dm thin: open dev read only when possible

If a thin pool is created in read-only-metadata mode then only open the
metadata device read-only.

Previously it was always opened with FMODE_READ | FMODE_WRITE.

(Note that dm_get_device() still allows read-only dm devices to be used
read-write at the moment: If I create a read-only linear device for the
metadata, via dmsetup load --readonly, then I can still create a rw pool
out of it.)

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 14 insertions, 11 deletions

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index 111c148fb1d0..ef021b0c8106 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -1945,6 +1945,7 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
         unsigned long block_size;
         dm_block_t low_water_blocks;
         struct dm_dev *metadata_dev;
+        fmode_t metadata_mode;
 
         /*
          * FIXME Remove validation from scope of lock.
@@ -1956,10 +1957,22 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
                 r = -EINVAL;
                 goto out_unlock;
         }
+
         as.argc = argc;
         as.argv = argv;
 
-        r = dm_get_device(ti, argv[0], FMODE_READ | FMODE_WRITE, &metadata_dev);
+        /*
+         * Set default pool features.
+         */
+        pool_features_init(&pf);
+
+        dm_consume_args(&as, 4);
+        r = parse_pool_features(&as, &pf, ti);
+        if (r)
+                goto out_unlock;
+
+        metadata_mode = FMODE_READ | ((pf.mode == PM_READ_ONLY) ? 0 : FMODE_WRITE);
+        r = dm_get_device(ti, argv[0], metadata_mode, &metadata_dev);
         if (r) {
                 ti->error = "Error opening metadata block device";
                 goto out_unlock;
@@ -1992,16 +2005,6 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
                 goto out;
         }
 
-        /*
-         * Set default pool features.
-         */
-        pool_features_init(&pf);
-
-        dm_consume_args(&as, 4);
-        r = parse_pool_features(&as, &pf, ti);
-        if (r)
-                goto out;
-
         pt = kzalloc(sizeof(*pt), GFP_KERNEL);
         if (!pt) {
                 r = -ENOMEM;