Booting GPT disk in BIOS systems using syslinux

Recently I was asked whether it is possible to boot GPT disk under a system with BIOS firmware.

I was somehow under the impression that GPT-partitioned disk (GPT disk for short), though perfectly usable in Linux, will not boot under BIOS systems. You need EFI or UEFI systems to boot GPT disk - that was the conventional wisdom that I subscribed to; and that was the piece of wisdom I was about to dump to the inquirer.

The good thing is that I am aware that the wisdom I subscribed to is rather old; that there may be some advancement in the fields; and especially that a GPT partition usually has a MBR protective partition which in theory could contain bootable code just like a standard MBR does; so I decided to do some quick research and tests first to avoid embarassing myself by giving the wrong answer.

It was a good decision as it turns out that GPT disk is indeed bootable under BIOS too! The answer actually depends on the BIOS and the bootloader.

As long as the BIOS in your system is happy to load MBR code and execute it - you're already halfway to the happy end. Some BIOS-es only check that the MBR is valid and will happily execute it if it does (these are the good ones), some are more tricky and check whether the MBR has a valid bootable partition (this is still workable though it needs more work), but some are recalcitrant and need a specific crafted MBR to boot from it - if you happen to have one of these, your luck may have run out.

The second part is the bootloader. GRUB2 definitely supports booting from GPT disk, and so is syslinux. Legacy Grub (Grub 0.97) does not, however there seems to be a patched version which does. Grub4Dos definitely does not support it, at least at the time of writing.

Okay, so how do one configure it? Every bootloader is different, of course, but I'll set the example with syslinux, as it is pretty easy.

For the following I will assume that your GPT disk is /dev/sda and your (soon-to-be) bootable partition is partition 3, that is, /dev/sda3. Obviously change all references to /dev/sda, /dev/sda3 and the partition 3 to your own setup.

1. Use GPT partitioning tool to mark that the partition is going to be bootable.
If you use gparted, choose "Manage Flags" and tick "legacy boot".

If you use sgdisk, type
sgdisk -A 3:set:2 /dev/sda
which means set bit 2 ("legacy BIOS partition") on partition 3 of disk /dev/sda to "on".

If you use the interactive gdisk (by typing "gdisk /dev/sda"), enter the "extra functionality (experts only)" by typing "x", and then choose "set attributes" by typing "a". You will be asked for which partition to update, choose "3", and from there you will be asked which bit to enable, so choose "legacy BIOS bootable" by typing "2". Don't forget to save your changes by typing "w".

2. Next, mount the partition somewhere, say in /mnt/data
mount /dev/sda3 /mnt/data

Let's assume that the partition (for the sake of the discussion) is a Linux filesystem such as ext3 or ext4.

3. Use syslinux to install the boot loader, pretending that it is a regular partition:
extlinux -i /mnt/data
or
extlinux -i /mnt/data/boot
if you want to keep syslinux files under /boot directory. Then please add the usual syslinux.conf configuration to boot your system - location of your kernel, initrd, rootfs, etc.

4. The final magic is to copy syslinux's MBR code which is capable of booting GPT partition. This code is stored (usually) in /usr/share/syslinux/gptmbr.bin. All you need to do is copy this to your disk's MBR.
dd if=/usr/share/syslinux/gptmbr.bin of=/dev/sda bs=440 count=1

5. Congratulations, you're done! Do the usual cleanup (un-mount /mnt/data, etc) and then reboot.

6. Assuming however, you have done all these correctly and BIOS still won't boot the system (saying no bootable disk found etc), you may need to mark the protective MBR as bootable. You need to do this using "fdisk". Start "fdisk" on the disk (fdisk /dev/sda) and ignore the warnings. Type "a" to toggle the bootable flag, and choose "1" for the partition to enable. ("1", not "3", because now we are talking about the protective MBR, and there is only one partition in the MBR - the protective partition). Don't forget to save your work by typing "w". Note that doing this is actually discouraged, see for example: http://mjg59.dreamwidth.org/8035.html (Matt Garret is the expert on all things UEFI, he is the author of the 'shim' bootloader used by many Linux distros which provides compatibility with UEFI and Secure Boot) - but if this is what it takes for your system to boot, then so be it.

Caution 1: Installing bootloader is not for the faint of heart, and you'd better have a backup plan if this is your *primary* computer, in case you're doing it wrong and the system cannot come to life after that. Installing a bootloader also means that ALL your previously installed operating system will not boot unless you configure the new bootloader to boot them. If you don't understand the consequences of the actions described in the above steps, you'd better stay away from doing this altogether. You have been warned.

Caution 2:The method described above assumes that you use a GPT with a standard protective MBR partition. In this scheme, you only have one partition defined on the MBR - partition of type 'ee' which covers the entire disk; and this is the recommended way to do it. gdisk actually offers another type of protective MBR called "hybrid MBR", in this case, the GPT partition definitions are copied and mirrored into the MBR partition definitions - if you have 3 partitions in GPT you will have 4 partitions in MBR (3 GPT + original protective). This method is highly discouraged, and the reason why is written here: http://www.rodsbooks.com/gdisk/hybrid.html (by the author of gptfdisk itself).


Posted on 23 May 2013, 5:41 - Categories: Linux
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Comments:

Posted on 18 Apr 2018, 08:01 by André
"GPT support"
so bios has to support GPT partitions?
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Posted on 31 May 2018, 00:53 by jamesbond
"No"
No.

The point of this post is to enable booting of GPT partitions on systems that cannot normally boot it (but can boot normal MBR disks).

However, as noted in the post, the BIOS must be "flexible" enough to be willing to boot a "valid looking" MBR.
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