Re: [PATCH v2] ceph: fix posix ACL hooks

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Feb 03, 2014 at 01:44:22PM -0800, Linus Torvalds wrote:
> On Mon, Feb 3, 2014 at 1:39 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > If we really have hardlinks, the result of permission check would better
> > be a function of inode itself - as in, "if it gives different results
> > for two pathnames reachable for the same user, we have a bug".
		      ^^^^^^^^^
> No. You're wrong.
> 
> EVEN ON A UNIX FILESYSTEM THE PATH IS MEANINGFUL.
> 
> Do this: create a hardlink in two different directories. Make the
> *directory* permissions for one of the directories be something you
> cannot traverse. Now try to check the permissions of the *same* inode
> through those two paths. Notice how you get *different* results.
> 
> Really.

Yes.  In one case we won't get to looking at the permissions of that thing
at all.
 
> Now, imagine that you are doing the same thing over a network. On the
> server, there may be a single inode for the file, but when the client
> gives the server a pathname, the two pathnames to that single inode
> ARE NOT EQUIVALENT.

Why do we pretend that those are links, in the first place?

> And the fact is, filesystems with hardlinks and path-name-based
> operations do exist. cifs with the unix extensions is one of them.

Pox on Tridge...

I really, really hate that change; I can buy "->getxattr() has inconvenient
interface because of lousy protocol design", but spreading the same to
->permission(), with everything that will fall out of that... <shudder>