linux-hardening - Re: [PATCH] drm/i915/gt: Remove const from struct i915

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Message-ID: <202504301345.659CB6B@keescook>
Date: Wed, 30 Apr 2025 13:56:04 -0700
From: Kees Cook <kees@...nel.org>
To: Tvrtko Ursulin <tursulin@...ulin.net>
Cc: Jani Nikula <jani.nikula@...ux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Matt Roper <matthew.d.roper@...el.com>,
	Gustavo Sousa <gustavo.sousa@...el.com>,
	Andi Shyti <andi.shyti@...ux.intel.com>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	Gnattu OC <gnattuoc@...com>, Nitin Gote <nitin.r.gote@...el.com>,
	Ranu Maurya <ranu.maurya@...el.com>,
	Ville Syrjälä <ville.syrjala@...ux.intel.com>,
	Angus Chen <angus.chen@...el.com>,
	Juha-Pekka Heikkila <juhapekka.heikkila@...il.com>,
	Yu Jiaoliang <yujiaoliang@...o.com>,
	Dnyaneshwar Bhadane <dnyaneshwar.bhadane@...el.com>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] drm/i915/gt: Remove const from struct i915_wa list
 allocation

On Mon, Apr 28, 2025 at 01:09:46PM +0100, Tvrtko Ursulin wrote:
> 
> On 26/04/2025 07:13, Kees Cook wrote:
> > In preparation for making the kmalloc family of allocators type aware,
> > we need to make sure that the returned type from the allocation matches
> > the type of the variable being assigned. (Before, the allocator would
> > always return "void *", which can be implicitly cast to any pointer type.)
> > 
> > The assigned type is "struct i915_wa *". The returned type, while
> > technically matching, will be const qualified. As there is no general
> > way to remove const qualifiers, adjust the allocation type to match
> > the assignment.
> > 
> > Signed-off-by: Kees Cook <kees@...nel.org>
> > ---
> > Cc: Jani Nikula <jani.nikula@...ux.intel.com>
> > Cc: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
> > Cc: Rodrigo Vivi <rodrigo.vivi@...el.com>
> > Cc: Tvrtko Ursulin <tursulin@...ulin.net>
> > Cc: David Airlie <airlied@...il.com>
> > Cc: Simona Vetter <simona@...ll.ch>
> > Cc: Matt Roper <matthew.d.roper@...el.com>
> > Cc: Gustavo Sousa <gustavo.sousa@...el.com>
> > Cc: Andi Shyti <andi.shyti@...ux.intel.com>
> > Cc: Lucas De Marchi <lucas.demarchi@...el.com>
> > Cc: <intel-gfx@...ts.freedesktop.org>
> > Cc: <dri-devel@...ts.freedesktop.org>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_workarounds.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> > index 116683ebe074..b37e400f74e5 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> > @@ -156,7 +156,7 @@ static void _wa_add(struct i915_wa_list *wal, const struct i915_wa *wa)
> >   	if (IS_ALIGNED(wal->count, grow)) { /* Either uninitialized or full. */
> >   		struct i915_wa *list;
> > -		list = kmalloc_array(ALIGN(wal->count + 1, grow), sizeof(*wa),
> > +		list = kmalloc_array(ALIGN(wal->count + 1, grow), sizeof(*list),
> 
> Will the sizeof stay, and if so, how will kmalloc be able to distinguish the
> type? Or we expect one more churn on the same line?

It is expected that when (if?) this happens, there will be a pre-rc1
treewide change to convert kmalloc to kmalloc_obj[1]. (So, yes, this
call would change, but it'll happen separately.)

-Kees

[1] Here's what v4 looked like:
    https://lore.kernel.org/lkml/20250315025852.it.568-kees@kernel.org/
    v5 is still under development, but will look like this:
    https://web.git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=dev/v6.15-rc3%2b/alloc_obj/v5

-- 
Kees Cook