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, 3 insertions, 5 deletions

diff --git a/drivers/md/dm-verity.c b/drivers/md/dm-verity.c
index 52cde982164a..6ad538375c3c 100644
--- a/drivers/md/dm-verity.c
+++ b/drivers/md/dm-verity.c
@@ -508,8 +508,8 @@ static int verity_map(struct dm_target *ti, struct bio *bio)
 /*
  * Status: V (valid) or C (corruption found)
  */
-static int verity_status(struct dm_target *ti, status_type_t type,
-                         unsigned status_flags, char *result, unsigned maxlen)
+static void verity_status(struct dm_target *ti, status_type_t type,
+                          unsigned status_flags, char *result, unsigned maxlen)
 {
         struct dm_verity *v = ti->private;
         unsigned sz = 0;
@@ -540,8 +540,6 @@ static int verity_status(struct dm_target *ti, status_type_t type,
                                 DMEMIT("%02x", v->salt[x]);
                 break;
         }
-
-        return 0;
 }
 
 static int verity_ioctl(struct dm_target *ti, unsigned cmd,
@@ -860,7 +858,7 @@ bad:
 
 static struct target_type verity_target = {
         .name           = "verity",
-        .version        = {1, 1, 0},
+        .version        = {1, 1, 1},
         .module         = THIS_MODULE,
         .ctr            = verity_ctr,
         .dtr            = verity_dtr,