[00/12] treewide: Fix GENMASK misuses
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Message ID cover.1562734889.git.joe@perches.com
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  • treewide: Fix GENMASK misuses
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Joe Perches July 10, 2019, 5:04 a.m. UTC
These GENMASK uses are inverted argument order and the
actual masks produced are incorrect.  Fix them.

Add checkpatch tests to help avoid more misuses too.

Joe Perches (12):
  checkpatch: Add GENMASK tests
  clocksource/drivers/npcm: Fix misuse of GENMASK macro
  drm: aspeed_gfx: Fix misuse of GENMASK macro
  iio: adc: max9611: Fix misuse of GENMASK macro
  irqchip/gic-v3-its: Fix misuse of GENMASK macro
  mmc: meson-mx-sdio: Fix misuse of GENMASK macro
  net: ethernet: mediatek: Fix misuses of GENMASK macro
  net: stmmac: Fix misuses of GENMASK macro
  rtw88: Fix misuse of GENMASK macro
  phy: amlogic: G12A: Fix misuse of GENMASK macro
  staging: media: cedrus: Fix misuse of GENMASK macro
  ASoC: wcd9335: Fix misuse of GENMASK macro

 drivers/clocksource/timer-npcm7xx.c               |  2 +-
 drivers/gpu/drm/aspeed/aspeed_gfx.h               |  2 +-
 drivers/iio/adc/max9611.c                         |  2 +-
 drivers/irqchip/irq-gic-v3-its.c                  |  2 +-
 drivers/mmc/host/meson-mx-sdio.c                  |  2 +-
 drivers/net/ethernet/mediatek/mtk_eth_soc.h       |  2 +-
 drivers/net/ethernet/mediatek/mtk_sgmii.c         |  2 +-
 drivers/net/ethernet/stmicro/stmmac/descs.h       |  2 +-
 drivers/net/ethernet/stmicro/stmmac/dwmac-sun8i.c |  4 ++--
 drivers/net/wireless/realtek/rtw88/rtw8822b.c     |  2 +-
 drivers/phy/amlogic/phy-meson-g12a-usb2.c         |  2 +-
 drivers/staging/media/sunxi/cedrus/cedrus_regs.h  |  2 +-
 scripts/checkpatch.pl                             | 15 +++++++++++++++
 sound/soc/codecs/wcd-clsh-v2.c                    |  2 +-
 14 files changed, 29 insertions(+), 14 deletions(-)

Comments

Johannes Berg July 10, 2019, 9:17 a.m. UTC | #1
On Tue, 2019-07-09 at 22:04 -0700, Joe Perches wrote:
> These GENMASK uses are inverted argument order and the
> actual masks produced are incorrect.  Fix them.
> 
> Add checkpatch tests to help avoid more misuses too.
> 
> Joe Perches (12):
>   checkpatch: Add GENMASK tests

IMHO this doesn't make a lot of sense as a checkpatch test - just throw
in a BUILD_BUG_ON()?

johannes
Russell King - ARM Linux admin July 10, 2019, 9:43 a.m. UTC | #2
On Wed, Jul 10, 2019 at 11:17:31AM +0200, Johannes Berg wrote:
> On Tue, 2019-07-09 at 22:04 -0700, Joe Perches wrote:
> > These GENMASK uses are inverted argument order and the
> > actual masks produced are incorrect.  Fix them.
> > 
> > Add checkpatch tests to help avoid more misuses too.
> > 
> > Joe Perches (12):
> >   checkpatch: Add GENMASK tests
> 
> IMHO this doesn't make a lot of sense as a checkpatch test - just throw
> in a BUILD_BUG_ON()?

My personal take on this is that GENMASK() is really not useful, it's
just pure obfuscation and leads to exactly these kinds of mistakes.

Yes, I fully understand the argument that you can just specify the
start and end bits, and it _in theory_ makes the code more readable.

However, the problem is when writing code.  GENMASK(a, b).  Is a the
starting bit or ending bit?  Is b the number of bits?  It's confusing
and causes mistakes resulting in incorrect code.  A BUILD_BUG_ON()
can catch some of the cases, but not all of them.

For example:

	GENMASK(6, 2)

would satisify the requirement that a > b, so a BUILD_BUG_ON() will
not trigger, but was the author meaning 0x3c or 0xc0?

Personally, I've decided I am _not_ going to use GENMASK() in my code
because I struggle to get the macro arguments correct - I'm _much_
happier, and it is way more reliable for me to write the mask in hex
notation.

I think this is where use of a ternary operator would come in use.  The
normal way of writing a number of bits tends to be "a:b", so if GENMASK
took something like GENMASK(6:2), then I'd have less issue with it,
because it's argument is then in a familiar notation.

Yes, I'm sure that someone will point out that the GENMASK arguments
are just in the same order, but that doesn't prevent _me_ frequently
getting it wrong - and that's the point.  The macro seems to me to
cause more problems than it solves.
Joe Perches July 10, 2019, 3:45 p.m. UTC | #3
On Wed, 2019-07-10 at 10:43 +0100, Russell King - ARM Linux admin wrote:
> On Wed, Jul 10, 2019 at 11:17:31AM +0200, Johannes Berg wrote:
> > On Tue, 2019-07-09 at 22:04 -0700, Joe Perches wrote:
> > > These GENMASK uses are inverted argument order and the
> > > actual masks produced are incorrect.  Fix them.
> > > 
> > > Add checkpatch tests to help avoid more misuses too.
> > > 
> > > Joe Perches (12):
> > >   checkpatch: Add GENMASK tests
> > 
> > IMHO this doesn't make a lot of sense as a checkpatch test - just throw
> > in a BUILD_BUG_ON()?

I tried that.

It'd can't be done as it's used in declarations
and included in asm files and it uses the UL()
macro.

I also tried just making it do the right thing
whatever the argument order.

Oh well.

> My personal take on this is that GENMASK() is really not useful, it's
> just pure obfuscation and leads to exactly these kinds of mistakes.
> 
> Yes, I fully understand the argument that you can just specify the
> start and end bits, and it _in theory_ makes the code more readable.
> 
> However, the problem is when writing code.  GENMASK(a, b).  Is a the
> starting bit or ending bit?  Is b the number of bits?  It's confusing
> and causes mistakes resulting in incorrect code.  A BUILD_BUG_ON()
> can catch some of the cases, but not all of them.

It's a horrid little macro and I agree with Russell.

I also think if it existed at all it should have been
GENMASK(low, high) not GENMASK(high, low).

I
Joe Perches July 10, 2019, 4:01 p.m. UTC | #4
On Wed, 2019-07-10 at 08:45 -0700, Joe Perches wrote:
> On Wed, 2019-07-10 at 10:43 +0100, Russell King - ARM Linux admin wrote:
> > On Wed, Jul 10, 2019 at 11:17:31AM +0200, Johannes Berg wrote:
> > > On Tue, 2019-07-09 at 22:04 -0700, Joe Perches wrote:
> > > > These GENMASK uses are inverted argument order and the
> > > > actual masks produced are incorrect.  Fix them.
> > > > 
> > > > Add checkpatch tests to help avoid more misuses too.
> > > > 
> > > > Joe Perches (12):
> > > >   checkpatch: Add GENMASK tests
> > > 
> > > IMHO this doesn't make a lot of sense as a checkpatch test - just throw
> > > in a BUILD_BUG_ON()?
> 
> I tried that.
> 
> It'd can't be done as it's used in declarations
> and included in asm files and it uses the UL()
> macro.
> 
> I also tried just making it do the right thing
> whatever the argument order.

I forgot.

I also made all those arguments when it was
introduced in 2013.

https://lore.kernel.org/patchwork/patch/414248/

> Oh well.

yeah.
David Miller July 11, 2019, 9:30 p.m. UTC | #5
From: Joe Perches <joe@perches.com>
Date: Tue,  9 Jul 2019 22:04:13 -0700

> These GENMASK uses are inverted argument order and the
> actual masks produced are incorrect.  Fix them.
> 
> Add checkpatch tests to help avoid more misuses too.

Patches #7 and #8 applied to 'net', with appropriate Fixes tags
added to #8.
Andrzej Hajda July 12, 2019, 12:54 p.m. UTC | #6
Hi Joe,

On 10.07.2019 07:04, Joe Perches wrote:
> These GENMASK uses are inverted argument order and the
> actual masks produced are incorrect.  Fix them.
>
> Add checkpatch tests to help avoid more misuses too.
>
> Joe Perches (12):
>   checkpatch: Add GENMASK tests
>   clocksource/drivers/npcm: Fix misuse of GENMASK macro
>   drm: aspeed_gfx: Fix misuse of GENMASK macro
>   iio: adc: max9611: Fix misuse of GENMASK macro
>   irqchip/gic-v3-its: Fix misuse of GENMASK macro
>   mmc: meson-mx-sdio: Fix misuse of GENMASK macro
>   net: ethernet: mediatek: Fix misuses of GENMASK macro
>   net: stmmac: Fix misuses of GENMASK macro
>   rtw88: Fix misuse of GENMASK macro
>   phy: amlogic: G12A: Fix misuse of GENMASK macro
>   staging: media: cedrus: Fix misuse of GENMASK macro
>   ASoC: wcd9335: Fix misuse of GENMASK macro
>
>  drivers/clocksource/timer-npcm7xx.c               |  2 +-
>  drivers/gpu/drm/aspeed/aspeed_gfx.h               |  2 +-
>  drivers/iio/adc/max9611.c                         |  2 +-
>  drivers/irqchip/irq-gic-v3-its.c                  |  2 +-
>  drivers/mmc/host/meson-mx-sdio.c                  |  2 +-
>  drivers/net/ethernet/mediatek/mtk_eth_soc.h       |  2 +-
>  drivers/net/ethernet/mediatek/mtk_sgmii.c         |  2 +-
>  drivers/net/ethernet/stmicro/stmmac/descs.h       |  2 +-
>  drivers/net/ethernet/stmicro/stmmac/dwmac-sun8i.c |  4 ++--
>  drivers/net/wireless/realtek/rtw88/rtw8822b.c     |  2 +-
>  drivers/phy/amlogic/phy-meson-g12a-usb2.c         |  2 +-
>  drivers/staging/media/sunxi/cedrus/cedrus_regs.h  |  2 +-
>  scripts/checkpatch.pl                             | 15 +++++++++++++++
>  sound/soc/codecs/wcd-clsh-v2.c                    |  2 +-
>  14 files changed, 29 insertions(+), 14 deletions(-)
>
After adding following compile time check:

------

diff --git a/Makefile b/Makefile
index 5102b2bbd224..ac4ea5f443a9 100644
--- a/Makefile
+++ b/Makefile
@@ -457,7 +457,7 @@ KBUILD_AFLAGS   := -D__ASSEMBLY__ -fno-PIE
 KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
                   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
                   -Werror=implicit-function-declaration
-Werror=implicit-int \
-                  -Wno-format-security \
+                  -Wno-format-security -Werror=div-by-zero \
                   -std=gnu89
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_AFLAGS_KERNEL :=
diff --git a/include/linux/bits.h b/include/linux/bits.h
index 669d69441a62..61d74b103055 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -19,11 +19,11 @@
  * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
  */
 #define GENMASK(h, l) \
-       (((~UL(0)) - (UL(1) << (l)) + 1) & \
+       (((~UL(0)) - (UL(1) << (l)) + 1 + 0/((h) >= (l))) & \
         (~UL(0) >> (BITS_PER_LONG - 1 - (h))))
 
 #define GENMASK_ULL(h, l) \
-       (((~ULL(0)) - (ULL(1) << (l)) + 1) & \
+       (((~ULL(0)) - (ULL(1) << (l)) + 1 + 0/((h) >= (l))) & \
         (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))
 
 #endif /* __LINUX_BITS_H */

-------

I was able to detect one more GENMASK misue (AARCH64, allyesconfig):

  CC      drivers/phy/rockchip/phy-rockchip-inno-hdmi.o
In file included from ../include/linux/bitops.h:5:0,
                 from ../include/linux/kernel.h:12,
                 from ../include/linux/clk.h:13,
                 from ../drivers/phy/rockchip/phy-rockchip-inno-hdmi.c:9:
../drivers/phy/rockchip/phy-rockchip-inno-hdmi.c: In function
‘inno_hdmi_phy_rk3328_power_on’:
../include/linux/bits.h:22:37: error: division by zero [-Werror=div-by-zero]
  (((~UL(0)) - (UL(1) << (l)) + 1 + 0/((h) >= (l))) & \
                                     ^
../drivers/phy/rockchip/phy-rockchip-inno-hdmi.c:24:42: note: in
expansion of macro ‘GENMASK’
 #define UPDATE(x, h, l)  (((x) << (l)) & GENMASK((h), (l)))
                                          ^~~~~~~
../drivers/phy/rockchip/phy-rockchip-inno-hdmi.c:201:50: note: in
expansion of macro ‘UPDATE’
 #define RK3328_TERM_RESISTOR_CALIB_SPEED_7_0(x)  UPDATE(x, 7, 9)
                                                  ^~~~~~
../drivers/phy/rockchip/phy-rockchip-inno-hdmi.c:1046:26: note: in
expansion of macro ‘RK3328_TERM_RESISTOR_CALIB_SPEED_7_0’
   inno_write(inno, 0xc6, RK3328_TERM_RESISTOR_CALIB_SPEED_7_0(v));


Of course I do not advise to add the check as is to Kernel - it is
undefined behavior area AFAIK.


Regards

Andrzej