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    SubjectRe: [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@chromium.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@gmail.com
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