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

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

On Wed, Feb 21, 2024 at 12:53:10PM +0100, Markus Armbruster wrote:
> Date: Wed, 21 Feb 2024 12:53:10 +0100
> From: Markus Armbruster <armbru@redhat.com>
> Subject: Re: [PATCH 6/6] hw/vfio/iommufd: Fix missing ERRP_GUARD() in
>  iommufd_cdev_getfd()
> 
> Zhao Liu <zhao1.liu@linux.intel.com> writes:
> 
> > 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.
> > * - It should not be passed to error_prepend() or
> > *   error_append_hint(), because that doesn't work with &error_fatal.
> > * 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.
> >
> > Currently, since vfio_attach_device() - the caller of
> > iommufd_cdev_getfd() - is always called in DeviceClass.realize() context
> > and won't get the NULL errp parameter, iommufd_cdev_getfd()
> > doesn't trigger the dereference issue.
> >
> > 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>
> > ---
> > Suggested by credit:
> >  Markus: Referred his explanation about ERRP_GUARD().
> > ---
> >  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;
> 
> The problematic use is
> 
>        if (*errp) {
>            error_prepend(errp, VFIO_MSG_PREFIX, path);
>        }
> 
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
>

Thanks! Will also add the description of problematic use in commit
message to make review easier. ;-)

Regards,
Zhao