Re: serial console _after_ boot ? - was: Redirect kernel console

Re: serial console _after_ boot ? - was: Redirect kernel console

[devzero]

hi !

i was wondering how to make kernel messages appear on /dev/ttyS0 without a reboot, i.e. kernelparam "console=ttyS0"

after playing for a while with setconsole, setterm and klogconsole i didn`t find a way to make that happen.

i can do "setconsole /dev/tty1 </dev/console" and then "echo test >/dev/console" and see "test" on tty1

"setterm -msg on -msglevel 8 >/dev/console"  (or >/dev/tty1), modprobe loop;rmmod loop - and i see the kernel message there
   
i can do "setconsole /dev/ttyS0 </dev/console" and then "echo test >/dev/console" and see "test" on ttyS0

BUT :

"setterm -msg on -msglevel 8 >/dev/console"  (or >/dev/ttyS0), modprobe loop;rmmod loop  ......  nothing!

since i came across jan`s posting from 2005, which didn`t get a reply i think it`s not that stupid to ask again...

any clue how to do that at runtime or what`s the issue here ? (see excerpts below) 

i can set 
regards
roland



List:       linux-kernel
Subject:    Redirect kernel console
From:       Jan Engelhardt <jengelh () linux01 ! gwdg ! de>
Date:       2005-08-13 12:48:22
Message-ID: Pine.LNX.4.61.0508131447220.4457 () yvahk01 ! tjqt ! qr
[Download message RAW]

Hi,


there is a klogconsole utiltity that allows to change the console to which 
kernel messages are printed. However, is it possible to redirect to ttyS0 
without a reboot?


Jan Engelhardt
------- 





>> Subject: Re: serial console _after_ boot ?

>> > Setterm outputs the correct escape sequences to stdout. You can 
>> > configure another tty using 'setterm -msglevel 8 -msg on > /dev/ttyS0'. 
>> > I tested this on the standard kernel consoles (/dev/tty1-9), and it 
>> > worked. You obviously need root access to write the escape sequences to 
>> > the tty's.
>> 
>> oh - yes, this works! but with tty1-9 only.
>> not with ttyS0 :(
>
>.....as the docs tell.....
>
> Some options  however  (marked  "virtual  consoles  only" below)  do  not  correspond to a terminfo(5) capability.
>
>      -msg [on|off] (virtual consoles only)
>              Enables or disables the sending of kernel printk() messages to the console.
>
>       -msglevel 1-8 (virtual consoles only)
>              Sets the console logging level for kernel printk() messages.  All messages strictly more important than  this
>              will  be  printed, so a logging level of 0 has the same effect as -msg on and a logging level of 8 will print
>              all kernel messages.  klogd(8) may be a more convenient interface to the logging of kernel messages.


> > > but i don`t have change_console. 
> > > searched the net and found that this eventually should be part of sysvinit package.
> > > but mine doesn`t provide that tool.
> > > 
> > > found "setconsole" which looks like something similar, but "setconsole /dev/ttyS0 < /dev/console"
> > > doesn`t have any effect
> > 
> > What distribution do you use? I have change_console on gentoo, yet I 
> > cannot find the tool on a debian etch. But that's a vserver, so it's 
> > probably not a reference.
> 
> i have opensuse 10.3

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Re: serial console _after_ boot ? - was: Redirect kernel console

[Jan Engelhardt]

On Jan 3 2008 13:43, devzero@web.de wrote:
>
>hi !
>
>i was wondering how to make kernel messages appear on /dev/ttyS0 without a reboot, i.e. kernelparam "console=ttyS0"

The solution is simple... the following piece of code is inside
opensuse-10.3/src/sysvinit-2.86-115.src.rpm#showconsole-1.08.tar.bz2#showconsole-1.08/blogd.c

    (void)ioctl(0, TIOCCONS, NULL);  /* Undo any current map if any */
    if (ioctl(pts, TIOCCONS, NULL) < 0)
        error("can not set console device to %s: %s\n", ptsname, strerror(errno));

so I suppose that's it. Write up a new program that calls the ioctl
on a tty, and you should be done. IOW:

#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>

int main(void)
{
	int fd = open("/dev/ttyS0", O_RDWR);
	ioctl(0, TIOCCONS, NULL);
	ioctl(fd, TIOCCONS, NULL);
}

Re: serial console _after_ boot ? - was: Redirect kernel console

[devzero]

fantastic, thanks!

unfortunately......

opensuse103:/home/roland/serialcons # ./mytioccons
ioctl: Device or resource busy

but, it`s not userspace:

opensuse103:/home/roland # lsof |egrep "console|ttyS0"
opensuse103:/home/roland #


in tty_io.c -> ticoccons() there is

        if (redirect) {
                spin_unlock(&redirect_lock);
                return -EBUSY;
        }


but i`m not deep enough into programming to understand this.

-------------------
mytioccons.c

#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <libio.h>
#include <errno.h>
#include <stdio.h>

int main(void)
{
int fd = open("/dev/ttyS0", O_RDWR);
int errno;

ioctl(0, TIOCCONS, NULL);

if (ioctl(fd, TIOCCONS, NULL) < 0)
        printf("ioctl: %s\n", strerror(errno));
}
-------------------




> -----Ursprüngliche Nachricht-----
> Von: "Jan Engelhardt" <jengelh@computergmbh.de>
> Gesendet: 03.01.08 14:28:45
> An: devzero@web.de
> CC: linux-kernel@vger.kernel.org
> Betreff: Re: serial console _after_ boot ? - was: Redirect kernel console


