Messing around with EFI02. Mar '14
Introduction
UEFI is a new way to boot PC-s. Most UEFI enabled computers also allow so called Legacy mode which emulates PC BIOS behaviour. Windows 7 recommended UEFI and Windows 8 requires that computers come with UEFI firmware.
Making distinction between UEFI and traditional PC BIOS is not that easy because the manufacturers have seriously messed up presenting that information.
Partition table
First of all the partition table differs. Traditional PC-s have DOS partition tables:
localhost ~ $ fdisk /dev/sda
Command (m for help): p
Disk /dev/sda: 128.0 GB, 128035676160 bytes
255 heads, 63 sectors/track, 15566 cylinders, total 250069680 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000d5238
Device Boot Start End Blocks Id System
/dev/sda1 * 2048 29296639 14647296 83 Linux
/dev/sda2 29296640 250068991 110386176 83 Linux
Command (m for help):One of the requirements of EFI is that instead DOS partition table a GPT partition table is used. Attempting to edit GPT partition table with fdisk will result in a warning. Also all the disk space is allocated to protective partition:
localhost ~ $ fdisk /dev/sda
WARNING: GPT (GUID Partition Table) detected on '/dev/sda'! The util fdisk doesn't support GPT. Use GNU Parted.
Command (m for help): p
Disk /dev/sda: 256.1 GB, 256060514304 bytes
255 heads, 63 sectors/track, 31130 cylinders, total 500118192 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000
Device Boot Start End Blocks Id System
/dev/sda1 1 500118191 250059095+ ee GPT
Command (m for help):To edit a GPT partition table you need gdisk:
localhost ~ $ gdisk /dev/sda
GPT fdisk (gdisk) version 0.8.5
Partition table scan:
MBR: protective
BSD: not present
APM: not present
GPT: present
Found valid GPT with protective MBR; using GPT.
Command (? for help): p
Disk /dev/sda: 500118192 sectors, 238.5 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 878637F3-2209-4C79-94EE-6C8DDB145D61
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 500118158
Partitions will be aligned on 2048-sector boundaries
Total free space is 2669 sectors (1.3 MiB)
Number Start (sector) End (sector) Size Code Name
1 2048 249855 121.0 MiB EF00
2 249856 29546495 14.0 GiB 0700
3 441524224 470820863 14.0 GiB 0700
4 470820864 500117503 14.0 GiB 0700
5 29546496 441524223 196.4 GiB 0700
Command (? for help):EFI partition
As can be seen from the partition listing above there is a partition with code EF00. This is a FAT32 formatted partition that contains the bootloaders of operating systems. It is usually mounted at /boot/efi:
localhost ~ $ mount | grep boot
/dev/sda1 on /boot/efi type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=utf8,shortname=mixed,errors=remount-ro)If you would run it on a cleanly installed Debian Wheezy box it would look something like this:
localhost ~ $ find /boot/efi
/boot/efi
/boot/efi/EFI
/boot/efi/EFI/debian
/boot/efi/EFI/debian/grubx64.efiSecure Boot feature of UEFI firmware checks the signature of those bootloader binaries and refuses to boot Debian because Debian's GRUB is not signed. On my Thinkpad T420 I could not locate Secure Boot toggle in the BIOS, appearently it's disabled by default.
EFI boot entries
You can list EFI entries using efibootmgr if you have booted your machine in EFI mode, that is - you have NOT enabled legacy mode in the BIOS:
lauri-t420 lauri $ efibootmgr
BootCurrent: 0019
Timeout: 0 seconds
BootOrder: 0019,000D,000B,000A,0008,0007,0006,000C,0013,0009,0011,0010,000F,000E,0012
Boot0000 Setup
Boot0001 Boot Menu
Boot0002 Diagnostic Splash Screen
Boot0003 Startup Interrupt Menu
Boot0004 ME Configuration Menu
Boot0005 Rescue and Recovery
Boot0006* USB CD
Boot0007* USB FDD
Boot0008* ATAPI CD0
Boot0009* ATA HDD2
Boot000A* ATA HDD0
Boot000B* ATA HDD1
Boot000C* USB HDD
Boot000D PCI LAN
Boot000E* ATAPI CD1
Boot000F* ATAPI CD2
Boot0010* Other CD
Boot0011* ATA HDD3
Boot0012* ATA HDD4
Boot0013* Other HDD
Boot0014* IDER BOOT CDROM
Boot0015* IDER BOOT Floppy
Boot0016* ATA HDD
Boot0017* ATAPI CD:
Boot0018* PCI LAN
Boot0019* debianThis of course assumes that efivars module has been loaded and the kernel has detected the presence of UEFI:
modprobe efivarsIf you have loaded the module but efibootmgr still fails to probe the EFI entries this means that your EFI firmware is not detected by the kernel:
Fatal: Couldn't open either sysfs or procfs directories for accessing EFI variables.
Try 'modprobe efivars' as root.Restoring EFI entries
On my Thinkpad T420 I happened to restore BIOS defaults which appearently also deletes all EFI entries (dafuq?!). Get a hold of a USB stick that boots with UEFI enabled BIOS, you may notice Debian Wheezy LiveCDs, Gentoo LiveCD and many others fail to boot if you have put the ISO on the usb key 1:1 with dd or cat. Sabayon Linux luckily has ISO which boots properly off USB memory stick. Note that I haven't used unetbootin to place ISO on a memory stick in a long time and I would not recommend it to anyone anyway.
I have to remind this again - efibootmgr complains that sysfs entries are missing if you attempt to boot in legacy mode.
Once Sabayon is up and running:
sudo mkdir /wheezy # Create mountpoint for my already installed Debian Wheezy
sudo mount /dev/sda2 /wheezy # Mount root filesystem of Debian
sudo mount /dev/sda1 /wheezy/boot/efi # Mount EFI partition
sudo mount --bind /dev /wheezy/dev
sudo mount --boot /dev/pts /wheezy/dev/pts
sudo mount --boot /sys /wheezy/sys
sudo mount --boot /proc /wheezy/proc
sudo chroot /wheezy apt-get install --reinstall grub-efi-amd64
sudo umount /wheezy/dev/pts
sudo umount /wheezy/dev/
sudo umount /wheezy/proc
sudo umount /wheezy/sys
sudo umount /wheezy/boot/efi
sudo umount /wheezy/
sudo rebootThis method also works if let's say you tried to install Ubuntu and it failed to detect that it is running on a UEFI enabled machine and the machine refuses to boot into GRUB.
Summary
Following flowchart summarises EFI debug: