Re: [PATCH v3 8/8] leds: leds-mt6323: Add support for WLEDs and MT6332

From: Nathan Chancellor <nathan@kernel.org>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: pavel@ucw.cz, lee@kernel.org, sean.wang@mediatek.com,
	robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
	matthias.bgg@gmail.com, linux-leds@vger.kernel.org,
	devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org,
	kernel@collabora.com, llvm@lists.linux.dev
Subject: Re: [PATCH v3 8/8] leds: leds-mt6323: Add support for WLEDs and MT6332
Date: Wed, 21 Jun 2023 21:31:24 +0000	[thread overview]
Message-ID: <20230621213124.GA2689001@dev-arch.thelio-3990X> (raw)
In-Reply-To: <20230601110813.2373764-9-angelogioacchino.delregno@collabora.com>

Hi Angelo,

On Thu, Jun 01, 2023 at 01:08:13PM +0200, AngeloGioacchino Del Regno wrote:
> Add basic code to turn on and off WLEDs and wire up MT6332 support
> to take advantage of it.
> This is a simple approach due to the aforementioned PMIC supporting
> only on/off status so, at the time of writing, it is impossible for me
> to validate more advanced functionality due to lack of hardware.
> 
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>

After this patch as commit 9bb0a9e0626c ("leds: leds-mt6323: Add support
for WLEDs and MT6332") in -next, I see the following warnings from
clang, which are basically flagging potential kernel Control Flow
Integrity [1] violations that will be visible at runtime (this warning
is not enabled for the kernel yet but we would like it to be):

  drivers/leds/leds-mt6323.c:598:49: error: incompatible function pointer types assigning to 'int (*)(struct led_classdev *, enum led_brightness)' from 'int (struct led_classdev *, unsigned int)' [-Werror,-Wincompatible-function-pointer-types-strict]
    598 |                         leds->led[reg]->cdev.brightness_set_blocking =
        |                                                                      ^
    599 |                                                 mt6323_wled_set_brightness;
        |                                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/leds/leds-mt6323.c:600:40: error: incompatible function pointer types assigning to 'enum led_brightness (*)(struct led_classdev *)' from 'unsigned int (struct led_classdev *)' [-Werror,-Wincompatible-function-pointer-types-strict]
    600 |                         leds->led[reg]->cdev.brightness_get =
        |                                                             ^
    601 |                                                 mt6323_get_wled_brightness;
        |                                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~
  2 errors generated.

From what I can tell/understand, 'enum led_brightness' is obsolete and
the value that is passed via ->brightness_set_blocking() is an 'unsigned
int' as well but it seems 'enum led_brightness' is used as the parameter
in a lot of different callback implementations, so the prototype cannot
be easily updated without a lot of extra work. Is there any reason not
to just do something like this to avoid this issue?

[1]: https://lwn.net/Articles/898040/

Cheers,
Nathan

diff --git a/drivers/leds/leds-mt6323.c b/drivers/leds/leds-mt6323.c
index e8fecfc2e90a..24f35bdb55fb 100644
--- a/drivers/leds/leds-mt6323.c
+++ b/drivers/leds/leds-mt6323.c
@@ -76,7 +76,7 @@ struct mt6323_led {
 	int			id;
 	struct mt6323_leds	*parent;
 	struct led_classdev	cdev;
-	unsigned int		current_brightness;
+	enum led_brightness	current_brightness;
 };
 
 /**
@@ -451,7 +451,7 @@ static int mtk_wled_hw_off(struct led_classdev *cdev)
 	return 0;
 }
 
-static unsigned int mt6323_get_wled_brightness(struct led_classdev *cdev)
+static enum led_brightness mt6323_get_wled_brightness(struct led_classdev *cdev)
 {
 	struct mt6323_led *led = container_of(cdev, struct mt6323_led, cdev);
 	struct mt6323_leds *leds = led->parent;
@@ -471,7 +471,7 @@ static unsigned int mt6323_get_wled_brightness(struct led_classdev *cdev)
 }
 
 static int mt6323_wled_set_brightness(struct led_classdev *cdev,
-				      unsigned int brightness)
+				      enum led_brightness brightness)
 {
 	struct mt6323_led *led = container_of(cdev, struct mt6323_led, cdev);
 	struct mt6323_leds *leds = led->parent;

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