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Date:   Mon, 16 Oct 2023 07:35:02 +0200
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Subject: Re: [PATCH v2 1/3] serial: core: Potential overflow of frame_time
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To:     Vamshi Gajjela <vamshigajjela@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        ilpo.jarvinen@linux.intel.com
Cc:     linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
        manugautam@google.com, Subhash Jadavani <sjadavani@google.com>,
        Channa Kadabi <kadabi@google.com>
References: <20231014181333.2579530-1-vamshigajjela@google.com>
From:   Jiri Slaby <jirislaby@kernel.org>
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In-Reply-To: <20231014181333.2579530-1-vamshigajjela@google.com>
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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