fix mount -a on btrfs bind mount

If I have a fstab that contains:

UUID=1f976144-6c23-4622-bf6c-cd9821acc49d /var/log btrfs subvol=@/var/log 0 0
/var/log /local/nfsv4/log none bind,private

When calling "mount -a" multiple times, it mounts the bind mount again and again.

Here are related mountinfo entries with my kernel:
44 34 0:30 /@/var/log /var/log rw,relatime shared:27 - btrfs /dev/vda2 rw,space_cache,subvolid=269,subvol=/@/var/log 95 34 0:30 /@/var/log /local/nfsv4/log rw,relatime - btrfs /dev/vda2 rw,space_cache,subvolid=269,subvol=/@/var/log

Debugging this case, I found that the btrfs specific code in mnt_table_get_fs_root() returns bad value:

after remove_mountpoint_from_path() root is "/" mnt_table_find_target(tb, "/var/log", MNT_ITER_BACKWARD) returns src_root set to "/@/var/log". startswith() does not match and the snprintf() attempts to construct the final root value by concatenating of both strings:

FS root result: /@/var/log/

It looks strange, and it does not work:

Then it returns to mnt_table_is_fs_mounted(), and the iterator while (mnt_table_next_fs(tb, &itr, &fs) == 0)
could never get a match.
In section starting with if (root)
and mnt_fs_get_root(fs) returns "/@/var/log"
This is compared with "/@/var/log/" above, and match is never got.

Here is a possible easy fix. If it is correct, the code should be simplified to remove string copying there.

I am not sure whether it has any side effect, but it helps in my case.

Note that 41510d26ddd9a4c83f8589dded539e2985535dcf added a patch to "else if" part of this if, that is also a bit
suspicious: btrfs path should not contain "//" instead of "/{subvolume}/", at least not in a correct use.

---
 libmount/src/tab.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 0df8d49..44894e9 100644
@@ -1252,7 +1252,7 @@ struct libmnt_fs *mnt_table_get_fs_root(struct libmnt_table *tb,
 
 			if (!tmp)
 				goto err;
-			snprintf(tmp, sz, "%s%s", src_root, root);
+			snprintf(tmp, sz, "%s", src_root);
 			free(root);
 			root = tmp;
 		}

2.6.3

-- 
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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