[apparmor] Splitting unlink from write

[Zygmunt Krynicki]

Hello

I'd like to propose splitting the textual permission "w", so that it 
does not imply AA_MAY_DELETE if the file is a device, fifo or socket. 
Profiles routinely grant "w" permission, but nobody in their right mind 
expects applications to delete such files.

Both userspace and kernel can already kind-of express this. The only 
question is how to do that in a way that doesn't force a painful profile 
transition. I think we need a new permission bit.

My suggestion would be to add a AA_MAY_DELETE_SPECIAL permission. 
Starting with some future ABI deleting devices, fifos and sockets would 
check AA_MAY_DELETE_SPECIAL. Compatibility layer in the kernel would 
then continue to grant AA_MAY_DELETE_SPECIAL for older ABIs.

On the userspace side we might define new syntax such as:

allow file PATH D,

Where D implies delete special.

I'm happy to take a stab at implementing it. The only thing I'm not sure 
is how to name the new feature "delete_special".

I'm grateful for your thoughts

Best regards
ZK