Re: Linux-2.6.11 can't disable CAD

["Richard B. Johnson" <linux-os@analogic.com>]

On Thu, 7 Apr 2005, Randy.Dunlap wrote:

> On Thu, 7 Apr 2005 15:46:20 -0400 (EDT) Richard B. Johnson wrote:
>
> | On Thu, 7 Apr 2005, Randy.Dunlap wrote:
> |
> | > On Thu, 7 Apr 2005 11:16:14 -0400 (EDT) Richard B. Johnson wrote:
> | >
> | > |
> | > | In the not-too distant past, one could disable Ctl-Alt-DEL.
> | > | Can't do it anymore.
> | >
> | > What should disabling C_A_D do?
> | >
> | > | Script started on Thu 07 Apr 2005 10:58:11 AM EDT
> | > | [SNIPPED leading stuff...]
> | > |
> | > | mprotect(0xb7fe4000, 28672, PROT_READ|PROT_EXEC) = 0
> | > | brk(0)                                  = 0x804a000
> | > | brk(0x8053000)                          = 0x8053000
> | > | reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_CAD_OFF)
> | > = 0
> | > | pause( <unfinished ...>
> | > | _exit(0)                                = ?
> | > | # exit
> | > | Script done on Thu 07 Apr 2005 10:58:21 AM EDT
> | >
> |
> | It's a program that executes the __NR_reboot syscall (88) after
> | putting the documented values into the appropriate registers.
>
> Yeah, I understood that much.  Just wondering if it was
> available somewhere...
>
> | > What program is that?  I'm just echoing 0 | 1 into
> | > /proc/sys/kernel/ctrl-alt-del , is that equivalent?
> | > or have you tried that?
> | >
> |
> | Doesn't matter. Many embedded systems don't have /proc because
> | they don't have any file-systems.
>
> It matters if they are equivalent (and I don't have that
> program above).
>
> | > | Observe that reboot() returns 0 and `strace` understands what
> | > | parameters were passed. The result is that, if I hit Ctl-Alt-Del,
> | > | `init` will still execute the shutdown-order (INIT 0).
> | >
> | > echo 0 > /proc/sys/kernel/ctrl-alt-del
> | >  is same as CAD_OFF
> | > echo 1
> | >  is same as CAD_ON
> | >
> | > I tested 2.4.28, 2.6.3, 2.6.9, 2.6.11, and all of them behaved
> | > the same way for me.  If it's an issue with using a syscall
> | > to change the setting, I'll be glad to look into that too.
> | >
> | > observed behaviors:
> | > CAD enabled + C_A_D keys => call machine_reboot()
> | >  to reboot quickly, no normal shutdown sequence;
> | > CAD disabled + C_A_D keys => kill init, go thru normal
> | >  clean shutdown sequence;
> | > are these the expected behaviors?
> |
> | The expected behavior of the reported operation is for
> | Ctl-Alt-Del to no longer do anything. If the system-call
> | has been depreciated, then the call should return -1 and
> | errno should be ENOSYS. In such a case, I would have
> | to trap the key-sequence in some other way, not that
> | I know how without modifying the kernel.
>
> Have you looked at 'man 2 reboot'?  It seems to agree with
> the observed behavior (above).
> or where should I look to find documentation of the
> expected behavior that you described?

man 2 reboot on my system documents the system call interface
through the C runtime library.

Bbuuutttt then tells how glib-C munged it into a one-parameter
'C' call.   Hmmm. Maybe it's screwing me.  I will check into
this.  I don't know why `strace` shows the correct stuff when
I am making the 4-parameter call. Maybe something is really
confused....

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
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