[PATCH 1/2] Input: Document and check on implicitly defined FF_MAX_EFFECTS

[Elias Vanderstuyft]

There is an undocumented upper bound for the total number of ff effects:
    FF_GAIN (= 96).
This can be found as follows:
- user: write(EV_FF, effect_id, iterations)
    calls kernel: ff->playback(effect_id, ...): starts effect "effect_id"
- user: write(EV_FF, FF_GAIN, gain)
    calls kernel: ff->set_gain(gain, ...): sets gain

A collision occurs when effect_id equals FF_GAIN.
According to input_ff_event(),
FF_GAIN is the smallest value where a collision occurs.
Therefore the greatest safe value for effect_id is FF_GAIN - 1,
and thus the total number of effects should never exceed FF_GAIN.

Define FF_MAX_EFFECTS as FF_GAIN and check on this limit in ff-core.

Signed-off-by: Elias Vanderstuyft <elias.vds@xxxxxxxxx>
---
 drivers/input/ff-core.c    | 5 +++++
 include/uapi/linux/input.h | 8 ++++++++
 2 files changed, 13 insertions(+)

diff --git a/drivers/input/ff-core.c b/drivers/input/ff-core.c
index c642082..0bf7008 100644
--- a/drivers/input/ff-core.c
+++ b/drivers/input/ff-core.c
@@ -318,6 +318,11 @@ int input_ff_create(struct input_dev *dev, unsigned int max_effects)
 		return -EINVAL;
 	}
 
+	if (max_effects > FF_MAX_EFFECTS) {
+		dev_err(&dev->dev, "cannot allocate more than FF_MAX_EFFECTS effects\n");
+		return -EINVAL;
+	}
+
 	ff_dev_size = sizeof(struct ff_device) +
 				max_effects * sizeof(struct file *);
 	if (ff_dev_size < max_effects) /* overflow */
diff --git a/include/uapi/linux/input.h b/include/uapi/linux/input.h
index 63915a7..42d7933 100644
--- a/include/uapi/linux/input.h
+++ b/include/uapi/linux/input.h
@@ -1200,6 +1200,14 @@ struct ff_effect {
 #define FF_GAIN		0x60
 #define FF_AUTOCENTER	0x61
 
+/*
+ * ff->playback(effect_id = FF_GAIN) is the first effect_id to
+ * cause a collision with another ff method, in this case ff->set_gain().
+ * Therefore the greatest safe value for effect_id is FF_GAIN - 1,
+ * and thus the total number of effects should never exceed FF_GAIN.
+ */
+#define FF_MAX_EFFECTS	FF_GAIN
+
 #define FF_MAX		0x7f
 #define FF_CNT		(FF_MAX+1)
 
-- 
1.9.3

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