Re: [PATCH] USB: cdc-acm: expose serial close_delay and closing_wait in sysfs

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On 24.08.23 20:02, Simon Arlott wrote:
It's not practical to use TIOCGSERIAL from a udev rule. Instead of a
sysfs attribute (that udev has built-in support for writing) it would
require a separate compiled process or other non-trivial dependencies
(e.g. Python) to modify the closing_wait value. There's no shell script
support for read-modify-write of a complex ioctl struct.

That, however, is a deficiency of udev.

The ioctl can't be used without opening and closing the tty, which has
side effects. On open() it'll raise DTR/RTS and activate it. For cdc-acm
that will indicate to the device that the serial port has been opened
which will be visible to the software running on the USB device. On
close() it'll be delayed by the close_delay if any process is currently
doing a blocking open() and there's no carrier, then the closing_wait
time if there's been any incomplete transmitted data (by any process).

And that is an issue of the generic serial layer.
Those two values require CAP_SYS_ADMIN to modify (which is separately
enforced by many of the tty drivers) so user applications can't change
them even if they're aware of them.

That is even more damning. Either something is protected by a capability
or it is not. Such a protection must not be circumvented.

	Regards
		Oliver




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