support InputMask for event filtering

[aktau]

My little program reads events from a bluetooth controller (it's a little kiosk application activator).

It uses non-zero CPU, and causes ~8% CPU usage in bluetoothd just reading all the events. The PS5 DualSense controllers sends a lot of events from its various sensors.

I only want button presses though.

I encountered InputMask in the code:

go-evdev/types.go

Lines 73 to 78 in 66d1748

	// InputMask ...
	type InputMask struct {
	Type uint32
	CodesSize uint32
	CodesPtr uint64
	}

But it's not used. Then I found:

https://github.com/srsholmes/loadout/blob/6af41dc3b0d627d661f58b34557671a45c476569/apps/loadout-overlay/src/bun/native/input-intercept.ts#L182-L187

Which shows that it can be used for the purpose I want. It would be nice to support this.

[zonque]

Sounds like a good addition. You wanna cook up a patch?

[aktau]

I had the patch ready, and then my eMMC decided to die. Code lost. At any rate, one thing that vexed me for some time was that the ioctl would fail with EINVAL for some event types. Both CodesSize and CodesPtr seemed accurate, and were working with other event types.

I ended up finding the root cause. The bits_from_user helper demands that the input codes size (maxlen) is a multiple of long (8 on 64-bit systems).

I've found no other client code that takes this into account. Mostly because they all focus on event types which have natural multiples of 8 in their *_CNT. Except for this StackOverflow example, which does it right, but doesn't point out the reason that unsigned long is used for the chunk size.

TL;DR: If I ever re-make this patch, I need to make sure to round up to the nearest power of uintpr.

---

• "docs": https://github.com/torvalds/linux/blob/94515f3a7d4256a5062176b7d6ed0471938cd51a/include/uapi/linux/input.h#L220-L241
• evdev_do_ioctl: https://github.com/torvalds/linux/blob/94515f3a7d4256a5062176b7d6ed0471938cd51a/drivers/input/evdev.c#L1115 calls...
• evdev_set_mask: https://github.com/torvalds/linux/blob/94515f3a7d4256a5062176b7d6ed0471938cd51a/drivers/input/evdev.c#L953-L986 calls...
• bits_from_user: https://github.com/torvalds/linux/blame/94515f3a7d4256a5062176b7d6ed0471938cd51a/drivers/input/evdev.c#L661-L696

Highlighting the important parts:

static long evdev_do_ioctl(struct file *file, unsigned int cmd, void __user *p, int compat_mode) {
// ...
 	case EVIOCSMASK: {
		const void __user *codes_ptr;

		if (copy_from_user(&mask, p, sizeof(mask)))
			return -EFAULT;

		codes_ptr = (const void __user *)(unsigned long)mask.codes_ptr;
		return evdev_set_mask(client, mask.type, codes_ptr, mask.codes_size, compat_mode); // // <------ HERE
	}
// ...
}

static int evdev_set_mask(struct evdev_client *client, unsigned int type, const void __user *codes, u32 codes_size, int compat) {
	/* we allow unknown types and 'codes_size > size' for forward-compat */
	size_t cnt = evdev_get_mask_cnt(type);
	if (!cnt)
		return 0;

       // ...

	error = bits_from_user(mask, cnt - 1, codes_size, codes, compat);// <------ HERE
	if (error < 0) {
		bitmap_free(mask);
		return error;
	}

        // ...
	return 0;
}

static int bits_from_user(unsigned long *bits, unsigned int maxbit, unsigned int maxlen, const void __user *p, int compat) {
	int len, i;

	if (maxlen % sizeof(long)) // <------ HERE
		return -EINVAL;
	len = BITS_TO_LONGS(maxbit) * sizeof(long);
	if (len > maxlen)
		len = maxlen;
	if (copy_from_user(bits, p, len))
		return -EFAULT;
	return len;
}

[aktau]

Sent #21. PTAL.

[aktau]

@zonque can you review #21?