How to exit from linux kernel and return to then grub?

How to exit from linux kernel and return to then grub?

[Z C]

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If I boot into a linux kernel using a grub command like "linux /vmlinuz;
inirtd /initrd; boot", how do I shutdown linux itself (but do not power off
or reboot the computer hardware) and return back to the grub directly?

What I meant is something equivalent to the "exit" command in most shells:
If you are within one shell and you enter another shell, then if you want
to quit the second shell and return back to the first shell, just simply
type exit. All env variables and commands you previous typed in the first
shell are completely intact.

Suppose I am now in the grub shell, and then I boot into a tiny linux
kernel, say, a busybox shell, then what can I do to exit the second shell
(i.e. busybox shell) and return to the first shell (i.e. grub shell)? Of
course I can enter the grub shell again by simply rebooting the hardware,
but this is not what I want.

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Re: How to exit from linux kernel and return to then grub?

[Lennart Sorensen]

On Sun, May 18, 2014 at 01:33:15PM +0800, Z C wrote:
> If I boot into a linux kernel using a grub command like "linux /vmlinuz;
> inirtd /initrd; boot", how do I shutdown linux itself (but do not power off
> or reboot the computer hardware) and return back to the grub directly?
> 
> What I meant is something equivalent to the "exit" command in most shells:
> If you are within one shell and you enter another shell, then if you want
> to quit the second shell and return back to the first shell, just simply
> type exit. All env variables and commands you previous typed in the first
> shell are completely intact.
> 
> Suppose I am now in the grub shell, and then I boot into a tiny linux
> kernel, say, a busybox shell, then what can I do to exit the second shell
> (i.e. busybox shell) and return to the first shell (i.e. grub shell)? Of
> course I can enter the grub shell again by simply rebooting the hardware,
> but this is not what I want.

Once linux boots, it does not preserve the boot loader in memory (whyever
would it do that?)

If you want to invent a way to load grub using kexec so you can switch
back to grub fropm a running linux system, well have fun, but why bother?
Where is the use?  It is so hard to deal with what the state of all the
hardware is without doing a reset.

-- 
Len Sorensen

Re: How to exit from linux kernel and return to then grub?

[SevenBits]

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On 05/18/2014 01:33 AM, Z C wrote:
> If I boot into a linux kernel using a grub command like "linux
> /vmlinuz; inirtd /initrd; boot", how do I shutdown linux itself
> (but do not power off or reboot the computer hardware) and return
> back to the grub directly?

You can't.

> 
> What I meant is something equivalent to the "exit" command in most
> shells: If you are within one shell and you enter another shell,
> then if you want to quit the second shell and return back to the
> first shell, just simply type exit. All env variables and commands
> you previous typed in the first shell are completely intact.
> 
> Suppose I am now in the grub shell, and then I boot into a tiny
> linux kernel, say, a busybox shell, then what can I do to exit the
> second shell (i.e. busybox shell) and return to the first shell
> (i.e. grub shell)? Of course I can enter the grub shell again by
> simply rebooting the hardware, but this is not what I want.

When Linux boots, the kernel takes over the management of the computer
completely. The boot loader is not preserved in memory. Why would it
be? The function of the boot loader really is to just boot up the
computer and then get the heck out.

There are many, many obstacles to implementing something like this.
Not to mention the fact that in order to return to GRUB, you would
need to set the processor to return from protected mode into real
mode. All modern processors can do from real mode to protected mode
(which you must do to enabled 32 bit processing).

So, no. It isn't possible.

> 
> 
> 
> _______________________________________________ Grub-devel mailing
> list Grub-devel@gnu.org 
> https://lists.gnu.org/mailman/listinfo/grub-devel
> 
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Re: How to exit from linux kernel and return to then grub?

