Re: [PATCH v1.2 0/5] IMA: making i_readcount a first class inode citizen (reposting)

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Nov 19, 2010 at 12:50:53PM -0500, J. Bruce Fields wrote:
> On Thu, Nov 18, 2010 at 03:31:10PM -0800, Linus Torvalds wrote:
> > On Thu, Nov 18, 2010 at 3:02 PM, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> > >
> > > This patchset separates the incrementing/decrementing of the i_readcount, in
> > > the VFS layer, from other IMA functionality, by replacing the current
> > > ima_counts_get() call with i_readcount_inc(). Its unclear whether this call to
> > > increment i_readcount should be made earlier, like i_writecount.  Currently the
> > > call is situated immediately after the switch from put_filp() to fput() for
> > > cleanup.
> > 
> > Well, it seems nicer than the situation we have now. So I'm certainly
> > ok with seeing this merged for 2.6.38 (through the security tree?) if
> > nobody has objections.
> > 
> > It's a bit sad to have another atomic in the open path, but if the
> > lease people want this and are ok with just the counter (no races?)
> > then it seems worth it.
> 
> Having thought about it more, I'm no longer convinced it will be useful
> for leases.
> 
> It seems attractive to replace the current d_count/i_count checks by an
> i_readcount check, but:
> 
> 	1) as long as break_lease() is called before i_readcount_inc(),
> 	   there's a window between the two where setlease has no way to
> 	   tell whether a read open is about to happen;
> 
> 	2) more importantly, it won't help file servers, which need more
> 	   than mutual exclusion between opens and leases.
> 
> Number 2 in more detail:
> 
> Write leases exist to let a file server (nfsd or Samba) tell a client
> that it has exclusive access to a file, so that the client can delay
> writes, knowing that it will be notified on lease break (and given a
> chance to write back dirty data) before someone else can look at the
> file.
> 
> But say someone modifies a file on a client and then runs "make" on the
> server.  The "make" needs to know about the modifications.  But make only
> stat's the file, doesn't open it....
> 
> We can break leases on stat, but on its own that's racy--setlease needs
> some way to determine whether a lease is in progress.  And i_readlease()

(err, I meant i_readcount).

> doesn't help there, unless we decide we're going to temporarily
> increment that around every stat.  (But if another atomic in the open
> path is bad, another in the stat path sounds worse--and it's probably
> not the semantics ima needs anyway.)

Anyway, so I've got nothing against i_readlease, but I don't see how to
use them for the write lease implementation--it looks to me like we're
better off living with d_count/i_count checks.  They give false
positives, but I don't think some false positives are really a problem.

--b.