Re: GSSAPI fix for pynfs nfs4.1 client code

[Volodymyr Khomenko <volodymyr@vastdata.com>]

> The seq_num field can start at any value below MAXSEQ
Yes, that's the statement I haven't found before in RFC...
Probably we also need to write a test starting the seq_num from a big
value (more than SEQUENCE_WINDOW)
to make sure that it is really implemented properly without
'is_inited' flag (so what's the initial value?).

However I still propose to keep the default behaviour of pynfs to be
the same as for linux NFS4 client.
I think the caller can change it when needed (to 0 or whatever
needed), but the default value should be good...

volodymyr.

On Fri, Oct 1, 2021 at 5:13 PM J. Bruce Fields <bfields@fieldses.org> wrote:
>
> On Fri, Oct 01, 2021 at 09:49:50AM +0300, Volodymyr Khomenko wrote:
> > > So, I can verify that --security=krb5 works after this patch but not
> > > before, good.  But why is that?  As you say, the server is supposed to
> > > ignore the sequence number on context creation requests.  And 0 is valid
> > > sequence number as far as I know.
> >
> > By design of RPCGSS we have a 'last seen seq_number' counter on the
> > server side per each GSS context
> > and we must not accept any packet that was already seen before (we
> > also have a bitmask of several last requests for this).
> > This 'last seen counter' is unsigned int32 (the same as seq_num) so we
> > can't just init it to -1 so next seq_num=0 will be valid.
> > The most obvious implementation is to init it last_seen_seq_num to 0
> > so the very 1st packet after NFS4 NULL must be 1 to differ from last
> > seen seq_number.
>
> Note in theory gssapi mechanisms can require multiple round trips (in
> the GSS_PROC_CONTINUE_INIT case), so this wouldn't actually avoid
> duplicate sequence numbers.
>
> In any case, the rfc is unambiguous here: "In a creation request, the
> seq_num and service fields are undefined and both must be ignored by the
> server."
>
> > A better implementation (theoretically) can set this counter to
> > 'undefined' state by additional flag, but this is  more
> > resource-consuming
> > (you need to process is_inited flag + last_seen_seq_num instead of
> > just one counter).
> > If the last seen seq_number is undefined during GSS initialization,
> > then strictly speaking we can send ANY seq_num for the very 1st
> > request after NFS4 NULL.
>
> Right, again, from RFC 2203, " The seq_num field can start at any value
> below MAXSEQ."
>
> It can be implemented without the need for an is_inited flag.
>
> The initial sequence number of 0 really did find an actual bug in the
> server, so pynfs is definitely doing its job in this case!
>
> --b.