linux-kernel - Re: [PATCH 4.19 027/125] HID: wacom: generic: Treat serial number and related fields as unsigned

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Message-ID: <nycvar.YFH.7.76.1911131201010.1799@cbobk.fhfr.pm>
Date:   Wed, 13 Nov 2019 12:02:11 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Pavel Machek <pavel@...x.de>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Jason Gerecke <jason.gerecke@...om.com>,
        Aaron Armstrong Skomra <aaron.skomra@...om.com>
Subject: Re: [PATCH 4.19 027/125] HID: wacom: generic: Treat serial number
 and related fields as unsigned

On Wed, 13 Nov 2019, Pavel Machek wrote:

> > From: Jason Gerecke <killertofu@...il.com>
> > 
> > commit ff479731c3859609530416a18ddb3db5db019b66 upstream.
> > 
> > The HID descriptors for most Wacom devices oddly declare the serial
> > number and other related fields as signed integers. When these numbers
> > are ingested by the HID subsystem, they are automatically sign-extended
> > into 32-bit integers. We treat the fields as unsigned elsewhere in the
> > kernel and userspace, however, so this sign-extension causes problems.
> > In particular, the sign-extended tool ID sent to userspace as ABS_MISC
> > does not properly match unsigned IDs used by xf86-input-wacom and libwacom.
> > 
> > We introduce a function 'wacom_s32tou' that can undo the automatic sign
> > extension performed by 'hid_snto32'. We call this function when processing
> > the serial number and related fields to ensure that we are dealing with
> > and reporting the unsigned form. We opt to use this method rather than
> > adding a descriptor fixup in 'wacom_hid_usage_quirk' since it should be
> > more robust in the face of future devices.
> 
> > +++ b/drivers/hid/wacom.h
> > @@ -205,6 +205,21 @@ static inline void wacom_schedule_work(s
> >  	}
> >  }
> >  
> > +/*
> > + * Convert a signed 32-bit integer to an unsigned n-bit integer. Undoes
> > + * the normally-helpful work of 'hid_snto32' for fields that use signed
> > + * ranges for questionable reasons.
> > + */
> > +static inline __u32 wacom_s32tou(s32 value, __u8 n)
> > +{
> > +	switch (n) {
> > +	case 8:  return ((__u8)value);
> > +	case 16: return ((__u16)value);
> > +	case 32: return ((__u32)value);
> > +	}
> > +	return value & (1 << (n - 1)) ? value & (~(~0U << n)) : value;
> > +}
> 
> Can we do something like:
> 
>     BUG_ON(n>32);

Please no BUG_ON()s in bitop helpers.

Thanks,

-- 
Jiri Kosina
SUSE Labs