From 14bb8e3ca6abaab9a584413d2dd74a6eb2cc18d0 Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
Date: Wed, 9 Sep 2020 12:18:07 +0200
Subject: [PATCH] lsblk: ignore only loopdevs without backing file

* do not ignore all empty devices, we need more smart solution

* ignore only loop devices without backing file, for example:
 # touch img
 # losetup -f img
 losetup: img: Warning: file is smaller than 512 bytes; the loop device may be useless or invisible for system tools.

 - old version display nothing
 - new version:

 # lsblk /dev/loop0
 NAME  MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
 loop0   7:0    0   0B  0 loop

Addresses: https://github.com/karelzak/util-linux/issues/1118
Signed-off-by: Karel Zak <kzak@redhat.com>
---
 include/loopdev.h  |  1 +
 lib/loopdev.c      | 11 +++++++++++
 misc-utils/lsblk.c | 12 +++++++++++-
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/loopdev.h b/include/loopdev.h
index 0e3a7517a..65bd579d2 100644
--- a/include/loopdev.h
+++ b/include/loopdev.h
@@ -134,6 +134,7 @@ extern int is_loopdev(const char *device);
 extern int loopdev_is_autoclear(const char *device);
 
 extern char *loopdev_get_backing_file(const char *device);
+extern int loopdev_has_backing_file(const char *device);
 extern int loopdev_is_used(const char *device, const char *filename,
 			   uint64_t offset, uint64_t sizelimit, int flags);
 extern char *loopdev_find_by_backing_file(const char *filename,
diff --git a/lib/loopdev.c b/lib/loopdev.c
index 1581f9cc2..c4b492dc5 100644
--- a/lib/loopdev.c
+++ b/lib/loopdev.c
@@ -1619,6 +1619,17 @@ char *loopdev_get_backing_file(const char *device)
 	return res;
 }
 
+int loopdev_has_backing_file(const char *device)
+{
+	char *tmp = loopdev_get_backing_file(device);
+
+	if (tmp) {
+		free(tmp);
+		return 1;
+	}
+	return 0;
+}
+
 /*
  * Returns: TRUE/FALSE
  */
diff --git a/misc-utils/lsblk.c b/misc-utils/lsblk.c
index 49c15ab78..82eae6bad 100644
--- a/misc-utils/lsblk.c
+++ b/misc-utils/lsblk.c
@@ -50,6 +50,7 @@
 #include "closestream.h"
 #include "optutils.h"
 #include "fileutils.h"
+#include "loopdev.h"
 
 #include "lsblk.h"
 
@@ -1149,6 +1150,15 @@ static void devtree_to_scols(struct lsblk_devtree *tr, struct libscols_table *ta
 		device_to_scols(dev, NULL, tab, NULL);
 }
 
+static int ignore_empty(struct lsblk_device *dev)
+{
+	if (dev->size != 0)
+		return 0;
+	if (dev->maj == LOOPDEV_MAJOR && loopdev_has_backing_file(dev->filename))
+		return 0;
+	return 1;
+}
+
 /*
  * Reads very basic information about the device from sysfs into the device struct
  */
@@ -1200,7 +1210,7 @@ static int initialize_device(struct lsblk_device *dev,
 		dev->size <<= 9;					/* in bytes */
 
 	/* Ignore devices of zero size */
-	if (!lsblk->all_devices && dev->size == 0) {
+	if (!lsblk->all_devices && ignore_empty(dev)) {
 		DBG(DEV, ul_debugobj(dev, "zero size device -- ignore"));
 		return -1;
 	}