Re: ff-core effect id handling in case of a failed effect upload

(added Dmitry to CC)

19.02.2014 13:42, Elias Vanderstuyft kirjoitti:

Hi,

Hi,

In the process of reviewing the Wine DInput translation layer, I
noticed an inconvenience (in the ff-core implementation?) that can
possibly lead to confusing problems to application developers (not
only for Wine), in short:
If a new (id==-1) effect was uploaded (look at
ff-core.c::input_ff_upload(...)) that failed (e.g. returning EINVAL),
ff-core will have assigned a positive number to the effect id. This
can be confusing because the dev->ff->effects[] array will not contain
an element at the index of that new effect id.

I agree that this is a bit confusing, and the kernel code should
probably be modified to not clobber the ioctl-provided effect on failure
(effect->id is set to an "undefined" value, i.e. next free effect slot).

Dmitry, WDYT?

The downside is that if changed, any new userspace code may unknowingly
depend on the fixed non-clobbering behavior (though apparently they
already do, as evidenced by Wine DInput, so might not be much of an
argument...).

Here is a more elaborated description/discussion:
- This is a bug that is either the responsibility of the Linux kernel,
or of Wine (and possibly other applications that do the same thing as
described below):
    It is caused by the following situation:
        When uploading an effect, the specific kernel device driver
may return an error,
        e.g. EINVAL for example when a periodic's effect period is set to zero.
        This error will then be returned by "ioctl(*(This->fd),
EVIOCSFF, &This->effect)".
        With or without error, one can find out that
/drivers/input/ff-core.c:input_ff_upload(...) is called,
        which will set effect->id >= 0, if it was set to -1 (=> new
effect created) by the user.

        But in case of an error:
        - Assume effect->id was set to -1 by the user:
            The error is reported by ff->upload(...) at
/drivers/input/ff-core.c:167,
            the effect->id will also be set >= 0 (*).
            The offending effect will not be saved in the
ff->effects[] array (***).
        - Assume effect->id was set >= 0 by the user (and
ff->effects[effect->id] is a valid existing effect):
            The error is reported by ff->upload(...) at
/drivers/input/ff-core.c:167,
            the effect->id will remain unchanged (**).
            The offending effect will not overwrite the
ff->effects[effect->id] element (****).

        Is this (see *, **, *** and ****) desired behaviour?
        - If yes:
            Change the following in Wine's dinput/effect_linuxinput.c:90 :
                if (ioctl(*(This->fd), EVIOCSFF, &This->effect) == -1) {
            to :
                int effectId_old = This->effect.id;
                if (ioctl(*(This->fd), EVIOCSFF, &This->effect) == -1) {
                    This->effect.id = effectId_old;
        - If no for *:
            Kernel code /drivers/input/ff-core.c:input_ff_upload(...)
has to be patched to revert "effect->id" back to its original value
set by the user,
            which is only needed when the initial (by user) value of
"effect->id" was equal to -1.
        - If no for **** (or maybe also ***):
            ff->effects[effect->id] could be replaced by an 'empty'
effect (however this can get complex because the effect's type has to
remain unchanged)
            This would be a change in the kernel code
/drivers/input/ff-core.c:input_ff_upload(...).
        - If no for **:
            I don't really know. Discussion is needed.

        - In my opinion, **, *** and **** are desired behaviour, while
* should leave the effect->id at -1.

Right.

            In that case, Wine's dinput implementation does not have
to be patched, and the kernel code only should apply a minor patch.

Well, maybe better to change dinput anyway so that it can work properly
with current kernel versions (assuming this gets changed in the future)?
Not my call, though.

-- 
Anssi Hannula