QEMU

QEMU wrongly translates newlines on serial output

Bug #1407813 reported by Nadav Har'El on 2015-01-05

This bug affects 3 people

Affects		Status	Importance	Assigned to	Milestone
	QEMU	Fix Released	Undecided	Unassigned

Bug Description

When using "-serial stdio", QEMU shows the guest serial port's output on the tty running qemu. As it should, QEMU sets the tty to raw mode. Or almost... Strangely, it neglects to remove one output-translation bit, ONLCR (see termios(3)) enabled on the tty. And it should have removed this output translation!

The problem is that with this ONLCR, the guest has no way of outputting a bare linefeed ('\n') - every time the guest tries to output a bare linefeed to the serial port, the host tty will translate it to \r\n which will be sent to the underlying terminal (e.g., xterm).

In most cases, this issue doesn't cause a problem: When the guest is running a Unix-like operating system which is itself in cooked mode, the guest itself will always output \r\n, and the hosts second translation (to \r\r\n) does no harm. But in certain cases, the guest can *really* want to output just \n, and have this \n reach the terminal emulator and do what a linefeed is supposed to do without a carriage-return - namely - just go one line down in the same column.

As an illustration of this bug, consider a guest running a Unix-like operating system running a curses-based application (e.g., "vi"). If you look at the output of "infocmp xterm", you'll notice that cud1=^J. This means that if the curses library decides to move one line down (it can happen in some cursor movement situations) it might decide to print a linefeed (\n) to move one line down. The guest's operating system will not mess with this linefeed (because the guest is in raw mode), but then qemu's tty, because it was wrongly left in ONLCR mode, will change this \n to \r\n before it reaches the terminal - causing wrong cursor movement (instead the cursor going straight down, it moves to the first column of the next line).