linux-kernel - Re: [PATCH v2] NFSD: Rework encoding and decoding of nfsd4

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Message-id: <175323408768.2234665.8262591875626168370@noble.neil.brown.name>
Date: Wed, 23 Jul 2025 11:28:07 +1000
From: "NeilBrown" <neil@...wn.name>
To: "Chuck Lever" <chuck.lever@...cle.com>
Cc: "Christoph Hellwig" <hch@...radead.org>,
 "Sergey Bashirov" <sergeybashirov@...il.com>,
 "Jeff Layton" <jlayton@...nel.org>, "Olga Kornievskaia" <okorniev@...hat.com>,
 "Dai Ngo" <Dai.Ngo@...cle.com>, "Tom Talpey" <tom@...pey.com>,
 linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] NFSD: Rework encoding and decoding of nfsd4_deviceid

On Wed, 23 Jul 2025, Chuck Lever wrote:
> On 7/22/25 1:36 AM, Christoph Hellwig wrote:
> > On Mon, Jul 21, 2025 at 05:48:55PM +0300, Sergey Bashirov wrote:
> >> Compilers may optimize the layout of C structures,
> > 
> > By interpreting the standard in the most hostile way: yes.
> > In practice: no.
> 
> Earnest question: Is NFSD/XDR properly insulated against the "randomize
> structure layout" option?
> 
> 
> > Just about every file system on-disk format and every network wire
> > protocol depends on the compiler not "optimizing" properly padded
> > C structures.
> It's an intrinsic assumption that is not documented in the code or
> anywhere else. IMO that is a latent banana peel.

We could document it in the code with __no_randomize_layout after the
structure definition.

But currently the only structures that are randomized in Linux are those
which are entirely function pointers, and those marked
__randomize_layout.

(See documentation for "RANDSTRUCT_FULL")

> 
> While not urgent, this is a defensive change and it improves code
> portability amongst compilers and languages (eg, Rust).
> 

I'm neither for nor against the change.  I would be interested to know
how much it changes the code side (if at all).

NeilBrown