Date:   Tue, 3 Dec 2019 18:38:21 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Abhishek Pandit-Subedi <abhishekpandit@...omium.org>,
        linux-input@...r.kernel.org, linux-bluetooth@...r.kernel.org,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Logan Gunthorpe <logang@...tatee.com>,
        Andrey Smirnov <andrew.smirnov@...il.com>,
        Kirill Smelkov <kirr@...edi.com>
Subject: Re: [PATCH] Input: uinput - Add UI_SET_UNIQ ioctl handler

On Tuesday 03 December 2019 00:09:47 Pali Rohár wrote:
> On Monday 02 December 2019 11:36:28 Dmitry Torokhov wrote:
> > On Mon, Dec 02, 2019 at 07:53:40PM +0100, Pali Rohár wrote:
> > > On Monday 02 December 2019 09:54:40 Dmitry Torokhov wrote:
> > > > On Mon, Dec 02, 2019 at 09:47:50AM +0100, Pali Rohár wrote:
> > > > > On Sunday 01 December 2019 17:23:05 Dmitry Torokhov wrote:
> > > > > > Hi Pali,
> > > > > > 
> > > > > > On Sun, Dec 01, 2019 at 03:53:57PM +0100, Pali Rohár wrote:
> > > > > > > Hello!
> > > > > > > 
> > > > > > > On Wednesday 27 November 2019 10:51:39 Abhishek Pandit-Subedi wrote:
> > > > > > > > Support setting the uniq attribute of the input device. The uniq
> > > > > > > > attribute is used as a unique identifier for the connected device.
> > > > > > > > 
> > > > > > > > For example, uinput devices created by BlueZ will store the address of
> > > > > > > > the connected device as the uniq property.
> > > > > > > > 
> > > > > > > > Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@...omium.org>
> > > > > > > 
> > > > > > > ...
> > > > > > > 
> > > > > > > > diff --git a/include/uapi/linux/uinput.h b/include/uapi/linux/uinput.h
> > > > > > > > index c9e677e3af1d..d5b7767c1b02 100644
> > > > > > > > --- a/include/uapi/linux/uinput.h
> > > > > > > > +++ b/include/uapi/linux/uinput.h
> > > > > > > > @@ -145,6 +145,7 @@ struct uinput_abs_setup {
> > > > > > > >  #define UI_SET_PHYS		_IOW(UINPUT_IOCTL_BASE, 108, char*)
> > > > > > > >  #define UI_SET_SWBIT		_IOW(UINPUT_IOCTL_BASE, 109, int)
> > > > > > > >  #define UI_SET_PROPBIT		_IOW(UINPUT_IOCTL_BASE, 110, int)
> > > > > > > > +#define UI_SET_UNIQ		_IOW(UINPUT_IOCTL_BASE, 111, char*)
> > > > > > > 
> > > > > > > I think that usage of char* as type in _IOW would cause compatibility
> > > > > > > problems like it is for UI_SET_PHYS (there is UI_SET_PHYS_COMPAT). Size
> > > > > > > of char* pointer depends on userspace (32 vs 64bit), so 32bit process on
> > > > > > > 64bit kernel would not be able to call this new UI_SET_UNIQ ioctl.
> > > > > > > 
> > > > > > > I would suggest to define this ioctl as e.g.:
> > > > > > > 
> > > > > > >   #define UI_SET_UNIQ		_IOW(_IOC_WRITE, UINPUT_IOCTL_BASE, 111, 0)
> > > > > > > 
> > > > > > > And then in uinput.c code handle it as:
> > > > > > > 
> > > > > > >   case UI_SET_UNIQ & ~IOCSIZE_MASK:
> > > > > > > 
> > > > > > > as part of section /* Now check variable-length commands */
> > > > > > 
> > > > > > If we did not have UI_SET_PHYS in its current form, I'd agree with you,
> > > > > > but I think there is benefit in having UI_SET_UNIQ be similar to
> > > > > > UI_SET_PHYS.
> > > > > 
> > > > > I thought that ioctl is just number, so we can define it as we want. And
> > > > > because uinput.c has already switch for variable-length commands it
> > > > > would be easy to use it. Final handling can be in separate function like
> > > > > for UI_SET_PHYS which can look like same.
> > > > 
> > > > Yes, we can define ioctl number as whatever we want. What I was trying
> > > > to say, right now users do this:
> > > > 
> > > > 	rc = ioctl(fd, UI_SET_PHYS, "whatever");
> > > > 	...
> > > > 
> > > > and with UI_SET_UNIQ they expect the following to work:
> > > > 
> > > > 	rc = ioctl(fd, UI_SET_UNIQ, "whatever");
> > > > 	...
> > > 
> > > And would not following definition
> > > 
> > >   #define UI_SET_UNIQ _IOW(_IOC_WRITE, UINPUT_IOCTL_BASE, 111, 0)
> > > 
> > > allow userspace to call
> > > 
> > >   rc = ioctl(fd, UI_SET_UNIQ, "whatever");
> > > 
> > > as you want?
> > 
> > OK, so what you are saying is that we can have whatever in the size
> > portion of ioctl number and simply ignore it in the driver
> 
> Yes.
> 
> > (and I do not
> > think we need to do any of "UI_SET_UNIQ & ~IOCSIZE_MASK" really).
> 
> You are right, we do not need to clear any IOCSIZE_MASK. As ioctl number
> would be always sam constant number. So it would be really simple. So
> original patch would work if UI_SET_UNIQ define would be changed to
> above with _IOW() macro.
> 
> > While this would work, I am not sure it is the best option as I think
> > we'd have to comment extensively why we have arbitrary number in place
> > of the size.
> 
> Comment can be added. But this is as ioctl is going to accept variable
> length array (not fixed array), zero value make sense for me (zero as we
> do not know exact size).
> 
> > And we still do not really save anything, as we still have to go through
> > compat ioctl handler (since we have it already) and it is very simple to
> > add a case for UI_SET_UNIQ there...
> 
> Yes, compat ioctl is still used. But my proposed solution does not
> involve to define a new compact ioctl number just for sizeof(char *).
> 
> I'm looking at this particular problem from side, that there is no
> reason to define two new ioctl numbers for UI_SET_UNIQ (one normal
> number and one compat number), when one number is enough. It is one new
> ioctl call, so one ioctl number should be enough.
> 
> And also with my proposed solution with ioctl size=0 it simplify
> implementation of UI_SET_UNIQ as it is not needed to implement also
> UI_SET_UNIQ_COMPAT ioctl nor touch compat ioct code path. Basically
> original patch (with changed UI_SET_UNIQ macro) is enough.
> 
> But of of course, this is my view of this problem and I would not be
> against your decision from maintainer position. Both solutions would
> work correctly and bring same behavior for userspace applications.


