Re: Spurious character typed on console every ~20 secs until logged in

[David Wright]

On Thu 29 Mar 2018 at 21:19:38 (+0100), Jonathan de Boyne Pollard wrote:
> David Wright:
> 
> >an extra ^@ character¹
> >
> 
> Unix & Linux Stack Exchange had this last year.

These look like very different symptoms. Taken in reverse order,

> * https://unix.stackexchange.com/questions/360830/

The screenshot looks as if an X session has just terminated,
returning to a VC-style screen.

I get this type of "problem" (though it's actually no problem at all)
occasionally when I terminate X (being a startx user). It's not
usually NUL characters, but always non-ASCII of some sort. For obvious
reasons, it's usually £, but can be § and suchlike, and it's always
characters that I'm fairly sure I typed in that X session. It's as if
the characters "leaked" through into the VC that's hosting the X
session. If it happened in the past, I was much less likely to spot
it as X used to run on VC7 which I'd only see by chance, whereas now
the X session typically runs in VC1, meaning I'm bound to notice it
when it happens. I'm uncertain if I've ever seen it outside jessie.

> * https://unix.stackexchange.com/questions/395494/
> 
> * https://unix.stackexchange.com/questions/396192/

(These two questions appear to have been merged.)

Their problem appears to be persistent and widespread in its effects,
whereas mine is very sporadic in occurrence, specific in effect, and
only lasts between boot time and the first login on the console.

The spurious NULs (assuming that's what they are) interfere with my
own typing, from the decryption passphrase until and including the
login password. Their reflection changes from * (passphrase) to
^@ (username) to nothing visible (during login password).

The timing is always 21 seconds between each NUL character, and that's
unaffected by whatever prompts and responses are taking place. As
soon as there's a successful login, the problem ceases: no more NULs.
I used the laptop all through yesterday after having the problem, and
there were no perceptible differences from normal behaviour.

I've checked the modules with and without the problem and
the same modules get loaded in either case (the order obviously
differs). Likewise the dmesg output is the same. I have a
/lib/modules/4.9.0-6-amd64/kernel/drivers/input/input-polldev.ko
but I've never seen it loaded.

I think the problem has only happened with the latest kernel. FWIW
the configuration differences are:

--- /boot/config-4.9.0-5-amd64  2018-01-04 05:12:40.0 -0600
+++ /boot/config-4.9.0-6-amd64  2018-03-02 01:52:22.0 -0600
@@ -3 +3 @@
-# Linux/x86 4.9.65 Kernel Configuration
+# Linux/x86 4.9.82 Kernel Configuration
@@ -222,0 +223 @@
+# CONFIG_BPF_JIT_ALWAYS_ON is not set
@@ -418,0 +420 @@
+CONFIG_RETPOLINE=y
@@ -1788,0 +1791 @@
+CONFIG_GENERIC_CPU_VULNERABILITIES=y

Cheers,
David.

Re: Spurious character typed on console every ~20 secs until logged in

[Jonathan de Boyne Pollard]

David Wright:


an extra ^@ character¹



Unix & Linux Stack Exchange had this last year.

* https://unix.stackexchange.com/questions/395494/

* https://unix.stackexchange.com/questions/396192/

* https://unix.stackexchange.com/questions/360830/

Re: Spurious character typed on console every ~20 secs until logged in

[David Wright]

On Thu 29 Mar 2018 at 17:20:58 (+0200), to...@tuxteam.de wrote:
> On Thu, Mar 29, 2018 at 10:10:55AM -0500, David Wright wrote:
> > On Thu 29 Mar 2018 at 10:46:17 (+0200), to...@tuxteam.de wrote:
> > > On Thu, Mar 29, 2018 at 08:24:03AM +, Curt wrote:
> > > 
> > > [...]
> > 
> > Thanks for the ideas. I hadn't come across the virtual buttons.
> > 
> > I think I'll look at what processes are running by the time it
> > tries to mount /home, and which of them might involve looping on
> > that timescale.
> 
> If it's something in user space, it'd be interesting who has
> the console open.

That comment made me remember about one possible candidate; I have
@reboot /sbin/kbdrate -r 8 -d 500 -s
in root's crontab as a matter of habit. But removing it had no effect.

I can definitely only observe two CRs, one at the unlocking prompt
(assuming I wait for it), and one at login. Of course, to be able
to observe a third, I have either start typing my username or press
 (in order to get the cursor out of column 1) and that very
keystroke could be enough to stop whatever's happening.

