Re: INPUT_COMPAT_TEST

["H. Peter Anvin" <hpa@zytor.com>]

On 07/08/2011 03:37 PM, Dmitry Torokhov wrote:
>>
>>> We also have similar issues with uinput API and uploading force-freedack
>>> effects.
>>
>> Those are ioctl, though, if I read the code right, or did I miss
>> something obvious?
> 
> Ah, yes, indeed.
> 

So the point still holds... you're right now using INPUT_COMPAT_TEST for
those, but what you *should* use is whether or not you were entered via
the compat ioctl entry point.

>>
>>>> but it looks like input also
>>>> does things like change the format(?!) of sysfs entries, all of which
>>>> makes me very concerned.
>>>
>>> Another historical unfortunate decision. /proc/bus/input (and later
>>> added sysfs entries) export bitmaps in "compressed" form so that
>>> userspace can not figure out the size of the segment (32 or 64 bit) on
>>> its own so we have to convert to userspace size for longs.
>>
>> "Compressed form"?  Could you give a concrete example?  They look like
>> they are emitted in text form.
> 
> We drop leading zeroes so if you get "1 0 0 1ffff" you do not know
> the bit position of the most significant '1' unless we keep segments of
> known size. Unfortunately we started with 32 bit segments on 32 bit
> kernels and 64 bit segments on 64 bit kernels so we coudl not simply say
> that we always split on 32 bit boundary when we discovered compat
> problem a few years later.

Ah yes, it is the "binary output masquerading as text, so we end up with
something that is worse a mess than either" problem.

>>
>> Do you have a program that someone could run to see the differences
>> between compat and non-compat paths?
> 
> Hmm, cat for /proc/bus/input/devices and sysfs nodes and evtest would
> either work or give garbage if compat code woudl not work.
> 

I'm desperately trying to come up with a solution which doesn't require
us to replicate every single system call (which is what relying on
is_compat_task() does -- it remembers the entry point used) in order
support one single misdesigned subsystem.  Do you have any kind of ideas
for what we might be able to do?

	-hpa