Boot hangs in single-user mode

Andrew Hamilton-Wright ahamiltonwright at
Mon Jun 24 17:10:12 UTC 2013

I have tracked down the issue.  Not sure whether this is a PR issue or not...

On 2013-06-06, at 11:18 AM, Polytropon wrote:

> On Thu, 6 Jun 2013 10:24:52 -0300, Andrew Hamilton-Wright wrote:
>> Strangely, it seems that I cannot boot single user, either
>> using "boot -s" from the boot loader, or using the boot menu. 
>> When I get to the point where the root filesystem is mounted,
>> it hangs right after printing the message:
>> Trying to mount root from ufs:/dev/ada0s1a
> Have you tried hitting the RETURN key several times? 

   [ ... ]

> It's important to identify if the system is _really_ hanging,
> or if the message "just isn't visible"...

This is indeed the crux of the issue.  While hammering on the RETURN key did not produce a prompt, it turns out that there was a prompt...

At some time in the relatively distant past, I had configured this machine to allow display to a serial console (long since disconnected) by adding these lines to /boot/loader.conf 


My notes say "These came from the serial console setup page, and do work for vt100", however I did not note exactly which man page they came from, unfortunately.  I do not see these lines on syscons(4), sio(4) or dcons(4).

Similar lines are mentioned in the handbook regarding setting up a serial console (there is no mention of single-user mode here):

The issue, as it relates to single-user mode, is essentially this:  if the system is configured to boot with multi-console options, then when the single user prompt is printed, it is only printed on the "second" console (which is also the only valid source of keyboard input) -- in this case, the configured but unattached serial port.

I'm not sure what the best strategy is here.  Having only one console that is accepting input for the single-user shell certainly makes sense.  The question is, which of potentially several consoles should it be?

IMO, it would be better/clearer if (for i386/amd64 anyway) the console was the one associated with the motherboard-based keyboard and video card.  An argument here would be that the [CTRL]-[ALT]-[DEL] sequence is still valid when associated with this keyboard, so it does seem odd that other input on that device is ignored.

I can see arguments for other setups, also, mostly revolving around the "why would you _have_ another console configured if you didn't need it, so the configured console must therefore be the important one" -- though the FreeBSD user base is certainly willing enough to experiment that I am sure I am not the only person who set up multi-console for a "fun" project.

Perhaps the best strategy would be to add a message printed on all consoles (as the rest of the boot information is) just before the prompt is printed (singly) to let people know that this is happening?  I'm not sure if a way to 100% predict the desired console is possible.

Thoughts?  If figure I will put a PR in, so that at least this is tracked, even if we don't change anything.  I will reference this thread in the PR, but if anyone has input as to what to suggest, I would appreciate it.  At the very least, the handbook should get updated to indicate that this may happen.


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