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

[Chuck Lever III <chuck.lever@oracle.com>]

> On Oct 5, 2024, at 12:51 PM, Pali Rohár <pali@kernel.org> wrote:
> 
> 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@kernel.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@suse.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?

On Linux, those operations are part of the NLMv1/3/4
protocol implementation, so essentially the NLM v2
functionality is a part of all NLM versions on Linux.


>> 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.

Generally this is an abuse of the purpose of the RPC
program versioning mechanism. Linux has a very similar
upcall mechanism, but uses NLM procedure numbers that
are set aside for this purpose instead of abusing a
moribund protocol version.


> 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.

How about this replacement:

 * XDR functions to encode/decode NLM version 1 and 3 RPC
 * arguments and results. NLM version 2 is not specified
 * by a standard, thus it is not implemented.


--
Chuck Lever