Re: [RFC, PATCH 1/4] svc: Fix skip computation in svc_defer and svc_revisit

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Oct 24, 2007 at 12:45:09PM +1000, Greg Banks wrote:
> On Tue, Oct 23, 2007 at 02:03:55PM -0400, J. Bruce Fields wrote:
> > On Tue, Oct 23, 2007 at 12:45:30PM +1000, Greg Banks wrote:
> > > On Fri, Oct 19, 2007 at 04:45:23PM -0500, Tom Tucker wrote:
> > > > 
> > > > The rq_arg.len includes the size of the transport header. The computations
> > > > assumed that it did not.
> > > 
> > > ok, assuming that your latest svc_rdma_xdr_decode_req(), which I
> > > haven't read yet, doesn't change rq_arg.len as it parses the RDMA
> > > chunking header.
> > > 
> > > The reason I mention that is that one set of patches I was working
> > > with some time ago had different semantics for rq_arg.len; when
> > > parsing the chunking header, rq_arg.len was adjusted downward as
> > > rq_head[0].iov_base was adjusted forward.  This kept rq_arg.len equal
> > > to the sum of rq_{head[0],pages[...],tail[0]}.iov_len, which seemed
> > > like a good idea.
> > > 
> > > But your way seems valid too.
> > 
> > The integrity and privacy code may be a particularly unpleasant source
> > of assumptions about the various length fields; see e.g.
> > unwrap_integ_data() and unwrap_priv_data().  (But maybe nobody cares
> > about krb5i/krb5p over rdma?)
> 
> More or less the entire point of NFS/RDMA is that the CPU never
> has to see the data that goes on the wire; no memory copies (*),
> no message digest calculations, no encryptions.  This makes krb5[ip]
> over RDMA rather an unhelpful combination.

Yeah, I figured.

But obviously we have to at least make sure that combination doesn't
cause a crash.

--b.

> But if there's some code there that makes actual assumptions about
> the xdr_buf.len field, we need to find it, document it and ensure
> we're maintaining that assumption in the new code.

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