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Date:   Sat, 14 Oct 2023 19:13:55 -0400
From:   Hugo Villeneuve <hugo@hugovil.com>
To:     Vamshi Gajjela <vamshigajjela@google.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        ilpo.jarvinen@linux.intel.com, linux-serial@vger.kernel.org,
        linux-kernel@vger.kernel.org, manugautam@google.com,
        Subhash Jadavani <sjadavani@google.com>,
        Channa Kadabi <kadabi@google.com>
Message-Id: <20231014191355.7f44aac3537a8d260225fa0c@hugovil.com>
In-Reply-To: <20231014181333.2579530-1-vamshigajjela@google.com>
References: <20231014181333.2579530-1-vamshigajjela@google.com>
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Subject: Re: [PATCH v2 1/3] serial: core: Potential overflow of frame_time
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On Sat, 14 Oct 2023 23:43:33 +0530
Vamshi Gajjela <vamshigajjela@google.com> wrote:

> From: VAMSHI GAJJELA <vamshigajjela@google.com>

Hi,
your commit title doesn't really explain what this patch is doing.

Please see: https://cbea.ms/git-commit/#imperative


> 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.

Again, refering to your title commit message, I would expect that you
would describe precisely how a potential overflow can happen here, and
I am not seeing it.

And based on your log message, it seems that your commit is simply some
kind of optimization, not a fix?

Hugo.


> 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;
> -- 
> 2.42.0.655.g421f12c284-goog
>