Re: Change default on boot

[Andrew Athan]

It turns out that the RedHat version of grub implements a useful feature:

savedefault: savedefault [--stage2=STAGE2_FILE] [--default=DEFAULT] [--once]
   Save DEFAULT as the default boot entry in STAGE2_FILE. If
   '--once' is specified, the default is reset after the next reboot.

So, lets say that menu item 0 is the new, possibly unstable kernel.  
Using this, one could add a line "savedefault --default=1" to the 
commands for the "0" menu item.  Then, the boot will revert back to "1" 
after one boot.  For example:

default=saved
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Xen 2.0 / XenLinux 2.6.10
       kernel /xen.gz dom0_mem=1048576 sched=bvt
       module /vmlinuz-2.6.10-xen0-syncraid 
root=/dev/VolGroup00/LogVol01 ro rhgb quiet
       module /initrd-2.6.10.img
title Fedora Core (2.6.10 NO DEBUG)
       root (hd0,0)
       savedefault --default=1
       kernel /vmlinuz-2.6.10 ro root=/dev/VolGroup00/LogVol01 rhgb quiet
       initrd /initrd-2.6.10.img

Then, in a command line, run grub, and at the prompt type "savedefault 
--default=0"

The next time the machine boots, it will boot into Xen.  But if it fails 
to boot, and/or on the next reboot, it will boot Fedora Core.

I was pointed in this direction by the email at this location:  
http://lists.gnu.org/archive/html/bug-grub/2004-06/msg00063.html  , 
"Subject: GRUB: extending savedefault" by Martin von Gagern.

If these features are not already part of GNU Grub, they certainly have 
my vote!  By the way, Martin also provides the source for a command-line 
utility that changes the savedefault value, excerpted here.  I am not 
sure if it is compatible with the current format of the grub stage2 file.

A.


--- grub-0.94/util/grub-savedefault.c   1970-01-01 01:00:00.000000000 +0100
+++ grub-0.94-savedefault/util/grub-savedefault.c       2004-06-19
00:33:01.871447584 +0200
@@ -0,0 +1,67 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 1999,2000,2001,2002,2003,2004  Free Software 
Foundation, Inc.
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the Free Software
+ *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define SAVED_ENTRY_POS 0x20c
+#define DEFAULT_STAGE2_LOCATION "/boot/grub/stage2"
+
+int main(int argc, char** argv) {
+       const char *filename=DEFAULT_STAGE2_LOCATION;
+       char* ptr;
+       int entry;
+       FILE* file;
+      
+       if (argc>=2) entry=strtoul(argv[1], &ptr, 0);
+       if (argc<2 || argc>3 || *ptr || entry > 0xff) {
+               ptr=strrchr(argv[0], '/');
+               if (ptr==NULL) ptr=strrchr(argv[0], '\\');
+               if (ptr==NULL) ptr=argv[0];
+               else ++ptr;
+               fprintf(stderr,
+                       "Usage: %s NUM [STAGE2]\n\t"
+                       "Modify grub stage2 file STAGE2 (default "
+                       DEFAULT_STAGE2_LOCATION
+                       ")\n\tto set default entry to NUM\n",
+                       ptr);
+               return argc==1 ? 0 : 1;
+       }
+       file=fopen(filename, "r+b");
+       if (file==NULL) {
+               perror(filename);
+               return 1;
+       }
+       if (fseek(file, SAVED_ENTRY_POS, SEEK_SET)) {
+               perror("seek to entry number location (20C)");
+               fclose(file);
+               return 2;
+       }
+       if (fputc(entry, file) != entry) {
+               perror("write number to stage2 file");
+               fclose(file);
+               return 2;
+       }
+       if (fclose(file)) {
+               perror("close stage2 file");
+               return 2;
+       }
+       return 0;
+}


A.


Andrew Athan wrote:

> I am trying to remotely debug a linux kernel problem which causes the 
> machine to reboot early in the bootup process.  When this happens, the 
> machine goes into an infinite loop attempting to boot.
>
> It would be nice to be be able to override the default menu entry that 
> grub boots into, but only for a specified number of boots (or 
> unsuccessful boots).  This could be hardcoded at "1".
>
> (Like a poor-man's "last known good" bootup capability )
>
> This would allow safer updates to the boot-loader configurations of 
> remote machines, since by setting the count to "1", one could make a 
> change and have some chance that if the boot is not successful, the 
> next boot will revert to the prior "known good" config.  
> Differentiation between successful and unsuccessful boots, which is 
> really an "extra credit" feature, could be signaled by a monotonically 
> incrementing count stored within a file in the boot partition.
>
>
> Or, someone could point me to an existing solution...
>
> Thanks!
> Andrew