linux-kernel - Re: [PATCH] Input: uinput - Add UI_SET

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Date:   Mon, 2 Dec 2019 19:53:40 +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 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?

> They would not expect a variable length IOCTL here, or expect a
> fixed-size string, nor do they expect to cast pointer to u64. So keeping
> the spirit of UI_SET_PHYS, even if it is not great from 64/32 bit point
> of view is beneficial here.
> 
> > 
> > > But you are absolutely correct that in current form the patch is
> > > deficient on 64/32 systems, and the compat handling needs to be added
> > > before it can be accepted.
> > 
> > Is not better to avoid usage of compat ioctl? Or it is OK to use compat
> > ioctl also for new features? I do not know if there are some kernel
> > rules for it or not... But for me it sounds like "compatibility layer
> > for older code".
> 
> Yes, if uinput driver did not have any compat code in it, we would not
> want to add it. But alas! we already need to handle compat cases for
> expsting API, so consistency is more important than purity (in my
> opinion) here.
> 
> Thanks.
> 

-- 
Pali Rohár
pali.rohar@...il.com

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