Re: [PATCH] Input: zforce_ts - fix playload length check

[Dirk Behme]

On 28.07.2015 12:23, Geert Uytterhoeven wrote:
> On Mon, Jul 27, 2015 at 11:06 PM, Dmitry Torokhov
> <dmitry.torokhov@xxxxxxxxx> wrote:
> > Commit 7d01cd261c76f95913c81554a751968a1d282d3a ("Input: zforce - don't
> > overwrite the stack") attempted to add a check for payload size being too
> > large for the supplied buffer. Unfortunately with the currently selected
> > buffer size the comparison is always false as buffer size is larger than
> > the value a single byte can hold, and that results in compiler warnings.
> > Additionally the check was incorrect as it was not accounting for the
> > already read 2 bytes of data stored in the buffer.
>
> The check was indeed incorrect.
>
> > Fixes: 7d01cd261c76f95913c81554a751968a1d282d3a
> > Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> > ---
> >
> > This seems to shut up my GCC, I wonder if it is going to work gfor
> > everyone or we better add BUILD_BUG_ON(FRAME_MAXSIZE < 257) and a
> > comment and remove check.
> >
> >   drivers/input/touchscreen/zforce_ts.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/input/touchscreen/zforce_ts.c b/drivers/input/touchscreen/zforce_ts.c
> > index 2554efd..542ff02 100644
> > --- a/drivers/input/touchscreen/zforce_ts.c
> > +++ b/drivers/input/touchscreen/zforce_ts.c
> > @@ -441,7 +441,9 @@ static int zforce_read_packet(struct zforce_ts *ts, u8 *buf)
> >                  goto unlock;
> >          }
> >
> > -       if (buf[PAYLOAD_LENGTH] == 0 || buf[PAYLOAD_LENGTH] > FRAME_MAXSIZE) {
> > +       if (buf[PAYLOAD_LENGTH] == 0 ||
> > +           (FRAME_MAXSIZE - 2 < 255 &&
> > +            buf[PAYLOAD_LENGTH] > FRAME_MAXSIZE - 2)) {
>
> Doesn't help with gcc 4.1.2 :-(
>
> Before:
>
> drivers/input/touchscreen/zforce_ts.c: In function âzforce_read_packetâ:
> drivers/input/touchscreen/zforce_ts.c:432: warning: comparison is
> always false due to limited range of data type
>
> After:
>
> drivers/input/touchscreen/zforce_ts.c: In function âzforce_read_packetâ:
> drivers/input/touchscreen/zforce_ts.c:434: warning: comparison is
> always false due to limited range of data type


If it's easier, then just revert 7d01cd261c76f95913c81.

Sorry! It seems that at least 4 people have overlooked this issue :(

Best regards

Dirk

Btw: Could anybody give me a hint how to get this warning? My GCC 4.8.1 
with kernel default ARM Cortex A9 kernel options doesn't give me 
anything about this.


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