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Input: document and check on implicitly defined FF_MAX_EFFECTS

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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>
This commit is contained in:
Elias Vanderstuyft 2015-10-14 17:29:37 -07:00 committed by Dmitry Torokhov
parent 52a9266788
commit 33b96d9349
2 changed files with 13 additions and 0 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

5
drivers/input/ff-core.c
View file

@ -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 */

8
include/uapi/linux/input.h
View file

@ -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)