[PATCH] staging: comedi: dt3000: Fix signed integer overflow 'divider * base'

[Ian Abbott]

In `dt3k_ns_to_timer()` the following lines near the end of the function
result in a signed integer overflow:

        prescale = 15;
        base = timer_base * (1 << prescale);
        divider = 65535;
        *nanosec = divider * base;

(`divider`, `base` and `prescale` are type `int`, `timer_base` and
`*nanosec` are type `unsigned int`.  The value of `timer_base` will be
either 50 or 100.)

The main reason for the overflow is that the calculation for `base` is
completely wrong.  It should be:

        base = timer_base * (prescale + 1);

which matches an earlier instance of this calculation in the same
function.

Reported-by: David Binderman <dcb...@hotmail.com>
Cc: <sta...@vger.kernel.org>
Signed-off-by: Ian Abbott <abbo...@mev.co.uk>
---
N.B. Greg: The original report suggested an actual build error, so
may be prudent to queue this on your 'staging-linus' queue.

 drivers/staging/comedi/drivers/dt3000.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/comedi/drivers/dt3000.c 
b/drivers/staging/comedi/drivers/dt3000.c
index 2edf3ee91300..4ad176fc14ad 100644
--- a/drivers/staging/comedi/drivers/dt3000.c
+++ b/drivers/staging/comedi/drivers/dt3000.c
@@ -368,7 +368,7 @@ static int dt3k_ns_to_timer(unsigned int timer_base, 
unsigned int *nanosec,
        }
 
        prescale = 15;
-       base = timer_base * (1 << prescale);
+       base = timer_base * (prescale + 1);
        divider = 65535;
        *nanosec = divider * base;
        return (prescale << 16) | (divider);
-- 
2.20.1