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

[J. Bruce Fields]

On Sun, Nov 21, 2010 at 08:18:18AM -0500, Mimi Zohar wrote:
> On Fri, 2010-11-19 at 12:50 -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@xxxxxxxxxxxxxxxxxx> 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....
> 
> Hi Bruce,
> 
> IMA (and the proposed EVM/IMA-appraisal patches) detects file change
> based on i_version. When the file is closed, if the file has changed,
> IMA marks the file as needing to be re-measured. Of course this requires
> the filesystem to be mounted with iversion. Don't know if this helps.

In this example, it's not poor time resolution or anything that prevents
make from noticing the modification; it's the simple fact that the copy
of the file on the server hasn't been modified at all; only the remote
client knows about the modification, and the only way to find out about
the modification is to synchronously call back to the client and let it
write out its dirty data before allowing a local user to read its data
or metadata.

--b.

> 
> Mimi
> 
> > 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()
> > 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.)
> > 
> > --b.
> 
> 
> 
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