[PATCH] drm/i915: Allow NULL memory region

[Cavitt, Jonathan]

-----Original Message-----
From: Tvrtko Ursulin <tursulin at ursulin.net> 
Sent: Monday, July 29, 2024 1:21 AM
To: Dan Carpenter <dan.carpenter at linaro.org>
Cc: Cavitt, Jonathan <jonathan.cavitt at intel.com>; intel-gfx at lists.freedesktop.org; Gupta, saurabhg <saurabhg.gupta at intel.com>; chris.p.wilson at linux.intel.com
Subject: Re: [PATCH] drm/i915: Allow NULL memory region
> 
> 
> On 26/07/2024 18:00, Dan Carpenter wrote:
> > On Fri, Jul 26, 2024 at 09:17:20AM +0100, Tvrtko Ursulin wrote:
> >>
> >> On 25/07/2024 16:58, Dan Carpenter wrote:
> >>> On Thu, Jul 25, 2024 at 08:48:35AM +0100, Tvrtko Ursulin wrote:
> >>>>
> >>>> Hi,
> >>>>
> >>>> On 12/07/2024 22:41, Jonathan Cavitt wrote:
> >>>>> Prevent a NULL pointer access in intel_memory_regions_hw_probe.
> >>>>
> >>>> For future reference please include some impact assessment in patches tagged
> >>>> as fixes. Makes maintainers job, and even anyone's who tries to backport
> >>>> stuff to stable at some future date, much easier if it is known how
> >>>> important is the fix and in what circumstances can the problem it is fixing
> >>>> trigger.
> >>>>
> >>>
> >>> As someone doing backport work, I think this patch is fine.  Everyone
> >>> knows the impact of a NULL dereference in probe().
> >>>
> >>> I guess with patches that add NULL dereferences, the trick is
> >>> understanding when people are adding NULL checks to make a static
> >>> checker happy or when it's a real bug.  But the fault lies with the
> >>> people adding NULL checks just to make the tools happy.  Some of these
> >>> pointless NULL checks end up in stable, but it's fine, extra NULL checks
> >>> never hurt anyone.  If the maintainer wants to be extra safe by adding
> >>> NULL checks then who are we to say otherwise.
> >>>
> >>> In other words normal patches shouldn't have to say. "I'm not lying" at
> >>> the end.  It should be the pointless patches which say, "I'm doing a
> >>> pointless thing.  Don't bother backporting."
> >>>
> >>> Most stable patch backports are done automatically and people have
> >>> various tools and scripts to do that.  If the tools don't handle this
> >>> patch automatically then they are defective.
> >>
> >> Right, and every few releases maintainers and authors get a bunch of emails
> >> for patches which did not apply to some stable tree.
> >>
> > 
> > I believe these emails are only sent for commits that are tagged for
> > stable.  For AUTOSEL patches, the backporting is done on a best effort
> > basis.  On the other hand, hopefully this patch would have been tagged
> > for stable if we hadn't fixed the bug so quickly.
> > 
> >> In which case someone has to do manual work and then it is good to know how
> >> important it is to backport something. For cases when it is not trivial. It
> >> does not apply to this patch, but as a _best practice_ it is good if the
> >> commit message explains the impacted platforms and scenarios.
> >>
> >> In this case I can follow the Fixes: tag and see the fix that this patches
> >> fixes is only about ATS-M. Which if it was a more complicated patch might be
> >> a reason to not need bother backporting past some kernel version where
> >> platform X wasn't even supported.
> >>
> >> Therefore I think my point is that best practice is to include this the
> >> commit text, so any future maintainer/backporter does not have to re-do
> >> detective work, stands.
> > 
> > This is a really elaborate hypothetical.  Are there kernels which are
> > affected by this bug but don't support ATS-M?
> 
> I am not sure why are we arguing against the value of putting a bit more 
> info in commit messages.
> 
> When I was writing up the drm-intel-next-fixes pull request I already 
> had to follow the Fixes: chain for this one to understand the impact.
> 
> This patch is already in and all but from my point of view best practice 
> still is for commit messages to be a bit more verbose than "fix null 
> pointer deref". At least when fixes are coming from inside Intel I think 
> we can assume people have enough info to asses and document.

For future reference, what kind of additional information would you have
preferred been added to this patch that was not originally provided, and in
what location should that information have been added (as a part of the
commit message itself, after the Fixes tag, etc.)?
-Jonathan Cavitt

> 
> Regards,
> 
> Tvrtko
>