[syslinux] Syslinux/Extlinux chain : Unexpected change of Extlinux configuration folder after reboot

[Gene Cumm]

On Tue, Oct 20, 2015 at 1:08 PM, Ady via Syslinux <syslinux at zytor.com> wrote:
>
>> Hello,
>>
>> We are experiencing a strange behaviour on an embedded system (i386 PC board).
>>
>> The structure of our disk is the following one :
>> - A 2GB fat16 partition with DOS and syslinux
>> - A 100GB Ext4 partition with Extlinux an several linux images to boot on ram
>> - A swap partition

Presumably FAT16 at the first (hd0 1; sda1) and EXT4 at second (hd0 2; sda2).

>> The system boot on syslinux to choose among Dos and Linux boot using chain.
>> When Linux is chosen, syslinux chains to the boot sector of the Ext4 partition (chain hd0 2)
>>
>> The Ext4 partition contains several folders each containing a linux ramdisk image, a kernel and his own extlinux configuration (ldlinux.sys, syslinux.cfg).

Quite odd since ldllinux.sys itself is essentially useless without the
corresponding VBR.

>> We use the following command to change the linux kernel and image to use at the next boot : extlinux -i /Path_to_the_configuration_folder_of_linux_image_to_launch
>>
>> After calling extlinux -i, the first next reboot use the good kernel and image.
>> But all the following reboot (without call to extlinux -i), roll back to the previous linux image (the one used before the previous call to extlinux -i).
>>
>> I'have check the boot sector content of the Ext4 partition. The pointers (found with offset computed with sect1ptr0, sect1ptr1)  to the extlinux configuration change between reboot without calling extlinux.
>>
>> I have backup the Ext4 partition boot sector between each reboot with this command:
>> dd if=/dev/sda2 of=$VERSION_DIR/boot/syslinux/bootsector.bak ibs=60928 count=1 conv=notrunc
>> The file comparison show an unwanted change of the sector address of the configuration folder (at the offset sect1ptr0=11A and sect1ptr1=120)
>>
>> If I call again extlinux -i with the wanted configuration folder, as the first time, all the following reboots finally use the wanted linux image.
>>
>> How to prevent unwanted change in partition bootsector between reboot ?

SYSLINUX won't do this on its own.

>> Does syslinux/extlinux bootloader overwrite these values during boot ?

No, but may touch the ADV at the end of ldlinux.sys.

>> Is there a bootsector recovery mecanism which restore an old bootsector ?

If your motherboard or distro does this, that would be why.

> The following is a question, but I think it could be read as a
> suggestion too.
>
> Instead of using the extlinux command to install a different boot
> sector each time, is there any reason not to use the CONFIG directive?
>
> The CONFIG directive can load a new configuration file while changing
> the Working Directory at the same time.
>
>  http://www.syslinux.org/wiki/index.php/Config#CONFIG

Keeping the initial config simple is also suggested as it should
minimize any impact it may have on subsequent configurations.  Some
directives are sticky (not reset when loading a new config).

> Which exact version of SYSLINUX / EXTLINUX are you using? Are both VBRs
> (sda1 and sda2) using the same exact version? Is your chain.c32 using
> the same version too?

-- 
-Gene