> > Then, when it happens again, I'll try a remote login to see if
> > I can catch some process at it. It hasn't yet happened (on the
> > three occasions) at an opportune time for me to investigate.
> > 
> > I agree, the first character is probably a NUL. The second is
> > probably CR. Both can be used to terminate strings in different
> > situations. Or perhaps my termcap has fallen into a 1960s wormhole:
> > anyone remember WRU, and the Here Is key, …
> 
> Faintly. I also dimly remember about some serial interfaces
> sending NULLs when idle, but I can't put my finger on that.

My memory of these machines is of a roomful of them all bursting
into life when they rebooted the PDP11 in town, two miles away.
Then waiting for peace to be restored as their motors timed out.

> The advantage of old age is that you tend to forget things :)

Cheers,
David.

Re: Spurious character typed on console every ~20 secs until logged in

[Curt]

On 2018-03-29, David Wright  wrote:
> On Thu 29 Mar 2018 at 10:46:17 (+0200), to...@tuxteam.de wrote:
>> On Thu, Mar 29, 2018 at 08:24:03AM +, Curt wrote:
>> 
>> [...]
>
> Thanks for the ideas. I hadn't come across the virtual buttons.
>
> I think I'll look at what processes are running by the time it
> tries to mount /home, and which of them might involve looping on
> that timescale.
> Then, when it happens again, I'll try a remote login to see if
> I can catch some process at it. It hasn't yet happened (on the
> three occasions) at an opportune time for me to investigate.
>
> I agree, the first character is probably a NUL. The second is
> probably CR. Both can be used to terminate strings in different
> situations. Or perhaps my termcap has fallen into a 1960s wormhole:
> anyone remember WRU, and the Here Is key, …

I found more grist:

https://www.reddit.com/r/linuxquestions/comments/7r7a5e/terminal_typing_on_its_own_only_in_tty_terminals/

(I personally have neither input_polldev nor peaq_wmi loaded here--in
fact the latter is inexistant on this machine according to modinfo).

Actually I'm uncertain peaq_wmi is a loadable module. If you enter,
however, "input_polldev spam" or "input_polldev bug" into a search
engine, it almost feels like hitting pay dirt.

> Cheers,
> David.
>
>


-- 
We are no longer the same, you wiser but not sadder, and I sadder but not
wiser, for wiser I could hardly become without grave personal inconvenience,
whereas sorrow is a thing you can keep adding to all your life long, is it not,
like a stamp or an egg collection, without feeling very much the worse for it,
is it not. --Samuel Beckett

Re: Spurious character typed on console every ~20 secs until logged in

[tomas]

-BEGIN PGP SIGNED MESSAGE-
Hash: SHA1

On Thu, Mar 29, 2018 at 10:10:55AM -0500, David Wright wrote:
> On Thu 29 Mar 2018 at 10:46:17 (+0200), to...@tuxteam.de wrote:
> > On Thu, Mar 29, 2018 at 08:24:03AM +, Curt wrote:
> > 
> > [...]
> 
> Thanks for the ideas. I hadn't come across the virtual buttons.
> 
> I think I'll look at what processes are running by the time it
> tries to mount /home, and which of them might involve looping on
> that timescale.

If it's something in user space, it'd be interesting who has
the console open.

> Then, when it happens again, I'll try a remote login to see if
> I can catch some process at it. It hasn't yet happened (on the
> three occasions) at an opportune time for me to investigate.
> 
> I agree, the first character is probably a NUL. The second is
> probably CR. Both can be used to terminate strings in different
> situations. Or perhaps my termcap has fallen into a 1960s wormhole:
> anyone remember WRU, and the Here Is key, …

Faintly. I also dimly remember about some serial interfaces
sending NULLs when idle, but I can't put my finger on that.

The advantage of old age is that you tend to forget things :)

Cheers
- -- tomás
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Re: Spurious character typed on console every ~20 secs until logged in

[David Wright]

On Thu 29 Mar 2018 at 10:46:17 (+0200), to...@tuxteam.de wrote:
> On Thu, Mar 29, 2018 at 08:24:03AM +, Curt wrote:
> 
> [...]

Thanks for the ideas. I hadn't come across the virtual buttons.

I think I'll look at what processes are running by the time it
tries to mount /home, and which of them might involve looping on
that timescale.
Then, when it happens again, I'll try a remote login to see if
I can catch some process at it. It hasn't yet happened (on the
three occasions) at an opportune time for me to investigate.

I agree, the first character is probably a NUL. The second is
probably CR. Both can be used to terminate strings in different
situations. Or perhaps my termcap has fallen into a 1960s wormhole:
anyone remember WRU, and the Here Is key, …

Cheers,
David.

