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From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Ben Skeggs <bskeggs@redhat.com>, David Airlie <airlied@linux.ie>,
	Antonino Daplas <adaplas@gmail.com>,
	Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: likely signedness bug in drm and nvidia drivers
Date: Mon, 20 Jul 2015 20:46:47 +0000	[thread overview]
Message-ID: <87fv4io8ug.fsf@rasmusvillemoes.dk> (raw)

Hi,

The files

  drivers/gpu/drm/nouveau/nv50_fbcon.c
  drivers/gpu/drm/nouveau/nvc0_fbcon.c
  drivers/video/fbdev/nvidia/nv_accel.c

all contain a right-shift of ~0 (aka -1) - just grep for '~0 >>'. gcc
always does arithmetic right shift of signed types, which means that the
result is always -1 again [type promotion/conversion doesn't kick in
until after the shift subexpression has been evaluated], independent of
the second operand. I can hardly believe that is intended (the result is
used as a mask, which thus consists of all 1s). If these are indeed
bugs, the patch is obvious (just make the literal an unsigned 0).

Rasmus

WARNING: multiple messages have this Message-ID (diff)
From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Ben Skeggs <bskeggs@redhat.com>, David Airlie <airlied@linux.ie>,
	Antonino Daplas <adaplas@gmail.com>,
	Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: likely signedness bug in drm and nvidia drivers
Date: Mon, 20 Jul 2015 22:46:47 +0200	[thread overview]
Message-ID: <87fv4io8ug.fsf@rasmusvillemoes.dk> (raw)

Hi,

The files

  drivers/gpu/drm/nouveau/nv50_fbcon.c
  drivers/gpu/drm/nouveau/nvc0_fbcon.c
  drivers/video/fbdev/nvidia/nv_accel.c

all contain a right-shift of ~0 (aka -1) - just grep for '~0 >>'. gcc
always does arithmetic right shift of signed types, which means that the
result is always -1 again [type promotion/conversion doesn't kick in
until after the shift subexpression has been evaluated], independent of
the second operand. I can hardly believe that is intended (the result is
used as a mask, which thus consists of all 1s). If these are indeed
bugs, the patch is obvious (just make the literal an unsigned 0).

Rasmus
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             reply	other threads:[~2015-07-20 20:46 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-07-20 20:46 Rasmus Villemoes [this message]
2015-07-20 20:46 ` likely signedness bug in drm and nvidia drivers Rasmus Villemoes
2015-07-21  2:44 ` Ilia Mirkin
2015-07-21  2:44   ` Ilia Mirkin
2015-07-21  8:56   ` Daniel Thompson
2015-07-21  8:56     ` Daniel Thompson

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