[Vladimir 'φ-coder/phcoder' Serbinenko]

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On 18.05.2014 19:16, SevenBits wrote:
> On 05/18/2014 01:33 AM, Z C wrote:
>> If I boot into a linux kernel using a grub command like "linux
>> /vmlinuz; inirtd /initrd; boot", how do I shutdown linux itself
>> (but do not power off or reboot the computer hardware) and return
>> back to the grub directly?
> 
> You can't.
> 
> 
>> What I meant is something equivalent to the "exit" command in most
>> shells: If you are within one shell and you enter another shell,
>> then if you want to quit the second shell and return back to the
>> first shell, just simply type exit. All env variables and commands
>> you previous typed in the first shell are completely intact.
> 
>> Suppose I am now in the grub shell, and then I boot into a tiny
>> linux kernel, say, a busybox shell, then what can I do to exit the
>> second shell (i.e. busybox shell) and return to the first shell
>> (i.e. grub shell)? Of course I can enter the grub shell again by
>> simply rebooting the hardware, but this is not what I want.
> 
> When Linux boots, the kernel takes over the management of the computer
> completely. The boot loader is not preserved in memory. Why would it
> be? The function of the boot loader really is to just boot up the
> computer and then get the heck out.
> 
> There are many, many obstacles to implementing something like this.
> Not to mention the fact that in order to return to GRUB, you would
> need to set the processor to return from protected mode into real
> mode. All modern processors can do from real mode to protected mode
> (which you must do to enabled 32 bit processing).
> 
Please update your sources. Last CPU that behaved like this is 286. 386
and later can switch between modes in any direction.
GRUB itself run in protected (but not paged mode) and switches to real
mode to call BIOS functions.
"Returning" to GRUB would be technically feasible but very cumbersome.
Original requester didn't even tell what his end goal is and it probably
can be achieved in much less cumbersome way.
> So, no. It isn't possible.
> 
> 
> 
> 
>> _______________________________________________ Grub-devel mailing
>> list Grub-devel@gnu.org 
>> https://lists.gnu.org/mailman/listinfo/grub-devel
> 
> 
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel
> .
> 



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Re: How to exit from linux kernel and return to then grub?

[SevenBits]

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On 05/18/2014 01:46 PM, Vladimir '?-coder/phcoder' Serbinenko wrote:
> On 18.05.2014 19:16, SevenBits wrote:
>> On 05/18/2014 01:33 AM, Z C wrote:
>>> If I boot into a linux kernel using a grub command like "linux 
>>> /vmlinuz; inirtd /initrd; boot", how do I shutdown linux
>>> itself (but do not power off or reboot the computer hardware)
>>> and return back to the grub directly?
>> 
>> You can't.
>> 
>> 
>>> What I meant is something equivalent to the "exit" command in
>>> most shells: If you are within one shell and you enter another
>>> shell, then if you want to quit the second shell and return
>>> back to the first shell, just simply type exit. All env
>>> variables and commands you previous typed in the first shell
>>> are completely intact.
>> 
>>> Suppose I am now in the grub shell, and then I boot into a
>>> tiny linux kernel, say, a busybox shell, then what can I do to
>>> exit the second shell (i.e. busybox shell) and return to the
>>> first shell (i.e. grub shell)? Of course I can enter the grub
>>> shell again by simply rebooting the hardware, but this is not
>>> what I want.
>> 
>> When Linux boots, the kernel takes over the management of the
>> computer completely. The boot loader is not preserved in memory.
>> Why would it be? The function of the boot loader really is to
>> just boot up the computer and then get the heck out.
>> 
>> There are many, many obstacles to implementing something like
>> this. Not to mention the fact that in order to return to GRUB,
>> you would need to set the processor to return from protected mode
>> into real mode. All modern processors can do from real mode to
>> protected mode (which you must do to enabled 32 bit processing).
>> 
> Please update your sources. Last CPU that behaved like this is 286.
> 386 and later can switch between modes in any direction.

