qemuDomainCreateDeviceRecursive: Support file mount points

https://bugzilla.redhat.com/show_bug.cgi?id=1462060

When building a qemu namespace we might be dealing with bare
regular files. Files that live under /dev. For instance
/dev/my_awesome_disk:

  <disk type='file' device='disk'>
    <driver name='qemu' type='qcow2'/>
    <source file='/dev/my_awesome_disk'/>
    <target dev='vdc' bus='virtio'/>
  </disk>

  # qemu-img create -f qcow2 /dev/my_awesome_disk 10M

So far we were mknod()-ing them which is
obviously wrong. We need to touch the file and bind mount it to
the original:

1) touch /var/run/libvirt/qemu/fedora.dev/my_awesome_disk
2) mount --bind /dev/my_awesome_disk /var/run/libvirt/qemu/fedora.dev/my_awesome_disk

Later, when the new /dev is built and replaces original /dev the
file is going to live at expected location.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_domain.c

@@ -7721,6 +7721,7 @@ qemuDomainCreateDeviceRecursive(const char *device,
     int ret = -1;
     bool isLink = false;
     bool isDev = false;
+    bool isReg = false;
     bool create = false;
 #ifdef WITH_SELINUX
     char *tcon = NULL;
@@ -7744,6 +7745,7 @@ qemuDomainCreateDeviceRecursive(const char *device,
 
     isLink = S_ISLNK(sb.st_mode);
     isDev = S_ISCHR(sb.st_mode) || S_ISBLK(sb.st_mode);
+    isReg = S_ISREG(sb.st_mode);
 
     /* Here, @device might be whatever path in the system. We
      * should create the path in the namespace iff it's "/dev"
@@ -7855,16 +7857,12 @@ qemuDomainCreateDeviceRecursive(const char *device,
             }
             goto cleanup;
         }
-
-        /* Set the file permissions again: mknod() is affected by the
-         * current umask, and as such might not have set them correctly */
+    } else if (isReg) {
         if (create &&
-            chmod(devicePath, sb.st_mode) < 0) {
-            virReportSystemError(errno,
-                                 _("Failed to set permissions for device %s"),
-                                 devicePath);
+            virFileTouch(devicePath, sb.st_mode) < 0)
             goto cleanup;
-        }
+        /* Just create the file here so that code below sets
+         * proper owner and mode. Bind mount only after that. */
     } else {
         virReportError(VIR_ERR_OPERATION_UNSUPPORTED,
                        _("unsupported device type %s 0%o"),
@@ -7884,6 +7882,15 @@ qemuDomainCreateDeviceRecursive(const char *device,
         goto cleanup;
     }
 
+    /* Symlinks don't have mode */
+    if (!isLink &&
+        chmod(devicePath, sb.st_mode) < 0) {
+        virReportSystemError(errno,
+                             _("Failed to set permissions for device %s"),
+                             devicePath);
+        goto cleanup;
+    }
+
     /* Symlinks don't have ACLs. */
     if (!isLink &&
         virFileCopyACLs(device, devicePath) < 0 &&
@@ -7916,6 +7923,11 @@ qemuDomainCreateDeviceRecursive(const char *device,
     }
 #endif
 
+    /* Finish mount process started earlier. */
+    if (isReg &&
+        virFileBindMountDevice(device, devicePath) < 0)
+        goto cleanup;
+
     ret = 0;
  cleanup:
     VIR_FREE(target);