> 
> 
> On Jan 3 2008 13:43, devzero@web.de wrote:
> >
> >hi !
> >
> >i was wondering how to make kernel messages appear on /dev/ttyS0 without a reboot, i.e. kernelparam "console=ttyS0"
> 
> The solution is simple... the following piece of code is inside
> opensuse-10.3/src/sysvinit-2.86-115.src.rpm#showconsole-1.08.tar.bz2#showconsole-1.08/blogd.c
> 
>     (void)ioctl(0, TIOCCONS, NULL);  /* Undo any current map if any */
>     if (ioctl(pts, TIOCCONS, NULL) < 0)
>         error("can not set console device to %s: %s\n", ptsname, strerror(errno));
> 
> so I suppose that's it. Write up a new program that calls the ioctl
> on a tty, and you should be done. IOW:
> 
> #include <sys/ioctl.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <fcntl.h>
> 
> int main(void)
> {
> 	int fd = open("/dev/ttyS0", O_RDWR);
> 	ioctl(0, TIOCCONS, NULL);
> 	ioctl(fd, TIOCCONS, NULL);
> }
> 


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Re: serial console _after_ boot ? - was: Redirect kernel console

[Jan Engelhardt]

On Jan 3 2008 15:39, devzero@web.de wrote:
>fantastic, thanks!
>
>unfortunately......
>
>opensuse103:/home/roland/serialcons # ./mytioccons
>ioctl: Device or resource busy
>
>but i`m not deep enough into programming to understand this.
>

>int main(void)
>{
>int fd = open("/dev/ttyS0", O_RDWR);
>int errno;

This errno is so out of place.

>
>ioctl(0, TIOCCONS, NULL);
>
>if (ioctl(fd, TIOCCONS, NULL) < 0)
>        printf("ioctl: %s\n", strerror(errno));
>}

Re: serial console _after_ boot ? - was: Redirect kernel console

[devzero]

> This errno is so out of place.
yes, that was stupid.

anyway:

# strace ./mytiocons
--snipp--
open("/dev/ttyS0", O_RDWR)              = 3
ioctl(0, TIOCCONS)                      = -1 EBUSY (Device or resource busy)
ioctl(3, TIOCCONS)                      = -1 EBUSY (Device or resource busy)
--snipp--

man tty_ioctl is telling:

Redirecting console output

TIOCCONS      void
    Redirect output that would have gone to
    /dev/console or /dev/tty0 to the given tty. If that was a pty master, send it to the slave. Anybody can do this as long as the output was not redirected yet. If it was redirected already EBUSY is returned, but root may stop redirection by using this ioctl with fd pointing at /dev/console or /dev/tty0.



but it doesn`t seem to make any difference if i do that ioctl on /dev/console or /dev/tty0 , because i`m getting EBUSY there, too


> 
> 
> On Jan 3 2008 15:39, devzero@web.de wrote:
> >fantastic, thanks!
> >
> >unfortunately......
> >
> >opensuse103:/home/roland/serialcons # ./mytioccons
> >ioctl: Device or resource busy
> >
> >but i`m not deep enough into programming to understand this.
> >
> 
> >int main(void)
> >{
> >int fd = open("/dev/ttyS0", O_RDWR);
> >int errno;
> 
> This errno is so out of place.
> 
> >
> >ioctl(0, TIOCCONS, NULL);
> >
> >if (ioctl(fd, TIOCCONS, NULL) < 0)
> >        printf("ioctl: %s\n", strerror(errno));
> >}
> 


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Re: serial console _after_ boot ? - was: Redirect kernel console

[Miquel van Smoorenburg]

In article <Pine.LNX.4.64.0801031423070.26693@fbirervta.pbzchgretzou.qr>,
Jan Engelhardt  <jengelh@computergmbh.de> wrote:
>
>On Jan 3 2008 13:43, devzero@web.de wrote:
>>
>>hi !
>>
>>i was wondering how to make kernel messages appear on /dev/ttyS0
>without a reboot, i.e. kernelparam "console=ttyS0"
>
>The solution is simple... the following piece of code is inside
>opensuse-10.3/src/sysvinit-2.86-115.src.rpm#showconsole-1.08.tar.bz2#showconsole-1.08/blogd.c
>
>    (void)ioctl(0, TIOCCONS, NULL);  /* Undo any current map if any */
>    if (ioctl(pts, TIOCCONS, NULL) < 0)
>        error("can not set console device to %s: %s\n", ptsname,
>strerror(errno));
>
>so I suppose that's it. Write up a new program that calls the ioctl
>on a tty, and you should be done. IOW:

TIOCCONS only works for pseudo terminals.

Besides, it only redirects stuff you send to /dev/console. It
doesn't change what devices printk() prints to. Currently there
is no way to change that on a running kernel.

Mike.

Re: serial console _after_ boot ? - was: Redirect kernel console

[devzero]

>>>i was wondering how to make kernel messages appear on /dev/ttyS0
>>without a reboot, i.e. kernelparam "console=ttyS0"
>>
>>The solution is simple... the following piece of code is inside
>>opensuse-10.3/src/sysvinit-2.86-115.src.rpm#showconsole-1.08.tar.bz2#showconsole-1.08/blogd.c
>>
>>    (void)ioctl(0, TIOCCONS, NULL);  /* Undo any current map if any */
>>    if (ioctl(pts, TIOCCONS, NULL) < 0)
>>        error("can not set console device to %s: %s\n", ptsname,
>>strerror(errno));
>>
>>so I suppose that's it. Write up a new program that calls the ioctl
>>on a tty, and you should be done. IOW:
>
>TIOCCONS only works for pseudo terminals.
>
>Besides, it only redirects stuff you send to /dev/console. It
>doesn't change what devices printk() prints to. Currently there
>is no way to change that on a running kernel.
>
>Mike.

too bad, but thanks for the info.
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