btrfs-progs: read partition size from sysfs as fallback

Reading partition size using an ioctl requires the device open, but that
does not work for unprivileged users. This leads to 0 size in device
info structures filled by device_get_partition_size.

As a consequence, this also misreports such devices as missing in 'fi
us' overview:

 $ btrfs fi us /
 WARNING: cannot read detailed chunk info, per-device usage will not be shown, run as root
 Overall:
     Device size:		 411.35GiB
     Device allocated:		  53.01GiB
     Device unallocated:	 358.34GiB
     Device missing:		 411.35GiB
     Used:			  31.99GiB
     Free (estimated):		 379.16GiB	(min: 379.16GiB)
     Free (statfs, df):		 379.35GiB
     Data ratio:		      1.00
     Metadata ratio:		      1.00
     Global reserve:		 194.77MiB	(used: 0.00B)
     Multiple profiles:		        no

There should be 0 for 'Device missing'.

Add a fallback to read the device size from sysfs in case the ioctl is
not available.

Issue: #395
Signed-off-by: David Sterba <dsterba@suse.com>

common/device-utils.c

@@ -26,6 +26,7 @@
 #include <dirent.h>
 #include <blkid/blkid.h>
 #include <linux/limits.h>
+#include <limits.h>
 #include "kernel-lib/sizes.h"
 #include "kernel-shared/disk-io.h"
 #include "kernel-shared/zoned.h"
@@ -303,13 +304,50 @@ u64 device_get_partition_size_fd(int fd)
 	return result;
 }
 
+u64 device_get_partition_size_sysfs(const char *dev)
+{
+	int ret;
+	char path[PATH_MAX] = {};
+	char sysfs[PATH_MAX] = {};
+	char sizebuf[128] = {};
+	char *name = NULL;
+	int sysfd;
+	unsigned long long size = 0;
+
+	name = realpath(dev, path);
+	if (!name)
+		return 0;
+	name = basename(path);
+
+	ret = path_cat3_out(sysfs, "/sys/class/block", name, "size");
+	if (ret < 0)
+		return 0;
+	sysfd = open(sysfs, O_RDONLY);
+	if (sysfd < 0)
+		return 0;
+	ret = sysfs_read_file(sysfd, sizebuf, sizeof(sizebuf));
+	if (ret < 0) {
+		close(sysfd);
+		return 0;
+	}
+	errno = 0;
+	size = strtoull(sizebuf, NULL, 10);
+	if (size == ULLONG_MAX && errno == ERANGE) {
+		close(sysfd);
+		return 0;
+	}
+	close(sysfd);
+	return size;
+}
+
 u64 device_get_partition_size(const char *dev)
 {
 	u64 result;
 	int fd = open(dev, O_RDONLY);
 
 	if (fd < 0)
-		return 0;
+		return device_get_partition_size_sysfs(dev);
+
 	if (ioctl(fd, BLKGETSIZE64, &result) < 0) {
 		close(fd);
 		return 0;