On Thu, Mar 12, 2020 at 02:31:01PM +0100, Anthony Mallet wrote: > 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 > > Fixes: ba2d8ce9db0a6 ("cdc-acm: implement TIOCSSERIAL to avoid blocking close(2)") > Signed-off-by: Anthony Mallet <anthony.mallet@xxxxxxx> > --- > Changed in v2: fix typo (extra closing brace) > Changed in v3: version this patch series > Changed in v4: diffed against e6e6ec4 from github.com/torvalds/linux.git > --- > drivers/usb/class/cdc-acm.c | 27 +++++++++++++++++---------- > 1 file changed, 17 insertions(+), 10 deletions(-) > > diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c > index da619176deca..a41a3d27016c 100644 > --- a/drivers/usb/class/cdc-acm.c > +++ b/drivers/usb/class/cdc-acm.c > @@ -907,6 +907,7 @@ static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss) > { > struct acm *acm = tty->driver_data; > unsigned int closing_wait, close_delay; > + unsigned int old_closing_wait, old_close_delay; > int retval = 0; > > close_delay = msecs_to_jiffies(ss->close_delay * 10); > @@ -914,18 +915,24 @@ static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss) > ASYNC_CLOSING_WAIT_NONE : > msecs_to_jiffies(ss->closing_wait * 10); > > + /* we must redo the rounding here, so that the values match */ > + old_close_delay = jiffies_to_msecs(acm->port.close_delay) / 10; > + old_closing_wait = acm->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ? > + ASYNC_CLOSING_WAIT_NONE : > + jiffies_to_msecs(acm->port.closing_wait) / 10; > + > mutex_lock(&acm->port.mutex); > > - if (!capable(CAP_SYS_ADMIN)) { > - if ((close_delay != acm->port.close_delay) || > - (closing_wait != acm->port.closing_wait)) > + if ((ss->close_delay != old_close_delay) || > + (ss->closing_wait != old_closing_wait)) { > + if (!capable(CAP_SYS_ADMIN)) > retval = -EPERM; > - else > - retval = -EOPNOTSUPP; > - } else { > - acm->port.close_delay = close_delay; > - acm->port.closing_wait = closing_wait; > - } > + else { > + acm->port.close_delay = close_delay; > + acm->port.closing_wait = closing_wait; > + } > + } else > + retval = -EOPNOTSUPP; Swapping the order of the capability / close settings checks introduced a rather unexpected regression: when running with CAP_SYS_ADMIN we now get -EOPNOTSUPP when the close settings are unchanged. This will break programs / scripts that set these settings to fixed values. eg (tested on vanilla 5.4.28) changing the settings works, but setting the same value again fails: # setserial -a /dev/ttyACM0 /dev/ttyACM0, Line 0, UART: unknown, Port: 0x0000, IRQ: 0 Baud_base: 9600, close_delay: 50, divisor: 0 closing_wait: 3000 Flags: spd_normal # setserial /dev/ttyACM0 closing_wait 2000 # setserial /dev/ttyACM0 closing_wait 2000 Cannot set serial info: Operation not supported >From the commit message it's not clear why this behaviour was changed so I suspect this might be a bug. so long, Hias > > mutex_unlock(&acm->port.mutex); > return retval; > -- > 2.17.1 >
