From: Matthias Reichl <hias@horus.com>
To: Anthony Mallet <anthony.mallet@laas.fr>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Oliver Neukum <oneukum@suse.com>, linux-usb@vger.kernel.org
Subject: Re: [PATCH v4 2/2] USB: cdc-acm: fix rounding error in TIOCSSERIAL
Date: Fri, 27 Mar 2020 13:37:24 +0100 [thread overview]
Message-ID: <20200327123722.GA3051@lenny.lan> (raw)
In-Reply-To: <20200312133101.7096-2-anthony.mallet@laas.fr>
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@laas.fr>
> ---
> 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
>
next prev parent reply other threads:[~2020-03-27 12:45 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-12 13:31 [PATCH v4 1/2] USB: cdc-acm: fix close_delay and closing_wait units in TIOCSSERIAL Anthony Mallet
2020-03-12 13:31 ` [PATCH v4 2/2] USB: cdc-acm: fix rounding error " Anthony Mallet
2020-03-27 12:37 ` Matthias Reichl [this message]
2020-03-27 12:53 ` Greg Kroah-Hartman
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