Re: [PATCH 2/2] USB: cdc-acm: fix rounding error in TIOCSSERIAL

[Oliver Neukum <oneukum@suse.com>]

Am Donnerstag, den 05.03.2020, 11:24 +0100 schrieb Anthony Mallet:
> By default, tty_port_init() initializes those parameters to a multiple
> of HZ. For instance in line 69 of tty_port.c:
>    port->close_delay = (50 * HZ) / 100;
> https://github.com/torvalds/linux/blob/master/drivers/tty/tty_port.c#L69
> 
> With e.g. CONFIG_HZ = 250 (as this is the case for Ubuntu 18.04
> linux-image-4.15.0-37-generic), the default setting for close_delay is
> thus 125.
> 
> When ioctl(fd, TIOCGSERIAL, &s) is executed, the setting returned in
> user space is '12' (125/10). When ioctl(fd, TIOCSSERIAL, &s) is then
> executed with the same setting '12', the value is interpreted as '120'
> which is different from the current setting and a EPERM error may be
> raised by set_serial_info() if !CAP_SYS_ADMIN.
> https://github.com/torvalds/linux/blob/master/drivers/usb/class/cdc-acm.c#L919
> 
> Signed-off-by: Anthony Mallet <anthony.mallet@laas.fr>
> Fixes: ba2d8ce9db0a6 ("cdc-acm: implement TIOCSSERIAL to avoid blocking close(2)")
Acked-by: Oliver Neukum <oneukum@suse.com>

	Regards
		Oliver