Re: [Qemu-devel] [PATCH 05/14] nbd/client: Drop pointless buf variable

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Nov 30, 2018 at 04:03:34PM -0600, Eric Blake wrote:
> There's no need to read into a temporary buffer (oversized
> since commit 7d3123e1) followed by a byteswap into a uint64_t
> to check for a magic number via memcmp(), when the code
> immediately below demonstrates reading into the uint64_t then
> byteswapping in place and checking for a magic number via
> integer math.  What's more, having a different error message
> when the server's first reply byte is 0 is unusual - it's no
> different from any other wrong magic number, and we already
> detected short reads.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  nbd/nbd-internal.h |  1 +
>  nbd/client.c       | 14 +++-----------
>  2 files changed, 4 insertions(+), 11 deletions(-)
> 
> diff --git a/nbd/nbd-internal.h b/nbd/nbd-internal.h
> index eeff78d3c98..306a533dcd1 100644
> --- a/nbd/nbd-internal.h
> +++ b/nbd/nbd-internal.h
> @@ -46,6 +46,7 @@
>  /* Size of oldstyle negotiation */
>  #define NBD_OLDSTYLE_NEGOTIATE_SIZE (8 + 8 + 8 + 4 + 124)
> 
> +#define NBD_INIT_MAGIC              0x4e42444d41474943LL
>  #define NBD_REQUEST_MAGIC           0x25609513
>  #define NBD_OPTS_MAGIC              0x49484156454F5054LL
>  #define NBD_CLIENT_MAGIC            0x0000420281861253LL
> diff --git a/nbd/client.c b/nbd/client.c
> index 0be89f9e641..17ee24492a4 100644
> --- a/nbd/client.c
> +++ b/nbd/client.c
> @@ -731,7 +731,6 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name,
>                            QIOChannel **outioc, NBDExportInfo *info,
>                            Error **errp)
>  {
> -    char buf[256];
>      uint64_t magic;
>      int rc;
>      bool zeroes = true;
> @@ -752,21 +751,14 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name,
>          goto fail;
>      }
> 
> -    if (nbd_read(ioc, buf, 8, errp) < 0) {
> +    if (nbd_read(ioc, &magic, sizeof(magic), errp) < 0) {
>          error_prepend(errp, "Failed to read data: ");
>          goto fail;
>      }
> -
> -    buf[8] = '\0';
> -    if (strlen(buf) == 0) {
> -        error_setg(errp, "Server connection closed unexpectedly");
> -        goto fail;
> -    }
> -
> -    magic = ldq_be_p(buf);
> +    magic = be64_to_cpu(magic);
>      trace_nbd_receive_negotiate_magic(magic);
> 
> -    if (memcmp(buf, "NBDMAGIC", 8) != 0) {
> +    if (magic != NBD_INIT_MAGIC) {
>          error_setg(errp, "Invalid magic received");
>          goto fail;
>      }

The original code is really odd.  What's the whole strlen about?
Anyway this is an obvious improvement so:

Reviewed-by: Richard W.M. Jones <rjones@redhat.com>

Rich.

-- 
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