mount -o union doesn't allow changes to sub directories?

[Jeremy Chadwick]

On Wed, Mar 23, 2011 at 04:48:21PM +1100, Lawrence Stewart wrote:
> Hi again,
> 
> If I run something like:
> 
> mount -o union /dev/md0 /etc
> 
> where md0 is a memory backed fs device, and /etc is fully populated but
> mounted RO off a CD, I can touch a new file in /etc, but can't in any
> subdir of etc (fails with "Read-only filesystem" reported).
> 
> As far as I can tell this is a bug, but wanted to check if I'm
> misunderstanding how the "-o union" mount option is supposed to work.

Please see mount_unionfs(8), specifically the explanations of "lower"
and "upper" layers, along with EROFS (errno 30, "Read-only filesystem").

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.               PGP 4BD6C0CB |