2014-10-13 15:09:25

by Frans Klaver

Subject: [PATCH] serial: 8250_core: use pre-decrement for iteration limit

In serial8250_rx_chars(), max_count is set to 256. Due to the
post-decrement operator used in the while() condition, the maximum
number of iterations actually 257. This is not a problem, but it is
mildly surprising if you're debugging. Use pre-decrement instead.

Signed-off-by: Frans Klaver <[email protected]>
---
drivers/tty/serial/8250/8250_core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index a06084d..cc9f1a8 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -1485,7 +1485,7 @@ serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)

ignore_char:
lsr = serial_in(up, UART_LSR);
- } while ((lsr & (UART_LSR_DR | UART_LSR_BI)) && (max_count-- > 0));
+ } while ((lsr & (UART_LSR_DR | UART_LSR_BI)) && (--max_count > 0));
spin_unlock(&port->lock);
tty_flip_buffer_push(&port->state->port);
spin_lock(&port->lock);
--
2.1.0

2014-10-14 11:53:00

by Frans Klaver

Subject: Re: [PATCH] serial: 8250_core: use pre-decrement for iteration limit

On Mon, Oct 13, 2014 at 05:09:11PM +0200, Frans Klaver wrote:
> In serial8250_rx_chars(), max_count is set to 256. Due to the
> post-decrement operator used in the while() condition, the maximum
> number of iterations actually 257. This is not a problem, but it is
> mildly surprising if you're debugging. Use pre-decrement instead.

I'm somewhat inclined to changing the title to something like:

serial: 8250_core: actually limit char reads to max_count

Frans

2014-11-06 18:32:18

by Greg Kroah-Hartman

Subject: Re: [PATCH] serial: 8250_core: use pre-decrement for iteration limit

On Tue, Oct 14, 2014 at 01:52:34PM +0200, Frans Klaver wrote:
> On Mon, Oct 13, 2014 at 05:09:11PM +0200, Frans Klaver wrote:
> > In serial8250_rx_chars(), max_count is set to 256. Due to the
> > post-decrement operator used in the while() condition, the maximum
> > number of iterations actually 257. This is not a problem, but it is
> > mildly surprising if you're debugging. Use pre-decrement instead.
>
> I'm somewhat inclined to changing the title to something like:
>
> serial: 8250_core: actually limit char reads to max_count

Sounds better, care to resend with that subject:?

2014-11-06 20:12:50

by Frans Klaver

Subject: [PATCH v2] serial: 8250_core: actually limit char reads to max_count

In serial8250_rx_chars(), max_count is set to 256. Due to the
post-decrement operator used in the while() condition, the maximum
number of iterations actually 257. This is not a problem, but it is
mildly surprising if you're debugging. Use pre-decrement instead.

Signed-off-by: Frans Klaver <[email protected]>
---
v1..v2: change subject to better reflect the goal of the patch

drivers/tty/serial/8250/8250_core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index a06084d..cc9f1a8 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -1485,7 +1485,7 @@ serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)

ignore_char:
lsr = serial_in(up, UART_LSR);
- } while ((lsr & (UART_LSR_DR | UART_LSR_BI)) && (max_count-- > 0));
+ } while ((lsr & (UART_LSR_DR | UART_LSR_BI)) && (--max_count > 0));
spin_unlock(&port->lock);
tty_flip_buffer_push(&port->state->port);
spin_lock(&port->lock);
--
2.1.0