> 
>   commit 78aeb23eded2 ("e1000: Pad short frames to minimum size (60 bytes)")
>   commit 40a87c6c9b11 ("vmxnet3: Pad short frames to minimum size (60 bytes)")
> 
> Signed-off-by: Bin Meng <bin.meng@windriver.com>
> ---
> 
>  net/net.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/net/net.c b/net/net.c
> index b038370..34004da 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -638,6 +638,7 @@ static ssize_t 
> qemu_send_packet_async_with_flags(NetClientState *sender,
>                                                   NetPacketSent *sent_cb)
>  {
>      NetQueue *queue;
> +    uint8_t min_buf[60];

Can you add a definition instead of a magic value?
Maybe ETH_FRAME_MIN_LEN in "net/eth.h"?

>      int ret;
>  
>  #ifdef DEBUG_NET
> @@ -649,6 +650,14 @@ static ssize_t 
> qemu_send_packet_async_with_flags(NetClientState *sender,
>          return size;
>      }
>  
> +    /* Pad to minimum Ethernet frame length */
> +    if (size < sizeof(min_buf)) {
> +        memcpy(min_buf, buf, size);
> +        memset(&min_buf[size], 0, sizeof(min_buf) - size);
> +        buf = min_buf;
> +        size = sizeof(min_buf);
> +    }
> +
>      /* Let filters handle the packet first */
>      ret = filter_receive(sender, NET_FILTER_DIRECTION_TX,
>                           sender, flags, buf, size, sent_cb);
>