Re: System reboot triggered by just reading a device file....!?

Re: System reboot triggered by just reading a device file....!?

[devzero]

Hi Clemens, 

thanks, but i know i could do this.

this thread is not meant to protect myself from this curiousity but it is meant to protect others.
it`s a trap. 
i stepped into that.
now i know that trap, so i can easily sidestep.

but most people using linux don`t know about the watchdog, so i don`t think they will know about this trap. 
you can`t make that become common knowledge.

and we can`t expect that they will find out _what`s_ the trap at all, if they step into.
having this behaviour documented is like putting a sign "don`t step into this" at the back of the trap 

so why shouldn`t we help them avoiding it ?

it maybe very seldom that someone steps into this. 
but it may happen and then someone will have trouble and spend time on this.
i think every admin can tell you about weird random reboots of his systems which he cannot explain what was the reason for it.
this maybe some of those reasons and this one could be avoided. 
i`m thinking of something simple like echo "now you`re armed" > /dev/watchdog

regards
roland


> -----Ursprüngliche Nachricht-----
> Von: "Clemens Koller" <clemens.koller@anagramm.de>
> Gesendet: 22.11.07 21:43:15
> An: devzero@web.de
> CC: Simon Arlott <simon@fire.lp0.eu>, Robert Hancock <hancockr@shaw.ca>,  linux-kernel@vger.kernel.org
> Betreff: Re: System reboot triggered by just reading a device file....!?


> 
> devzero@web.de schrieb:
> 
>  > [was: reading /dev/watchdog triggers reboot as intended]
>  > need to change my own philosophy now, because i learned that reading isn`t harmless.   ;)
> 
> If you want to protect you from your curiosity (or from reading anything),
> you could just disable the watchdog in the kernel.
> See: Device Drivers -> Character devices -> Watchdog Timer Support -> ...
> 
> Regards,
> -- 
> Clemens Koller
> __________________________________
> R&D Imaging Devices
> Anagramm GmbH
> Rupert-Mayer-Straße 45/1
> Linhof Werksgelände
> D-81379 München
> Tel.089-741518-50
> Fax 089-741518-19
> http://www.anagramm-technology.com
> 


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OT: Re: System reboot triggered by just reading a device file....!?

[Clemens Koller]

Hi, Roland!

Please don't top-post.

devzero@web.de schrieb:
 > > [was: it would be easy to disable the kernel watchdog]
 > thanks, but i know i could do this.

Good. I was also curious and just checked again. The watchdog subsystem
is by default _disabled_ in the kernel configuration. If you use some
distro's kernel, where they turned it on, complain to them!
If you turned it on yourself, you are really on your own...
the Kconfig help there is IMO sufficient and very clear and,
"If unsure, say N". Hmm... sorry?!

 > this thread is not meant to protect myself from this curiousity but it is meant
 > to protect others. it`s a trap.

I guess I understand your position. But I don't see no way to improve
the kernel in that point.
Complain to the guys who enabled the watchdog / setup this trap for
any reason.

 > i stepped into that.
 > now i know that trap, so i can easily sidestep.
 > it maybe very seldom that someone steps into this.
 > but it may happen and then someone will have trouble and spend time on this.
 > i think every admin can tell you about weird random reboots of his systems
 > which he cannot explain what was the reason for it.

That's one possible way of "learning by doing suicide (tm);"

 > this maybe some of those reasons and this one could be avoided.
 > i`m thinking of something simple like echo "now you`re armed" > /dev/watchdog

Read some details about watchdogs to get more background and why the
watchdog is triggered so easily and why it's good this way.
i.e: http://www.ganssle.com/watchdogs.pdf

Regards,
-- 
Clemens Koller
__________________________________
R&D Imaging Devices
Anagramm GmbH
Rupert-Mayer-Straße 45/1
Linhof Werksgelände
D-81379 München
Tel.089-741518-50
Fax 089-741518-19
http://www.anagramm-technology.com

Re: OT: Re: System reboot triggered by just reading a device file....!?

[devzero]

Hi Clemens, 

> 
> Hi, Roland!
> 
> Please don't top-post.

sorry!

>  > > [was: it would be easy to disable the kernel watchdog]
>  > thanks, but i know i could do this.
> 
> Good. I was also curious and just checked again. The watchdog subsystem
> is by default _disabled_ in the kernel configuration. If you use some
> distro's kernel, where they turned it on, complain to them!
> If you turned it on yourself, you are really on your own...
> the Kconfig help there is IMO sufficient and very clear and,
> "If unsure, say N". Hmm... sorry?!

whoops - sorry for that. i should have checked that, but i think i just didn`t expect some distro vendor to change that default.
sure i will complain to suse now. stopping getting on your nerves here, now.


>  > this thread is not meant to protect myself from this curiousity but it is meant
>  > to protect others. it`s a trap.
> 
> I guess I understand your position. But I don't see no way to improve
> the kernel in that point.
> Complain to the guys who enabled the watchdog / setup this trap for
> any reason.

sure. you`re completely right.


>  > i stepped into that.
>  > now i know that trap, so i can easily sidestep.
>  > it maybe very seldom that someone steps into this.
>  > but it may happen and then someone will have trouble and spend time on this.
>  > i think every admin can tell you about weird random reboots of his systems
>  > which he cannot explain what was the reason for it.
> 
> That's one possible way of "learning by doing suicide (tm);"

:)

>  > this maybe some of those reasons and this one could be avoided.
>  > i`m thinking of something simple like echo "now you`re armed" > /dev/watchdog
> 
> Read some details about watchdogs to get more background and why the
> watchdog is triggered so easily and why it's good this way.
> i.e: http://www.ganssle.com/watchdogs.pdf

thanks for your help and for that very useful link. that`s the very best stuff i every read about watchdogs!

regards
Roland

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