Add -f FMT / --format FMT arg to qemu-nbd

Currently the qemu-nbd program will auto-detect the format of
any disk it is given. This behaviour is known to be insecure.
For example, if qemu-nbd initially exposes a 'raw' file to an
unprivileged app, and that app runs

   'qemu-img create -f qcow2 -o backing_file=/etc/shadow /dev/nbd0'

then the next time the app is started, the qemu-nbd will now
detect it as a 'qcow2' file and expose /etc/shadow to the
unprivileged app.

The only way to avoid this is to explicitly tell qemu-nbd what
disk format to use on the command line, completely disabling
auto-detection. This patch adds a '-f' / '--format' arg for
this purpose, mirroring what is already available via qemu-img
and qemu commands.

  qemu-nbd --format raw -p 9000 evil.img

will now always use raw, regardless of what format 'evil.img'
looks like it contains

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
[Use errx, not err. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-nbd.c

@@ -306,6 +306,7 @@ static void nbd_accept(void *opaque)
 int main(int argc, char **argv)
 {
     BlockDriverState *bs;
+    BlockDriver *drv;
     off_t dev_offset = 0;
     uint32_t nbdflags = 0;
     bool disconnect = false;
@@ -313,7 +314,7 @@ int main(int argc, char **argv)
     char *device = NULL;
     int port = NBD_DEFAULT_PORT;
     off_t fd_size;
-    const char *sopt = "hVb:o:p:rsnP:c:dvk:e:t";
+    const char *sopt = "hVb:o:p:rsnP:c:dvk:e:f:t";
     struct option lopt[] = {
         { "help", 0, NULL, 'h' },
         { "version", 0, NULL, 'V' },
@@ -333,6 +334,7 @@ int main(int argc, char **argv)
 #endif
         { "discard", 1, NULL, QEMU_NBD_OPT_DISCARD },
         { "shared", 1, NULL, 'e' },
+        { "format", 1, NULL, 'f' },
         { "persistent", 0, NULL, 't' },
         { "verbose", 0, NULL, 'v' },
         { NULL, 0, NULL, 0 }
@@ -351,6 +353,7 @@ int main(int argc, char **argv)
     bool seen_aio = false;
 #endif
     pthread_t client_thread;
+    const char *fmt = NULL;
 
     /* The client thread uses SIGTERM to interrupt the server.  A signal
      * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
@@ -454,6 +457,9 @@ int main(int argc, char **argv)
                 errx(EXIT_FAILURE, "Shared device number must be greater than 0\n");
             }
             break;
+        case 'f':
+            fmt = optarg;
+            break;
 	case 't':
 	    persistent = 1;
 	    break;
@@ -555,9 +561,19 @@ int main(int argc, char **argv)
     bdrv_init();
     atexit(bdrv_close_all);
 
+    if (fmt) {
+        drv = bdrv_find_format(fmt);
+        if (!drv) {
+            errx(EXIT_FAILURE, "Unknown file format '%s'", fmt);
+        }
+    } else {
+        drv = NULL;
+    }
+
     bs = bdrv_new("hda");
     srcpath = argv[optind];
-    if ((ret = bdrv_open(bs, srcpath, NULL, flags, NULL)) < 0) {
+    ret = bdrv_open(bs, srcpath, NULL, flags, drv);
+    if (ret < 0) {
         errno = -ret;
         err(EXIT_FAILURE, "Failed to bdrv_open '%s'", argv[optind]);
     }

qemu-nbd.texi

@@ -45,6 +45,8 @@ Export QEMU disk image using NBD protocol.
   disconnect the specified device
 @item -e, --shared=@var{num}
   device can be shared by @var{num} clients (default @samp{1})
+@item -f, --format=@var{fmt}
+  force block driver for format @var{fmt} instead of auto-detecting
 @item -t, --persistent
   don't exit on the last connection
 @item -v, --verbose