[PATCH?] fix mount -a on btrfs bind mount

[Stanislav Brabec <sbrabec@suse.cz>]

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(-)

diff --git a/libmount/src/tab.c b/libmount/src/tab.c
index 0df8d49..44894e9 100644
--- a/libmount/src/tab.c
+++ b/libmount/src/tab.c
@@ -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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