linux-kernel - Re: [PATCH v2] cxl: Check for invalid addresses returned from translation functions on errors

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Message-ID: <aWTkFpaR_T4eVSsp@rric.localdomain>
Date: Mon, 12 Jan 2026 13:07:50 +0100
From: Robert Richter <rrichter@....com>
To: Alison Schofield <alison.schofield@...el.com>
Cc: Davidlohr Bueso <dave@...olabs.net>,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ira Weiny <ira.weiny@...el.com>,
	Dan Williams <dan.j.williams@...el.com>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] cxl: Check for invalid addresses returned from
 translation functions on errors

On Thu, Jan 08, 2026 at 10:07:06AM -0800, Alison Schofield wrote:
> On Wed, Jan 07, 2026 at 01:03:00PM +0100, Robert Richter wrote:
> > On 06.01.26 10:42:36, Alison Schofield wrote:
> > > On Tue, Jan 06, 2026 at 06:23:58PM +0100, Robert Richter wrote:
> > 
> > > > @@ -3207,8 +3220,13 @@ static int region_offset_to_dpa_result(struct cxl_region *cxlr, u64 offset,
> > > >  		cxled = p->targets[i];
> > > >  		if (cxled->pos != pos)
> > > >  			continue;
> > > > +
> > > > +		dpa = cxl_dpa_resource_start(cxled);
> > > 
> > > We want to return -ENXIO, not 0 in this case.
> > > So jump out here immediately - right?
> > > 
> > > 		if (dpa == REsOURCE_SIZE_MAX)
> > > 			return -ENXIO;
> > 
> > The users of region_offset_to_dpa_result() handle the
> > RESOURCE_SIZE_MAX case. For that reason the dpa is not checked here.
> > I did not want to change the function interface with that fix.
> 
> region_offset_to_dpa_result() intends to return an rc when it knows
> it would return a bad result. This is that case. With this proposed
> change, we avoid adding to a RESOURCE_SIZE_MAX, which is defensive,
> but isn't quitting immediately correct? These fixups are all about
> stopping when any ingredient to the calc looks bad. It looks bad,
> let's stop right away.
> 
> Seeing that the callsites do this:
> 
> 	rc = region_offset_to_dpa_result(cxlr, offset, &result);
>         if (rc || !result.cxlmd || result.dpa == ULLONG_MAX) {
> 		...this failed
> 
> shows that they do some defensive programming and don't rely only 
> on the rc. But the inverse does not seem right - relying on that.

The logic here says that it is not an error to have result.dpa ==
ULLONG_MAX, e.g. you still could take result.cxlmd.

> 
> Staring at it more, I guess you could refactor the function to be
> a void and get rid of the rc entirely.
> 
> Is that the real simplification needed here?

This patch aims to only fix the use of bad address values. No rework
at all.

-Robert

> 
> -- Alison
> 
> > 
> > Thanks for review.
> > 
> > -Robert
> > 
> > > 
> > > 
> > > > +		if (dpa != RESOURCE_SIZE_MAX)
> > > > +			dpa += dpa_offset;
> > > > +
> > > >  		result->cxlmd = cxled_to_memdev(cxled);
> > > > -		result->dpa = cxl_dpa_resource_start(cxled) + dpa_offset;
> > > > +		result->dpa = dpa;
> > > >  
> > > >  		return 0;
> > > >  	}