[parted-devel] [PATCH 2/3] libparted: improve support for partitions on loopback devices

[Petr Uzel]

Since kernel 2.6.26, kernel allows partitions on loopback devices.
Implement support for this feature in parted.

* libparted/arch/linux.c (_sysfs_int_entry_from_dev): New function.
(_loop_get_partition_range): New function.
(_device_get_partition_range): Add special handling for loop devices.
* NEWS: Mention this change.
---
 NEWS                   |    4 ++
 libparted/arch/linux.c |   77 +++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 68 insertions(+), 13 deletions(-)

diff --git a/NEWS b/NEWS
index 6c55ec9..293dad8 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,10 @@ GNU parted NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** New features
+
+  parted has improved support for partitionable loopback devices
+
 ** Bug fixes
 
   libparted: no longer aborts (failed assertion) due to a nilfs2_probe bug
diff --git a/libparted/arch/linux.c b/libparted/arch/linux.c
index 8488b7f..ad3ee07 100644
--- a/libparted/arch/linux.c
+++ b/libparted/arch/linux.c
@@ -2429,31 +2429,82 @@ _blkpg_remove_partition (PedDisk* disk, int n)
 }
 
 /*
- * The number of partitions that a device can have depends on the kernel.
- * If we don't find this value in /sys/block/DEV/range, we will use our own
- * value.
+ * Read the integer from /sys/block/DEV_BASE/ENTRY and set *val
+ * to that value, where DEV_BASE is the last component of
+ * DEV->path. Upon success, return true. Otherwise, return false.
  */
-static unsigned int
-_device_get_partition_range(PedDevice* dev)
+static bool
+_sysfs_int_entry_from_dev(PedDevice const* dev, const char *entry, int *val)
 {
-        int         range, r;
+        int         r;
         char        path[128];
         FILE*       fp;
         bool        ok;
 
-        r = snprintf(path, sizeof(path), "/sys/block/%s/range",
-                     last_component(dev->path));
+        r = snprintf(path, sizeof(path), "/sys/block/%s/%s",
+                     last_component(dev->path), entry);
         if (r < 0 || r >= sizeof(path))
-                return MAX_NUM_PARTS;
+                return false;
 
         fp = fopen(path, "r");
         if (!fp)
-                return MAX_NUM_PARTS;
+                return false;
 
-        ok = fscanf(fp, "%d", &range) == 1;
+        ok = fscanf(fp, "%d", val) == 1;
         fclose(fp);
 
-        /* (range <= 0) is none sense.*/
+        return ok;
+}
+
+/*
+ * Returns maximum number of partitions that the loopback device can hold.
+ * First checks if the loop module exports max_part parameter (since kernel
+ * 3.0), if it does not succeed, it falls back to checking ext_range, which
+ * seems to have (for some reason) different semantics compared to other
+ * devices; specifically, ext_range <= 1 means that the loopback device does
+ * not support partitions.
+ */
+static unsigned int
+_loop_get_partition_range(PedDevice const* dev)
+{
+        int         max_part;
+        bool        ok = 0;
+
+        /* max_part module param is exported since kernel 3.0 */
+        FILE *fp = fopen("/sys/module/loop/parameters/max_part", "r");
+        if (fp) {
+                ok = fscanf(fp, "%d", &max_part) == 1;
+                fclose(fp);
+        }
+
+        if (ok)
+                return max_part > 0 ? max_part : 0;
+
+        /*
+         * max_part is not exported - check ext_range;
+         * device supports partitions if ext_range > 1
+         */
+        int range;
+        ok = _sysfs_int_entry_from_dev(dev, "range", &range);
+
+        return ok && range > 1 ? range : 0;
+}
+
+/*
+ * The number of partitions that a device can have depends on the kernel.
+ * If we don't find this value in /sys/block/DEV/range, we will use our own
+ * value.
+ */
+static unsigned int
+_device_get_partition_range(PedDevice const* dev)
+{
+        /* loop handling is special */
+        if (dev->type == PED_DEVICE_LOOP)
+                return _loop_get_partition_range(dev);
+
+        int range;
+        bool ok = _sysfs_int_entry_from_dev(dev, "range", &range);
+
         return ok && range > 0 ? range : MAX_NUM_PARTS;
 }
 
@@ -2478,7 +2529,7 @@ _disk_sync_part_table (PedDisk* disk)
         PED_ASSERT(disk->dev != NULL);
         int lpn;
 
-	unsigned int part_range = _device_get_partition_range(disk->dev);
+        unsigned int part_range = _device_get_partition_range(disk->dev);
 
         /* lpn = largest partition number. */
         if (ped_disk_get_max_supported_partition_count(disk, &lpn))
-- 
1.7.3.4