Commit 33b96d93 authored Oct 14, 2015 by Elias Vanderstuyft Committed by Dmitry Torokhov Oct 16, 2015

Input: document and check on implicitly defined FF_MAX_EFFECTS



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@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

parent 52a92667

drivers/input/ff-core.c

+5 −0

Original line number	Diff line number	Diff line
		@@ -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 */

include/uapi/linux/input.h

+8 −0

Original line number	Diff line number	Diff line
		@@ -414,6 +414,14 @@ 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)

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