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

From: Pali Rohár
Date: Sat Oct 05 2024 - 12:51:47 EST


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@xxxxxxxxxx>
> > > > > ---
> > > > > 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@xxxxxxx>
> > > >
> > > > 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.