On Fri, Mar 17, 2017 at 12:30:53PM +0100, Michael Tretter wrote:
> Hello,
>
> agetty sometimes changes the order of the characters of the username in
> the login prompt. For example, if I enter 'root' as username, the echo
> reads 'orot' and the login fails, because the user does not exist.
>
> A little bit background on the issue: I am running automated tests on
> an embedded device with an i.MX6 processor which connects to the board
> via serial console with 115200 baud. The test waits for the login
> prompt and then sends the username via the serial console. In about
> 20 % of the test runs the echoed username slightly differs from the
> sent username and the test fails.
>
> I can also reproduce the issue when connecting to the serial console
> via screen and pasting the username into the prompt. If I enter the
> username manually or connect strace to agetty, the username is always
> correct. I reproduced the problem on multiple different device
> (although all with the same SoC).
>
> I tested it with util-linux 2.29 with Linux 4.11-rc2 and 4.4.52.
>
> I was able to trace the issue to the wait_for_term_input() function.
> This function reads the first (few) bytes from the input queue and
> after reading reinserts the bytes with the TIOCSTI ioctl back into the
> queue. If new bytes are inserted into the input queue before the
> TIOCSTI happens, the input queue will contain the bytes in the wrong
> order and later reads on the input queue will not yield the original
> input.
Hmm... the issue is that we use non-canonical mode when wait for user
activity, then we need to switch back to canonical mode. I guess
(according to the code) that we can lost unread data by tcsetattr().
So agetty read before tcsetattr() and re-enter by TIOCSTI after
tcsetattr(). This is probably pretty fragile.
Maybe it would be better to remember already read bytes within agetty
rather than use TIOCSTI.
Stef, any idea?
> I also opened a bug in the kernel Bugzilla [1], because I am not sure
> about the correct way to report bugs in util-linux.
We do no use kernel BZ, see "BUG REPORTING" in the README.
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
http://karelzak.blogspot.com
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