Re: [Qemu-devel] [PATCH] Fix NBD unsupported options

[Paolo Bonzini]

On 05/04/2016 20:55, Alex Bligh wrote:
> nbd-client.c currently fails to handle unsupported options properly.
> If during option haggling the server finds an option that is
> unsupported, it returns an NBD_REP_ERR_UNSUP reply.
> 
> According to nbd's proto.md, the format for such a reply
> should be:
> 
>   S: 64 bits, 0x3e889045565a9 (magic number for replies)
>   S: 32 bits, the option as sent by the client to which this is a reply
>   S: 32 bits, reply type (e.g., NBD_REP_ACK for successful completion,
>      or NBD_REP_ERR_UNSUP to mark use of an option not known by this server
>   S: 32 bits, length of the reply. This may be zero for some replies,
>      in which case the next field is not sent
>   S: any data as required by the reply (e.g., an export name in the case
>      of NBD_REP_SERVER)
> 
> However, in nbd-client.c, the reply type was being read, and if it
> contained an error, it was bailing out and issuing the next option
> request without first reading the length. This meant that the
> next option / handshake read had an extra 4 bytes of data in it.
> In practice, this makes Qemu incompatible with servers that do not
> support NBD_OPT_LIST.
> 
> To verify this isn't an error in the specification or my reading of
> it, replies are sent by the reference implementation here:
>  https://github.com/yoe/nbd/blob/master/nbd-server.c#L1232
> and as is evident it always sends a 'datasize' (aka length) 32 bit
> word. Unsupported elements are replied to here:
>  https://github.com/yoe/nbd/blob/master/nbd-server.c#L1371
> 
> Signed-off-by: Alex Bligh <address@hidden>
> ---
>  nbd/client.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/nbd/client.c b/nbd/client.c
> index d9b7a9b..6f0541d 100644
> --- a/nbd/client.c
> +++ b/nbd/client.c
> @@ -138,12 +138,6 @@ static int nbd_receive_list(QIOChannel *ioc, char 
> **name, Error **errp)
>          return -1;
>      }
>      type = be32_to_cpu(type);
> -    if (type == NBD_REP_ERR_UNSUP) {
> -        return 0;
> -    }
> -    if (nbd_handle_reply_err(opt, type, errp) < 0) {
> -        return -1;
> -    }
>  
>      if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {
>          error_setg(errp, "failed to read option length");
> @@ -151,6 +145,13 @@ static int nbd_receive_list(QIOChannel *ioc, char 
> **name, Error **errp)
>      }
>      len = be32_to_cpu(len);
>  
> +    if (type == NBD_REP_ERR_UNSUP) {
> +        return 0;
> +    }
> +    if (nbd_handle_reply_err(opt, type, errp) < 0) {
> +        return -1;
> +    }
> +
>      if (type == NBD_REP_ACK) {
>          if (len != 0) {
>              error_setg(errp, "length too long for option end");
> 

Thanks, queued.

Paolo