dm: fix truncated status strings

Avoid returning a truncated table or status string instead of setting
the DM_BUFFER_FULL_FLAG when the last target of a table fills the
buffer.

When processing a table or status request, the function retrieve_status
calls ti->type->status. If ti->type->status returns non-zero,
retrieve_status assumes that the buffer overflowed and sets
DM_BUFFER_FULL_FLAG.

However, targets don't return non-zero values from their status method
on overflow. Most targets returns always zero.

If a buffer overflow happens in a target that is not the last in the
table, it gets noticed during the next iteration of the loop in
retrieve_status; but if a buffer overflow happens in the last target, it
goes unnoticed and erroneously truncated data is returned.

In the current code, the targets behave in the following way:
* dm-crypt returns -ENOMEM if there is not enough space to store the
  key, but it returns 0 on all other overflows.
* dm-thin returns errors from the status method if a disk error happened.
  This is incorrect because retrieve_status doesn't check the error
  code, it assumes that all non-zero values mean buffer overflow.
* all the other targets always return 0.

This patch changes the ti->type->status function to return void (because
most targets don't use the return code). Overflow is detected in
retrieve_status: if the status method fills up the remaining space
completely, it is assumed that buffer overflow happened.

Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 6 insertions, 10 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 10079e07edf4..6e45e3774eab 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1836,8 +1836,8 @@ static void snapshot_merge_resume(struct dm_target *ti)
         start_merge(s);
 }
 
-static int snapshot_status(struct dm_target *ti, status_type_t type,
-                           unsigned status_flags, char *result, unsigned maxlen)
+static void snapshot_status(struct dm_target *ti, status_type_t type,
+                            unsigned status_flags, char *result, unsigned maxlen)
 {
         unsigned sz = 0;
         struct dm_snapshot *snap = ti->private;
@@ -1883,8 +1883,6 @@ static int snapshot_status(struct dm_target *ti, status_type_t type,
                                           maxlen - sz);
                 break;
         }
-
-        return 0;
 }
 
 static int snapshot_iterate_devices(struct dm_target *ti,
@@ -2138,8 +2136,8 @@ static void origin_resume(struct dm_target *ti)
         ti->max_io_len = get_origin_minimum_chunksize(dev->bdev);
 }
 
-static int origin_status(struct dm_target *ti, status_type_t type,
-                         unsigned status_flags, char *result, unsigned maxlen)
+static void origin_status(struct dm_target *ti, status_type_t type,
+                          unsigned status_flags, char *result, unsigned maxlen)
 {
         struct dm_dev *dev = ti->private;
 
@@ -2152,8 +2150,6 @@ static int origin_status(struct dm_target *ti, status_type_t type,
                 snprintf(result, maxlen, "%s", dev->name);
                 break;
         }
-
-        return 0;
 }
 
 static int origin_merge(struct dm_target *ti, struct bvec_merge_data *bvm,
@@ -2180,7 +2176,7 @@ static int origin_iterate_devices(struct dm_target *ti,
 
 static struct target_type origin_target = {
         .name    = "snapshot-origin",
-        .version = {1, 8, 0},
+        .version = {1, 8, 1},
         .module  = THIS_MODULE,
         .ctr     = origin_ctr,
         .dtr     = origin_dtr,
@@ -2193,7 +2189,7 @@ static struct target_type origin_target = {
 
 static struct target_type snapshot_target = {
         .name    = "snapshot",
-        .version = {1, 11, 0},
+        .version = {1, 11, 1},
         .module  = THIS_MODULE,
         .ctr     = snapshot_ctr,
         .dtr     = snapshot_dtr,