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

Dirk Behme
2015-07-28T11:28:37Z

On 28.07.2015 12:23, Geert Uytterhoeven wrote:
> On Mon, Jul 27, 2015 at 11:06 PM, Dmitry Torokhov
> <dmitry.torokhov@gmail.com> 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@intel.com>
>> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
>> ---
>>
>> 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 ‘zforce_read_packet’:
> 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 ‘zforce_read_packet’:
> 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.