Hi Dmitry!

I was looking again at those _IOW defines for ioctl calls and I have
another argument why not specify 'char *' in _IOW:

All ioctls in _IOW() specify as a third macro argument type which is
passed as pointer to the third argument for ioctl() syscall.

So e.g.:

  #define EVIOCSCLOCKID _IOW('E', 0xa0, int)

is called from userspace as:

  int val;
  ioctl(fd, EVIOCSCLOCKID, &val);

Or

  #define EVIOCSMASK _IOW('E', 0x93, struct input_mask)

is called as:

  struct input_mask val;
  ioctl(fd, EVIOCSMASK, &val);

So basically third argument for _IOW specify size of byte buffer passed
as third argument for ioctl(). In _IOW is not specified pointer to
struct input_mask, but struct input_mask itself.

And in case you define

  #define MY_NEW_IOCTL _IOW(UINPUT_IOCTL_BASE, 200, char*)

then you by above usage you should pass data as:

  char *val = "DATA";
  ioctl(fd, MY_NEW_IOCTL, &val);

Which is not same as just:

  ioctl(fd, MY_NEW_IOCTL, "DATA");

As in former case you passed pointer to pointer to data and in later
case you passed only pointer to data.

It just mean that UI_SET_PHYS is already defined inconsistently which is
also reason why compat ioctl for it was introduced.

-- 
Pali Rohár
pali.rohar@...il.com

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