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

[Chuck Lever]

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.

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.

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

-- 
Chuck Lever