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: <20191113104724.GD32553@amd>
Date:   Wed, 13 Nov 2019 11:47:24 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Jason Gerecke <jason.gerecke@...om.com>,
        Aaron Armstrong Skomra <aaron.skomra@...om.com>,
        Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH 4.19 027/125] HID: wacom: generic: Treat serial number
 and related fields as unsigned

Hi!

> 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);
    if (n==32) return ((__u32)value);
    return value & ((1 << n) - 1);

instead?

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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