discover/devmapper: Read device size from sysfs

If udev doesn't export the ID_PART_ENTRY_SIZE variable for a device we
skip creating a snapshot for it. However in most cases the sysfs
attribute which udev reads to find ID_PART_ENTRY_SIZE is still
available. Therefore if we don't have access to ID_PART_ENTRY_SIZE try
to find the size in sysfs directly.
This allows us to create snapshots for devices which often don't have
this udev variable set, such as software raid (md) devices and NVMe
devices.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>

1 files changed, 77 insertions, 9 deletions

diff --git a/discover/devmapper.c b/discover/devmapper.c
index d3179f1..c1a5492 100644
--- a/discover/devmapper.c
+++ b/discover/devmapper.c
@@ -4,6 +4,10 @@
 #include <errno.h>
 #include <string.h>
 
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
 #include "libdevmapper.h"
 #include "devmapper.h"
 #include "platform.h"
@@ -17,27 +21,91 @@ struct target {
 	char		*params;
 };
 
-/* Return the number of sectors on a block device. Zero represents an error */
-static uint64_t get_block_sectors(struct discover_device *device)
+static unsigned long read_param_uint(struct discover_device *device,
+				const char *param)
 {
-	unsigned long long sectors;
+	unsigned long value = 0;
 	const char *tmp;
 
-	tmp = discover_device_get_param(device, "ID_PART_ENTRY_SIZE");
+	tmp = discover_device_get_param(device, param);
 	if (!tmp) {
-		pb_debug("Could not retrieve ID_PART_ENTRY_SIZE for %s\n",
+		pb_debug("Could not retrieve parameter '%s' for %s\n",
+			 param, device->device_path);
+		errno = EINVAL;
+	} else {
+		errno = 0;
+		value = strtoul(tmp, NULL, 0);
+	}
+
+	/* Return errno and result directly */
+	return value;
+}
+
+/* Return the number of sectors on a block device. Zero represents an error */
+static uint64_t get_block_sectors(struct discover_device *device)
+{
+	unsigned long major, minor, sectors = 0;
+	char *attr, *buf = NULL;
+	struct stat sb;
+	int fd = -1;
+	ssize_t sz;
+
+	sectors = read_param_uint(device, "ID_PART_ENTRY_SIZE");
+	if (!errno)
+		return (uint64_t)sectors;
+	else
+		pb_debug("Error reading sector count for %s: %m\n",
 		       device->device_path);
+
+	/* Either the udev property is missing or we failed to parse it.
+	 * Instead try to directly read the size attribute out of sysfs */
+	major = read_param_uint(device, "MAJOR");
+	if (errno) {
+		pb_debug("Error reading %s major number\n", device->device_path);
+		return 0;
+	}
+	minor = read_param_uint(device, "MINOR");
+	if (errno) {
+		pb_debug("Error reading %s minor number\n", device->device_path);
 		return 0;
 	}
 
-	errno = 0;
-	sectors = strtoull(tmp, NULL, 0);
+	attr = talloc_asprintf(device, "/sys/dev/block/%lu:%lu/size",
+			       major, minor);
+	if (stat(attr, &sb)) {
+		pb_debug("Failed to stat %s, %m\n", attr);
+		goto out;
+	}
+
+	fd = open(attr, O_RDONLY);
+	if (fd < 0) {
+		pb_debug("Failed to open sysfs attribute for %s\n",
+			 device->device_path);
+		goto out;
+	}
+
+	buf = talloc_array(device, char, sb.st_size);
+	if (!buf) {
+		pb_debug("Failed to allocate space for attr\n");
+		goto out;
+	}
+
+	sz = read(fd, buf, sb.st_size);
+	if (sz <= 0) {
+		pb_debug("Failed to read sysfs attr: %m\n");
+		goto out;
+	}
+
+	sectors = strtoul(buf, NULL, 0);
 	if (errno) {
-		pb_debug("Error reading sector count for %s: %s\n",
-		       device->device_path, strerror(errno));
+		pb_debug("Failed to read sectors from sysfs: %m\n");
 		sectors = 0;
 	}
 
+out:
+	close(fd);
+	talloc_free(buf);
+	talloc_free(attr);
 	return (uint64_t)sectors;
 }