nbd/client: Drop pointless buf variable

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. That whole strlen() issue has been
present and useless since commit 1d45f8b5 in 2010; perhaps it
was leftover debugging (since the correct magic number happens
to be ASCII)?  Make the error messages more consistent and
detailed while touching things.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181215135324.152629-9-eblake@redhat.com>

1 files changed, 7 insertions, 15 deletions

diff --git a/nbd/client.c b/nbd/client.c
index 5a03a84418..f625c207c5 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -733,7 +733,6 @@ 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;
@@ -754,27 +753,20 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name,
         goto fail;
     }
 
-    if (nbd_read(ioc, buf, 8, 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");
+    if (nbd_read(ioc, &magic, sizeof(magic), errp) < 0) {
+        error_prepend(errp, "Failed to read initial magic: ");
         goto fail;
     }
-
-    magic = ldq_be_p(buf);
+    magic = be64_to_cpu(magic);
     trace_nbd_receive_negotiate_magic(magic);
 
-    if (memcmp(buf, "NBDMAGIC", 8) != 0) {
-        error_setg(errp, "Invalid magic received");
+    if (magic != NBD_INIT_MAGIC) {
+        error_setg(errp, "Bad initial magic received: 0x%" PRIx64, magic);
         goto fail;
     }
 
     if (nbd_read(ioc, &magic, sizeof(magic), errp) < 0) {
-        error_prepend(errp, "Failed to read magic: ");
+        error_prepend(errp, "Failed to read server magic: ");
         goto fail;
     }
     magic = be64_to_cpu(magic);
@@ -913,7 +905,7 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name,
         }
         info->flags = oldflags;
     } else {
-        error_setg(errp, "Bad magic received");
+        error_setg(errp, "Bad server magic received: 0x%" PRIx64, magic);
         goto fail;
     }