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    SubjectRe: Linux-2.6.11 can't disable CAD
    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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