On Mon, Jul 27, 2015 at 11:53:27PM +0200, Heiko Stübner wrote:

Am Montag, 27. Juli 2015, 14:44:42 schrieb Dmitry Torokhov:

quoted

On Mon, Jul 27, 2015 at 11:35:23PM +0200, Heiko Stübner wrote:

quoted

Hi Dmitry,

Am Montag, 27. Juli 2015, 14:06:19 schrieb Dmitry Torokhov:

quoted

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.

Fixes: 7d01cd261c76f95913c81554a751968a1d282d3a
Reported-by: kbuild test robot <redacted>
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.

needed a bit to get to know my old zforce driver again ;-)


I may be blind, but currently I fail to see what problem the original
patch
actually tries to fix.

buf[PAYLOAD_LENGTH] is an u8, so the max value it can contain is 255. The
i2c_master_recv reads buf[PAYLOAD_LENGTH]-bytes into the buffer starting
at
buf[PAYLOAD_BODY] (= buf[2]). So it reads at max 255 bytes into a 257 byte
big buffer starting at index 2.

zforce_read_packet, also is an internal function used only by the
interrupt
handler, which always only calls it with a buffer of FRAME_MAXSIZE size.


The original patch said "If we get a corrupted packet with PAYLOAD_LENGTH

quoted

FRAME_MAXSIZE, we will silently overwrite the stack." but payload_length
can never actually be greater than the buffer size?

Right, not unless we for some reason decide to adjust FRAME_MAXSIZE to
make it smaller than 257 and then fail to add the check to make sure we
do not go past the buffer.

So everything is fine now, but I guess we'd like to be more safe in the
future...

I would argue that FRAME_MAXSIZE already indicates that it should not be 
changed. It's the maximum size a single frame can be. And this size is a 
property of the hardware itself, because of the format, 257 bytes is always 
the maximum you could get (2 bytes header + at max 255 bytes payload).

So this second check (while only taking up a minimal amount of time)

It does not take any time as it gets optimized out completely (with
current FRAME_MAXSIZE value).

actually 
only checks against kernel-developer making errors in the future and not 
something the hardware can cause.

Right.

But your change itself looks correct, so if you prefer to keep that check you 
can also add my
Reviewed-by: Heiko Stuebner <redacted>

I guess I'll sit on it. Another option is to revert the original change
and be done with it.

Thanks.

-- 
Dmitry