Re: [PATCH] Input: joydev - prevent potential write out of bounds in ioctl

I'm still studying "git send-email", so the first intro part of prev msg deleted, sorry, again:

Continuying my previous message, the JSIOCGBTNMAP always returns 1024 return code,
but not "amount of buttons" like I said before
(that is probably the size of the keymap that is _u16 keymap[KEY_MAX - BTN_MISC + 1] ).
Is it correct?
Reading the line of kernel joydev.c
579	len = min_t(size_t, _IOC_SIZE(cmd), sizeof(joydev->keypam)),
, why the min is always sizeof(joydev->keypam) ?
If I try to call from the userspace
ioctl(fd, JSIOCGBTNMAP, buttons)
where the buttons is "__u16 buttons[5]", then still I get 1024.

Also I did userspace test (that shows how kernel overwrites (out of array bound) the userspace):
1. The buttons is "__u16 buttons[5]" in userspace,
2. buttons[5] = 1;
3. ioctl(fd, JSIOCGBTNMAP, buttons)
4. printf("new %i\n", buttons[5]),
and the output is "new 0", so the value is being overwritten by kernel (so 1024 bytes copied
to 10 bytes buffer).

It looks like I don't understand what is the expected value of the _IOC_SIZE(cmd),
why not 10 for this read ioctl example? Is it possible to make this ioctl safe, so
it doesn't copy more data than user can handle?