Re: [PATCH 0/7] Permit filesystem local caching and NFS superblock sharing [try #13]

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Tue, 2006-09-05 at 10:55 +0800, Ian Kent wrote:
> On Mon, 2006-09-04 at 21:57 -0400, Trond Myklebust wrote:
> > On Mon, 2006-09-04 at 21:24 +0800, Ian Kent wrote:
> > 
> > > > [pid  3838] mkdir("/net", 0555)         = -1 EEXIST (File exists)
> > > > [pid  3838] stat64("/net", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
> > > > [pid  3838] mkdir("/net/trash", 0555)   = -1 EEXIST (File exists)
> > > > [pid  3838] stat64("/net/trash", {st_mode=S_IFDIR|0555, st_size=1024, ...}) = 0
> > > > [pid  3838] mkdir("/net/trash/mnt", 0555) = -1 EACCES (Permission denied)
> > > 
> > > This is the point I'm trying to make.
> > > I'm able to reproduce this with exports that don't have "nohide".
> > > The mkdir used to return EEXIST, possibly before getting to the EACCES
> > > test. It appears to be a change in semantic behavior and I can't see
> > > where it is coming from. autofs expects an EEXIST but not an EACCES and
> > > so doesn't perform the mount. I could ignore the EACCES but that would
> > > be cheating.
> > 
> > Why the hell is it doing a mkdir in the first place? ...and why the hell
> > is it not able to cope with EACCES? The latter is hardly an unlikely
> > reply: it means that the automounter should not be doing this in the
> > first place, 'cos it doesn't have the privileges. That is not the same
> > as saying that it doesn't have the privileges to do a lookup.
> 
> Why the hell shouldn't it be able to do an mkdir!

Firstly, if the call to mkdir actually _was_ successful, it would be
creating a new directory on the NFS server, and it would be doing so
with the automounter's privileges instead of the user's privileges. Why
would I want it to do that?

Secondly, and more pertinently to this case, you have no guarantee that
the automounter has _any_ privileges on the server at all other than
what is required to mount a filesystem. selinux is enforcing that on the
client side here, but the server could just as well be set up to do the
same (in fact, you could set up selinux to do the exact same thing on
the server).

IOW, the automounter should just be calling stat('/net/trash/mnt'). It
shouldn't be trying to create directories on the server at all.

> It is coping with the EACCESS return by not mounting the filesystem
> which is the correct response in this case.

No it isn't. The directory exists. It can be looked up. There is no
reason why you can't mount something on top of it.

Being permitted to do mkdir() or not has nothing to do with anything.