linux-kernel - Re: uinput: waiting for UI_FF_UPLOAD events will not inform user when allocation is required

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Message-ID: <CA+fyA4Qsm6ydyxNQL4cLak+1eNYBvOJzJwonts84YTavBKB8vg@mail.gmail.com>
Date:   Wed, 11 Oct 2023 17:10:09 +1030
From:   John Salamon <salamonj9@...il.com>
To:     dmitry.torokhov@...il.com, rydberg@...math.org
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: uinput: waiting for UI_FF_UPLOAD events will not inform user when
 allocation is required

Correction, "old" is a pointer to struct ff_effect, which after being
set to NULL looks like it gets pushed out by uinput on a struct
uinput_request.


On Tue, Oct 10, 2023 at 5:38 PM John Salamon <salamonj9@...il.com> wrote:
>
> Currently the "fake" input events generated by uinput in response to
> effect uploads will return an effect with an id that has already been
> handled by input_ff_upload in ff-core.c, which can modify the effect
> id. This causes a problem specifically when the effect originally
> uploaded via the EVIOCSFF ioctl contained an effect with -1, as the
> userspace code handling UI_FF_UPLOAD receives an effect with an id
> other than -1, and therefore will not know an allocation was
> requested.
>
> I notice that the "old" field on the ff_effect struct is set to NULL
> when the -1 id is changed (in input_ff_upload), which can serve as a
> flag that an allocation was requested. If it is the intention is that
> uinput users check if old == NULL to know when allocations are needed
> I think uinput documentation should describe this.
>
> I first noticed this using python-evdev, see my issue report here:
> https://github.com/gvalkov/python-evdev/issues/199