rfkill: use short writes and accept long reads

rfkill: use short writes and accept long reads

I'm seeing the following with kernel-core-5.10.16-200.fc33.x86_64:

$ sudo SYSTEMD_LOG_LEVEL=debug build/systemd-rfkill
Reading struct rfkill_event: got 8 bytes.
A new rfkill device has been added with index 0 and type bluetooth.
Found cgroup2 on /sys/fs/cgroup/, full unified hierarchy
Found container virtualization none.
rfkill0: Operating on rfkill device 'tpacpi_bluetooth_sw'.
Writing struct rfkill_event successful (8 of 9 bytes).
Loaded state '0' from /var/lib/systemd/rfkill/platform-thinkpad_acpi:bluetooth.
Reading struct rfkill_event: got 8 bytes.
A new rfkill device has been added with index 1 and type wwan.
rfkill1: Operating on rfkill device 'tpacpi_wwan_sw'.
Writing struct rfkill_event successful (8 of 9 bytes).
Loaded state '0' from /var/lib/systemd/rfkill/platform-thinkpad_acpi:wwan.
Reading struct rfkill_event: got 8 bytes.
A new rfkill device has been added with index 2 and type bluetooth.
rfkill2: Operating on rfkill device 'hci0'.
Writing struct rfkill_event successful (8 of 9 bytes).
Loaded state '0' from /var/lib/systemd/rfkill/pci-0000:00:14.0-usb-0:7:1.0:bluetooth.
Reading struct rfkill_event: got 8 bytes.
A new rfkill device has been added with index 3 and type wlan.
rfkill3: Operating on rfkill device 'phy0'.
Writing struct rfkill_event successful (8 of 9 bytes).
Loaded state '0' from /var/lib/systemd/rfkill/pci-0000:04:00.0:wlan.
All events read and idle, exiting.

We were expecting a read of exactly RFKILL_EVENT_SIZE_V1==8 bytes. But the
structure has 9 after [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=14486c82612a177cb910980c70ba900827ca0894

For some reason the kernel does not accept the full structure size, but cuts
the write short after 8 bytes:

static ssize_t rfkill_fop_write(struct file *file, const char __user *buf,
				size_t count, loff_t *pos)
{
	struct rfkill_event ev;

	/* we don't need the 'hard' variable but accept it */
	if (count < RFKILL_EVENT_SIZE_V1 - 1)
		return -EINVAL;

	/*
	 * Copy as much data as we can accept into our 'ev' buffer,
	 * but tell userspace how much we've copied so it can determine
	 * our API version even in a write() call, if it cares.
	 */
	count = min(count, sizeof(ev));
	if (copy_from_user(&ev, buf, count))
		return -EFAULT;

... so it should accept the full size. I'm not sure what is going on here.

But we don't care about the extra fields, so let's accept a write as long as
it's at least RFKILL_EVENT_SIZE_V1.

Fixes #18677.

(cherry picked from commit a71c09685021cbcecb7566a00342421f635cc002)