btrfs-progs: Fixing invalid device size output when "btrfs device usage /" called as normal user

When "btrfs device usage /" is being called as a normal user,
the function device_get_partition_size in device-utils.c calls
device_get_partition_size_sysfs() as a fallback, which reads
the size of the partition from "/sys/class/block/[partition]/size".

The problem here is that the size read is not actually the size of
the partition but rather a number of how many 512B (or whatever the
devices sector size is) sectors the partition contains.

Ex: if read value is 104857600 the size is not 100MB but 104857600 *
512B = 50GB

This patch adds a function named get_partition_sector_size_sysfs which
based on the partition name returns the sector size of the device, and
replaces "return size" with "return size * sector_size" in
device_get_partition_size_sysfs.

Issue: #979
Reviewed-by: Qu Wenruo <wqu@suse.com>

Author: raczzoli

common/device-utils.c

@@ -30,6 +30,7 @@
 #include <fcntl.h>
 #include <dirent.h>
 #include <errno.h>
+#include <libgen.h>
 #include <blkid/blkid.h>
 #include "kernel-lib/sizes.h"
 #include "kernel-shared/disk-io.h"
@@ -342,6 +343,50 @@ u64 device_get_partition_size_fd(int fd)
 	return result;
 }
 
+static ssize_t get_partition_sector_size_sysfs(const char *name)
+{
+	int ret = 0;
+	char real_path[PATH_MAX] = {};
+	char link_path[PATH_MAX] = {};
+	char *dev_name = NULL;
+	int sysfd;
+	char sysfs[PATH_MAX] = {};
+	char sizebuf[128];
+
+	snprintf(link_path, PATH_MAX, "/sys/class/block/%s/..", name);
+
+	if (!realpath(link_path, real_path)) {
+		error("Failed to resolve realpath of %s: %s\n", link_path, strerror(errno));
+		return -1;
+	}
+
+	dev_name = basename(real_path);
+
+	if (!dev_name) {
+		error("Failed to determine basename for path %s\n", real_path);
+		return -1;
+	}
+
+	snprintf(sysfs, PATH_MAX, "/sys/class/block/%s/queue/hw_sector_size", dev_name);
+
+	sysfd = open(sysfs, O_RDONLY);
+	if (sysfd < 0) {
+		error("Error opening %s to determine dev sector size: %s\n", real_path, strerror(errno));
+		return -1;
+	}
+
+	ret = sysfs_read_file(sysfd, sizebuf, sizeof(sizebuf));
+	close(sysfd);
+
+	if (ret < 0) {
+		error("Error reading hw_sector_size from sysfs for device %s!\n", dev_name);
+		return -1;
+	}
+
+	return atoi(sizebuf);
+}
+
+
 static u64 device_get_partition_size_sysfs(const char *dev)
 {
 	int ret;
@@ -351,6 +396,7 @@ static u64 device_get_partition_size_sysfs(const char *dev)
 	const char *name = NULL;
 	int sysfd;
 	unsigned long long size = 0;
+	ssize_t sector_size = 0;
 
 	name = realpath(dev, path);
 	if (!name)
@@ -375,7 +421,13 @@ static u64 device_get_partition_size_sysfs(const char *dev)
 		return 0;
 	}
 	close(sysfd);
-	return size;
+
+	sector_size = get_partition_sector_size_sysfs(name);
+
+	if (sector_size < 0)
+		return 0;
+
+	return size * sector_size;
 }
 
 u64 device_get_partition_size(const char *dev)