Hi, yes, I actually realized this and posted an addendum to this
message shortly afterwards. I don't know if you received it, but here
is what I wrote:

> Actually, a correction to what I just said. A processor can return 
> from protected mode to real mode. Read more here:
> 
> https://en.wikipedia.org/wiki/Protected_mode#Entering_and_exiting_protected_mode
>
>  I don't know if GRUB supports this (I would assume so) but
> generally speaking it is still not possible to do what you want
> because the Linux kernel as far as I know does not have the
> functionality that would be required. If you want to try and do
> this, go ahead and attempt it, but the roadblocks will be so large
> that it probably is not worth trying, and just simply rebooting the
> machine is the best way to get to GRUB.
> 
> If you tell us what are _trying_ to do, i.e what you need this 
> functionality for, perhaps we can suggest an alternative way to 
> achieve what you need.

So, yes, I realized after I posted my message that what I said was
wrong. My apologies.

> GRUB itself run in protected (but not paged mode) and switches to
> real mode to call BIOS functions. "Returning" to GRUB would be
> technically feasible but very cumbersome. Original requester didn't
> even tell what his end goal is and it probably can be achieved in
> much less cumbersome way.

My thoughts exactly.

>> So, no. It isn't possible.
>> 
>> 
>> 
>> 
>>> _______________________________________________ Grub-devel
>>> mailing list Grub-devel@gnu.org 
>>> https://lists.gnu.org/mailman/listinfo/grub-devel
>> 
>> 
>> _______________________________________________ Grub-devel
>> mailing list Grub-devel@gnu.org 
>> https://lists.gnu.org/mailman/listinfo/grub-devel .
>> 
> 
> 
> 
> 
> _______________________________________________ Grub-devel mailing
> list Grub-devel@gnu.org 
> https://lists.gnu.org/mailman/listinfo/grub-devel
> 
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Re: How to exit from linux kernel and return to then grub?

[SevenBits]

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Actually, a correction to what I just said. A processor can return
from protected mode to real mode. Read more here:

https://en.wikipedia.org/wiki/Protected_mode#Entering_and_exiting_protected_mode

I don't know if GRUB supports this (I would assume not) but generally
speaking it is still not possible to do what you want because the
Linux kernel as far as I know does not have the functionality that
would be required. If you want to try and do this, go ahead and
attempt it, but the roadblocks will be so large that it probably is
not worth trying, and just simply rebooting the machine is the best
way to get to GRUB.

If you tell us what are _trying_ to do, i.e what you need this
functionality for, perhaps we can suggest an alternative way to
achieve what you need.

- -- SevenBits

P.S: Since this topic is not about the development of GRUB, it
probably doesn't belong in this list. Please consider posting to
grub-users instead.

On 05/18/2014 01:33 AM, Z C wrote:
> If I boot into a linux kernel using a grub command like "linux
> /vmlinuz; inirtd /initrd; boot", how do I shutdown linux itself
> (but do not power off or reboot the computer hardware) and return
> back to the grub directly?
> 
> What I meant is something equivalent to the "exit" command in most
> shells: If you are within one shell and you enter another shell,
> then if you want to quit the second shell and return back to the
> first shell, just simply type exit. All env variables and commands
> you previous typed in the first shell are completely intact.
> 
> Suppose I am now in the grub shell, and then I boot into a tiny
> linux kernel, say, a busybox shell, then what can I do to exit the
> second shell (i.e. busybox shell) and return to the first shell
> (i.e. grub shell)? Of course I can enter the grub shell again by
> simply rebooting the hardware, but this is not what I want.
> 
> 
> 
> _______________________________________________ Grub-devel mailing
> list Grub-devel@gnu.org 
> https://lists.gnu.org/mailman/listinfo/grub-devel
> 
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Re: How to exit from linux kernel and return to then grub?

[Chris Jones]

On Sun, May 18, 2014 at 01:33:15AM EDT, Z C wrote:

[..]

> If you are within one shell and you enter another shell, then if you want
> to quit the second shell and return back to the first shell, just simply
> type exit. All env variables and commands you previous typed in the first
> shell are completely intact.

A more valid metaphor would be to start an xterm (or such)... type 'exec
bash'.. type 'exit'... see what happens.

CJ