[PATCH v3] leds: pwm: Annotate leds[] with __counted_by()

[Mert Seftali <mertsftl@gmail.com>]

Add the __counted_by() attribute to the flexible array member leds[] in
struct led_pwm_priv so the compiler and runtime (e.g. FORTIFY_SOURCE,
UBSAN_BOUNDS) can bounds-check accesses against num_leds.

For the annotation to be correct, num_leds must equal the number of
allocated elements before leds[] is accessed. Even taking the address
&priv->leds[i] is bounds-checked against num_leds under UBSAN_BOUNDS. The
driver allocates device_get_child_node_count() elements up front, so set
num_leds to that count right after allocation.

While here, pass the led_pwm_data element into led_pwm_add() instead of
the whole led_pwm_priv, so the helper no longer needs to index the array;
the caller walks a led_pwm_data pointer over priv->leds.

No functional change intended.

Suggested-by: Lee Jones <lee@kernel.org>
Signed-off-by: Mert Seftali <mertsftl@gmail.com>
---
Changes in v3:
- v2 followed the review suggestion to pass the element and increment
  num_leds in the caller. That is exactly what Sashiko caught: with
  __counted_by(num_leds), &priv->leds[num_leds] is bounds-checked, so
  incrementing num_leds from 0 as the loop runs trips UBSAN_BOUNDS on
  probe. num_leds is now set to the full count up front and the caller
  walks a led_pwm_data pointer, which keeps the element-passing and the
  dropped index variable, just without the running counter.

Changes in v2 (per Lee Jones review):
- Pass the led_pwm_data element into led_pwm_add() so it drops the priv and
  index arguments.

Build-tested only (no PWM-LED hardware). The __counted_by bounds behaviour
that motivated v3 was verified with a standalone UBSAN reproducer on gcc and
clang.

 drivers/leds/leds-pwm.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/leds/leds-pwm.c b/drivers/leds/leds-pwm.c
index 6c1f2f50ff85..d1116a5cb2b2 100644
--- a/drivers/leds/leds-pwm.c
+++ b/drivers/leds/leds-pwm.c
@@ -36,7 +36,7 @@ struct led_pwm_data {
 
 struct led_pwm_priv {
 	int num_leds;
-	struct led_pwm_data leds[];
+	struct led_pwm_data leds[] __counted_by(num_leds);
 };
 
 static int led_pwm_set(struct led_classdev *led_cdev,
@@ -81,10 +81,9 @@ static int led_pwm_default_brightness_get(struct fwnode_handle *fwnode,
 }
 
 __attribute__((nonnull))
-static int led_pwm_add(struct device *dev, struct led_pwm_priv *priv,
+static int led_pwm_add(struct device *dev, struct led_pwm_data *led_data,
 		       struct led_pwm *led, struct fwnode_handle *fwnode)
 {
-	struct led_pwm_data *led_data = &priv->leds[priv->num_leds];
 	struct led_init_data init_data = { .fwnode = fwnode };
 	int ret;
 
@@ -167,12 +166,12 @@ static int led_pwm_add(struct device *dev, struct led_pwm_priv *priv,
 		}
 	}
 
-	priv->num_leds++;
 	return 0;
 }
 
 static int led_pwm_create_fwnode(struct device *dev, struct led_pwm_priv *priv)
 {
+	struct led_pwm_data *led_data = priv->leds;
 	struct led_pwm led;
 	int ret;
 
@@ -193,7 +192,7 @@ static int led_pwm_create_fwnode(struct device *dev, struct led_pwm_priv *priv)
 
 		led.default_state = led_init_default_state_get(fwnode);
 
-		ret = led_pwm_add(dev, priv, &led, fwnode);
+		ret = led_pwm_add(dev, led_data++, &led, fwnode);
 		if (ret)
 			return ret;
 	}
@@ -217,6 +216,8 @@ static int led_pwm_probe(struct platform_device *pdev)
 	if (!priv)
 		return -ENOMEM;
 
+	priv->num_leds = count;
+
 	ret = led_pwm_create_fwnode(&pdev->dev, priv);
 
 	if (ret)
-- 
2.55.0