Re: xdr_stream interface oddities with pages vs stream

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Dec 17, 2025 at 08:54:36AM -0500, Chuck Lever wrote:
> 
> 
> On Tue, Dec 16, 2025, at 5:26 PM, Mike Snitzer wrote:
> > Hi,
> >
> > I've been working on enhancing the NFS client and server such that
> > nfs4_{set,set}acl utilities to set/get 4.0 ACLs will work when NFSD
> > v4.1 is reexporting NFS v4.2.  Its soul sucking, but that aside...
> >
> > In my journey I'm finding that xdr_stream_decode_u32() followed by
> > xdr_stream_subsegment() doesn't continue from the point where
> > xdr_stream_decode_u32() advanced xdr_stream's ->p
> >
> > xdr_stream_subsegment() doesn't appear in any way interlocked with the
> > pages offset (->p), it starts with xdr_stream_pos() but  ends with
> > advancing ->p.  So xdr_stream_subsegment() does what it should, I'm
> > concerned about interfaces like xdr_stream_decode_u32() not advancing
> > xdr->nwords
> >
> > Shouldn't both the xdr->p and xdr->nwords be interlocked?  SO that
> > xdr_stream_subsegment() continues from the point where
> > xdr_stream_decode_u32() advanced the stream?
> 
> From the wasteland that is my memory... I think only one side
> uses nwords, and I'm betting it's the encode side. These aren't
> exactly symmetrical APIs, and the differences can be subtle.

It was just user error on my part.. (I had expected the acl_pages to
be aligned at the start of the first page in the subsegment I
extracted, but it was offset within that first page).

Move along nothing to see here! ;)

Thannks,
Mike