[PATCH v2 6/7] hw/vfio/iommufd: Fix missing ERRP_GUARD() in iommufd_cdev_getfd()

[Zhao Liu <zhao1.liu@linux.intel.com>]

From: Zhao Liu <zhao1.liu@intel.com>

As the comment in qapi/error, dereferencing @errp requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
* - It must not be dereferenced, because it may be null.
...
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.
*
* Using it when it's not needed is safe, but please avoid cluttering
* the source with useless code.

But in iommufd_cdev_getfd(), @errp is dereferenced without ERRP_GUARD():

if (*errp) {
    error_prepend(errp, VFIO_MSG_PREFIX, path);
}

Currently, since vfio_attach_device() - the caller of
iommufd_cdev_getfd() - is always called in DeviceClass.realize() context
and doesn't get the NULL @errp parameter, iommufd_cdev_getfd()
hasn't triggered the bug that dereferencing the NULL @errp.

To follow the requirement of @errp, add missing ERRP_GUARD() in
iommufd_cdev_getfd().

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
---
Suggested by credit:
 Markus: Referred his explanation about ERRP_GUARD().
---
v2:
 * Add the @errp dereference code in commit message to make review
   easier. (Markus)
---
 hw/vfio/iommufd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/vfio/iommufd.c b/hw/vfio/iommufd.c
index 9bfddc136089..7baf49e6ee9e 100644
--- a/hw/vfio/iommufd.c
+++ b/hw/vfio/iommufd.c
@@ -116,6 +116,7 @@ static void iommufd_cdev_unbind_and_disconnect(VFIODevice *vbasedev)
 
 static int iommufd_cdev_getfd(const char *sysfs_path, Error **errp)
 {
+    ERRP_GUARD();
     long int ret = -ENOTTY;
     char *path, *vfio_dev_path = NULL, *vfio_path = NULL;
     DIR *dir = NULL;
-- 
2.34.1