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

[Eric Blake <eblake@redhat.com>]

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;
     }
-- 
2.17.2