linux-kernel - Re: [PATCH] lockd: Fix comment about NLMv3 backwards compatibility

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Message-ID: <20241005165125.rbtgxzz4olvv4sqn@pali>
Date: Sat, 5 Oct 2024 18:51:25 +0200
From: Pali Rohár <pali@...nel.org>
To: Chuck Lever <chuck.lever@...cle.com>
Cc: NeilBrown <neilb@...e.de>, Jeff Layton <jlayton@...nel.org>,
	Olga Kornievskaia <okorniev@...hat.com>,
	Dai Ngo <Dai.Ngo@...cle.com>, Tom Talpey <tom@...pey.com>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lockd: Fix comment about NLMv3 backwards compatibility

On Thursday 12 September 2024 19:34:02 Chuck Lever wrote:
> On Fri, Sep 13, 2024 at 01:28:20AM +0200, Pali Rohár wrote:
> > On Friday 13 September 2024 01:22:07 Pali Rohár wrote:
> > > On Friday 13 September 2024 09:10:45 NeilBrown wrote:
> > > > On Fri, 13 Sep 2024, Pali Rohár wrote:
> > > > > NLMv2 is completely different protocol than NLMv1 and NLMv3, and in
> > > > > original Sun implementation is used for RPC loopback callbacks from statd
> > > > > to lockd services. Linux does not use nor does not implement NLMv2.
> > > > > 
> > > > > Hence, NLMv3 is not backward compatible with NLMv2. But NLMv3 is backward
> > > > > compatible with NLMv1. Fix comment.
> > > > > 
> > > > > Signed-off-by: Pali Rohár <pali@...nel.org>
> > > > > ---
> > > > >  fs/lockd/clntxdr.c | 4 +++-
> > > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/fs/lockd/clntxdr.c b/fs/lockd/clntxdr.c
> > > > > index a3e97278b997..81ffa521f945 100644
> > > > > --- a/fs/lockd/clntxdr.c
> > > > > +++ b/fs/lockd/clntxdr.c
> > > > > @@ -3,7 +3,9 @@
> > > > >   * linux/fs/lockd/clntxdr.c
> > > > >   *
> > > > >   * XDR functions to encode/decode NLM version 3 RPC arguments and results.
> > > > > - * NLM version 3 is backwards compatible with NLM versions 1 and 2.
> > > > > + * NLM version 3 is backwards compatible with NLM version 1.
> > > > > + * NLM version 2 is different protocol used only for RPC loopback callbacks
> > > > > + * from statd to lockd and is not implemented on Linux.
> > > > >   *
> > > > >   * NLM client-side only.
> > > > >   *
> > > > 
> > > > Reviewed-by: NeilBrown <neilb@...e.de>
> > > > 
> > > > Do you have a reference for that info about v2?  I hadn't heard of it
> > > > before.
> > > > 
> > > > NeilBrown
> > > 
> > > I have just this information in my notes. I guess it should be possible
> > > to gather more information about v2 from released Sun/Solaris source
> > > code via OpenSolaris / Illumos projects.
> > 
> > Just very quickly I found this Illumos XDR file for NLM:
> > https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/rpcsvc/nlm_prot.x
> > 
> > And it defines NLMv2 with two procedures numbered 17 and 18, plus there
> > is a comment in file header about v2.
> > 
> > So probably the best reference would be the Illumos source code.
> 
> What you see in the Illumos code is not something that is part
> of the standard NLM protocol, but rather a private upcall protocol
> between the kernel and user space that is special sauce added
> by each implementation of NLM/NSM.

Ok. But this applies for v2, no?

> Also note the way NLMv3 is defined in this file: it defines only
> a handful of new operations. The other operations are inherited
> from NLMv1.

Yes, v3 is there and is inherited from v1. This is also what I pointed
in the comment. That v3 inherits from v1, not v2.

In header file of that nlm_prot.x is written:

 * There are currently 3 versions of the protocol in use.  Versions 1
 * and 3 are used with NFS version 2.  Version 4 is used with NFS
 * version 3.
 *
 * (Note: there is also a version 2, but it defines an orthogonal set of
 * procedures that the status monitor uses to notify the lock manager of
 * changes in monitored systems.)

Which sounds like version 3 has nothing with version 2.

My understanding of that comment is that version 2 contains only those
private upcall protocol between kernel and userspace about which you
wrote, and therefore version 3 is not backward compatible with version 2.

> IMO the comment is accurate and does not warrant a change.