Re: Spurious character typed on console every ~20 secs until logged in

[tomas]

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On Thu, Mar 29, 2018 at 08:24:03AM +, Curt wrote:

[...]
> Wild geese, it's spring, and they're migrating!

:-)
- -- t
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Re: Spurious character typed on console every ~20 secs until logged in

[Curt]

On 2018-03-29,   wrote:
>
> On Thu, Mar 29, 2018 at 07:50:13AM +, Curt wrote:
>> On 2018-03-29,   wrote:
>
> [link blaming NVidia]
>
>> I found another thread blaming the phenomenon on the "virtual lid
>> buttons" (whatever the hell those are)--workaround involved xmodmap:
>> 
>> https://bbs.archlinux.org/viewtopic.php?id=213094
>> 
>> Anyway hope this is not yet another of my wild geese.
>
> A downside of your and my hunches is that David's symptom seems
> to occur at the very beginning at the Linux console, while our
> candidates seem to manifest themselves even on X terminals...

I'm saw "logging in at tty". But I gave the wrong link!

https://bbs.archlinux.org/viewtopic.php?id=222316

Wild geese, it's spring, and they're migrating!

> Cheers
> - -- t
>
>


-- 
We are no longer the same, you wiser but not sadder, and I sadder but not
wiser, for wiser I could hardly become without grave personal inconvenience,
whereas sorrow is a thing you can keep adding to all your life long, is it not,
like a stamp or an egg collection, without feeling very much the worse for it,
is it not. --Samuel Beckett

Re: Spurious character typed on console every ~20 secs until logged in

[tomas]

-BEGIN PGP SIGNED MESSAGE-
Hash: SHA1

On Thu, Mar 29, 2018 at 07:50:13AM +, Curt wrote:
> On 2018-03-29,   wrote:

[link blaming NVidia]

> I found another thread blaming the phenomenon on the "virtual lid
> buttons" (whatever the hell those are)--workaround involved xmodmap:
> 
> https://bbs.archlinux.org/viewtopic.php?id=213094
> 
> Anyway hope this is not yet another of my wild geese.

A downside of your and my hunches is that David's symptom seems
to occur at the very beginning at the Linux console, while our
candidates seem to manifest themselves even on X terminals...

Cheers
- -- t
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Re: Spurious character typed on console every ~20 secs until logged in

[Curt]

On 2018-03-29,   wrote:
>
> On Wed, Mar 28, 2018 at 11:30:17PM -0500, David Wright wrote:
>> This has happened a couple of times or so in the past week:
>> On booting up, I get to the prompt to unlock my /home partition.
>> The first attempt fails, not because I mistyped the passphrase,
>> but because an extra ^@ character¹ was typed by the system.
>
> Bizarre. ^@ is (most probably) your terminal's way of spelling
> a NULL. Searching my brain extension (aka The Tubes) yields
> things:
>
>   
> https://superuser.com/questions/1254863/null-character-appears-on-its-own-in-tty
>
> Blamed on an NVidia -- seems even more bizarre. It might point
> you into a totally wrong direction.

I found another thread blaming the phenomenon on the "virtual lid
buttons" (whatever the hell those are)--workaround involved xmodmap:

https://bbs.archlinux.org/viewtopic.php?id=213094

Anyway hope this is not yet another of my wild geese.

> Cheers
> - -- t
>
>


-- 
We are no longer the same, you wiser but not sadder, and I sadder but not
wiser, for wiser I could hardly become without grave personal inconvenience,
whereas sorrow is a thing you can keep adding to all your life long, is it not,
like a stamp or an egg collection, without feeling very much the worse for it,
is it not. --Samuel Beckett

Re: Spurious character typed on console every ~20 secs until logged in

[tomas]

-BEGIN PGP SIGNED MESSAGE-
Hash: SHA1

On Wed, Mar 28, 2018 at 11:30:17PM -0500, David Wright wrote:
> This has happened a couple of times or so in the past week:
> On booting up, I get to the prompt to unlock my /home partition.
> The first attempt fails, not because I mistyped the passphrase,
> but because an extra ^@ character¹ was typed by the system.

Bizarre. ^@ is (most probably) your terminal's way of spelling
a NULL. Searching my brain extension (aka The Tubes) yields
things:

  
https://superuser.com/questions/1254863/null-character-appears-on-its-own-in-tty

Blamed on an NVidia -- seems even more bizarre. It might point
you into a totally wrong direction.

Cheers
- -- t
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