Re: Linux-2.6.11 can't disable CAD

["Randy.Dunlap" <rddunlap@osdl.org>]

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?

---
~Randy
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