Don't cross the (tty) streams

[Matthew Wilcox <matthew@wil.cx>]

Connect two machines with a serial cable.  On the victim:

willy@rowlf:~$ cat </dev/ttyS1

Now, let's find out which ttyS on the other machine is connected ...

willy@teeth:~$ echo foo >/dev/ttyS1
-bash: echo: write error: Input/output error

Oops, that's not a serial port.  No output on rowlf, as expected.
OK, carrying on ...

willy@teeth:~$ echo bar >/dev/ttyS0

And yet on rowlf, we now see:

foo
bar

Looks like there's an error path that doesn't throw away data on -EIO.

This bug exists on at least these two kernels (because I tested it in
both directions:
Linux rowlf 2.6.19-rc6-g70d6673f #1 SMP Thu Nov 16 20:49:15 EST 2006 ia64 GNU/Linux
Linux teeth 2.6.23-rc3-g68dba7a9-dirty #209 SMP PREEMPT Thu Sep 20 15:57:42 EDT 2007 i686 GNU/Linux

Neither kernel has any modifications to its serial code.

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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