Hi, A TIOCSSERIAL ioctl(2) performed on a file descriptor corresponding to a cdc-acm driver may wrongly report EPERM even when no CAP_SYS_ADMIN is required. This concerns almost all recent kernel versions, as I explain below. This happens line 921 of file cdc-acm.c https://github.com/torvalds/linux/blob/master/drivers/usb/class/cdc-acm.c#L921 with for instance the following pseudo-code snippet when it is run as a non CAP_SYS_ADMIN user. The code does not try to change anything but EPERM might still be returned in some configurations (in particular with CONFIG_HZ = 250): struct serial_struct s; int fd; int e; e = ioctl(fd, TIOCGSERIAL, &s); if (e != 0) abort(); e = ioctl(fd, TIOCSSERIAL, &s); if (e != 0) printf("wrong EPERM report\n"); The issue is caused by the rounding to 10 (integer division or multiplication) of the closing_wait and close_delay members of the struct serial_struct information, lines 899 and 912 of the file cdc-acm.c: https://github.com/torvalds/linux/blob/master/drivers/usb/class/cdc-acm.c#L899 https://github.com/torvalds/linux/blob/master/drivers/usb/class/cdc-acm.c#L912 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 This is particularly misleading for doing proper error handling when trying to set another setting, like e.g. the ASYNC_LOW_LATENCY flag which is not supported by the driver and should thus report EOPNOTSUPP as expected from the line 923 of cdc-acm.c: https://github.com/torvalds/linux/blob/master/drivers/usb/class/cdc-acm.c#L923 The fix might be to just round the default values to a multiple of 10 in the intialization part, like right after line 1301 of file cdc-acm.c https://github.com/torvalds/linux/blob/master/drivers/usb/class/cdc-acm.c#L1301 If a patch is needed to clarify my point, I can of course provide one. Cheers, Anthony Mallet
