From: Julio Faracco <jcfaracco(a)gmail.com&gt; This is an issue for LXC loop devices when you are trying to get loop devices info using `ioctl`. Modern apps uses `/sys/dev/block` to grab information about devices, but if you use the method mention you won't be able to retrive the associated file with that loop device. See example below from cryptsetup sources: static char *_ioctl_backing_file(const char *loop) { struct loop_info64 lo64 = {0}; int loop_fd; loop_fd = open(loop, O_RDONLY); if (loop_fd < 0) return NULL; if (ioctl(loop_fd, LOOP_GET_STATUS64, &lo64) < 0) { close(loop_fd); return NULL; } lo64.lo_file_name[LO_NAME_SIZE-2] = '*'; lo64.lo_file_name[LO_NAME_SIZE-1] = 0; close(loop_fd); return strdup((char*)lo64.lo_file_name); } It will return an empty string because lo_file_name was not set. Signed-off-by: Julio Faracco <jcfaracco(a)gmail.com&gt; --- src/util/virfile.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/src/util/virfile.c b/src/util/virfile.c index 81a3c096eb..fe0aa54eb6 100644 --- a/src/util/virfile.c +++ b/src/util/virfile.c @@ -781,6 +781,16 @@ int virFileLoopDeviceAssociate(const char *file, memset(&lo, 0, sizeof(lo)); lo.lo_flags = LO_FLAGS_AUTOCLEAR; + /* Set backing file name for LOOP_GET_STATUS64 queries */ + if (virStrncpy((char *) lo.lo_file_name, file, + strlen(file), LO_NAME_SIZE) < 0) { + virReportSystemError(errno, + _("Unable to set backing file %s"), file); + goto cleanup; + } + lo.lo_file_name[LO_NAME_SIZE-2] = '*'; + lo.lo_file_name[LO_NAME_SIZE-1] = 0; + if ((fsfd = open(file, O_RDWR)) < 0) { virReportSystemError(errno, _("Unable to open %s"), file); -- 2.20.1