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SubjectRe: [PATCH v2 1/3] serial: core: Potential overflow of frame_time
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On 14. 10. 23, 20:13, Vamshi Gajjela wrote:
> From: VAMSHI GAJJELA <vamshigajjela@google.com>
>
> uart_update_timeout() sets a u64 value to an unsigned int frame_time.
> While it can be cast to u32 before assignment, there's a specific case
> where frame_time is cast to u64. Since frame_time consistently
> participates in u64 arithmetic, its data type is converted to u64 to
> eliminate the need for explicit casting.

And make the divisions by the order of magnutude slower for no good
reason? (Hint: have you looked what DIV64_U64_ROUND_UP() looks like on
32bit yet?)

Unless you provide a reason it can overflow in real (in fact you spell
the opposite in the text above):
NACKED-by: Jiri Slaby <jirislaby@kernel.org>

> Signed-off-by: VAMSHI GAJJELA <vamshigajjela@google.com>
> ---
> v2:
> - use DIV64_U64_ROUND_UP with frame_time
>
> drivers/tty/serial/8250/8250_port.c | 2 +-
> include/linux/serial_core.h | 4 ++--
> 2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 141627370aab..d1bf794498c4 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1510,7 +1510,7 @@ static inline void __stop_tx(struct uart_8250_port *p)
> * rather than after it is fully sent.
> * Roughly estimate 1 extra bit here with / 7.
> */
> - stop_delay = p->port.frame_time + DIV_ROUND_UP(p->port.frame_time, 7);
> + stop_delay = p->port.frame_time + DIV64_U64_ROUND_UP(p->port.frame_time, 7);
> }
>
> __stop_tx_rs485(p, stop_delay);
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index bb6f073bc159..b128513b009a 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -558,7 +558,7 @@ struct uart_port {
>
> bool hw_stopped; /* sw-assisted CTS flow state */
> unsigned int mctrl; /* current modem ctrl settings */
> - unsigned int frame_time; /* frame timing in ns */
> + unsigned long frame_time; /* frame timing in ns */
> unsigned int type; /* port type */
> const struct uart_ops *ops;
> unsigned int custom_divisor;
> @@ -764,7 +764,7 @@ unsigned int uart_get_divisor(struct uart_port *port, unsigned int baud);
> */
> static inline unsigned long uart_fifo_timeout(struct uart_port *port)
> {
> - u64 fifo_timeout = (u64)READ_ONCE(port->frame_time) * port->fifosize;
> + u64 fifo_timeout = READ_ONCE(port->frame_time) * port->fifosize;
>
> /* Add .02 seconds of slop */
> fifo_timeout += 20 * NSEC_PER_MSEC;

--
js
suse labs

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