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

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

On Mon, Feb 03, 2014 at 01:31:19PM -0800, Linus Torvalds wrote:

> If the protocol is path-based (and it happens, and it's actually the
> *correct* thing to do for a network filesystem, rather than the
> idiotic "file handle" crap that tries to emulate the unix inode
> semantics in the protocol), then the inode is simply not sufficient.
> 
> And no, d_find_alias() is not correct or sufficient either. It can
> work in practice (and probably does perfectly fine 99.9% of the time),
> but it can equally well give the *wrong* dentry: yes, the dentry it
> returns would have been a valid dentry for somebody at some time, but
> it might be a stale dentry *now*, and it might be the wrong dentry for
> the current user (because the current user may not have permissions to
> that particular path, even if the user has permissions through his
> *own* path).
> 
> So I really think you're *fundamentally* incorrect when you say
> "result *is* a function of inode alone".

Which fs are you talking about?  For 9P it *is* a function of inode alone.
For CIFS there's no wrong dentry to pick - it doesn't have links to start
with.

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".