[media] v4l2-ctrls: fix corner case in round-to-range code

If you have a maximum that is at the limit of what the type supports,
and the step is > 1, then you can get wrap-around errors since the
code assumes that the maximum that the type supports is
ctrl->maximum + ctrl->step / 2.

In practice this is always fine, but in artificially crafted ranges
you will hit this bug. Since this is core code it should just work.

This bug has always been there but since it doesn't cause problems in
practice it was never noticed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
index 1acc7aa9d7e2e00aeddf50e1d5191528de896e10..004e7e82e1e033a1615ebfbc96f83a3cac83d387 100644 (file)
--- a/drivers/media/v4l2-core/v4l2-ctrls.c
+++ b/drivers/media/v4l2-core/v4l2-ctrls.c
@@ -1295,11 +1295,19 @@ static void std_log(const struct v4l2_ctrl *ctrl)
        }
 }
 
-/* Round towards the closest legal value */
+/*
+ * Round towards the closest legal value. Be careful when we are
+ * close to the maximum range of the control type to prevent
+ * wrap-arounds.
+ */
 #define ROUND_TO_RANGE(val, offset_type, ctrl)                 \
 ({                                                             \
        offset_type offset;                                     \
-       val += (ctrl)->step / 2;                                \
+       if ((ctrl)->maximum >= 0 &&                             \
+           val >= (ctrl)->maximum - ((ctrl)->step / 2))        \
+               val = (ctrl)->maximum;                          \
+       else                                                    \
+               val += (ctrl)->step / 2;                        \
        val = clamp_t(typeof(val), val,                         \
                      (ctrl)->minimum, (ctrl)->maximum);        \
        offset = (val) - (ctrl)->minimum;                       \
@@ -1325,7 +1333,10 @@ static int std_validate(const struct v4l2_ctrl *ctrl, u32 idx,
                 * the u64 divide that needs special care.
                 */
                val = ptr.p_s64[idx];
-               val += ctrl->step / 2;
+               if (ctrl->maximum >= 0 && val >= ctrl->maximum - ctrl->step / 2)
+                       val = ctrl->maximum;
+               else
+                       val += ctrl->step / 2;
                val = clamp_t(s64, val, ctrl->minimum, ctrl->maximum);
                offset = val - ctrl->minimum;
                do_div(offset, ctrl->step);