GNU EFI lib and Hello World

Writing UEFI applications takes a little bit of effort to get started, fortunately the GNU EFI library provides enough infrastructure to make it far less painful.  The crunch points are:

1. Understanding which UEFI protocols to use, fortunately these are well documented in the UEFI specification. Unfortunately the specification is a couple of thousand pages long, so be prepared to spend some time working out how it all hangs together.

2. Figuring out how to call the protocols using the GNU EFI wrapper shim.

3. Generating a UEFI PE COFF image using some objcopy runes.

So, the best way to start was to write a "Hello World" program and to work up from there.  First, I installed the gnu-efi development library using:

sudo apt-get install gnu-efi

Next I hacked up the classic "Hello World", this can be found here.  Then considerable amount of Googling was required to figure out how to use objcopy to help create the desired executable image.  The Makefile containing these runes can be found here.

So what next?  Well, this little adventure is my first step in a quest to write a bunch of tests so ensure UEFI firmware has functional protocols to allow grub to load a kernel.  I was hoping to re-use the firmware test suite framework but it may take some effort since I don't have any POSIX support on UEFI, so I'm going to have to scope out how much effort is required to port the framework to UEFI.

Anyhow, it's a start.

Dumping UEFI variables

UEFI variables in Linux can be found in /sys/firmware/efi/vars on UEFI firmware based machine, however, the raw variable data is in a binary format and hence not in a human readable form.   The Ubuntu Natty firmware test suite contains the uefidump tool to extract and decode the binary data into a more human readable form.

To run, use:

sudo fwts uefidump -

and you will see something similar to the following:

Name: AuthVarKeyDatabase.
  GUID: aaf32c78-947b-439a-a180-2e144ec37792
  Attr: 0x17 (NonVolatile,BootServ,RunTime).
  Size: 1 bytes of data.
  Data: 0000: 00                                               .

Name: Boot0000.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x7 (NonVolatile,BootServ,RunTime).
  Active: Yes
  Info: Primary Master Harddisk
  Path: \BIOS(2,0,Primary Master Harddisk).

Name: Boot0001.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x7 (NonVolatile,BootServ,RunTime).
  Active: Yes
  Info: EFI Internal Shell
  Path: \Unknown-MEDIA-DEV-PATH(0x7)\Unknown-MEDIA-DEV-PATH(0x6).

Name: Boot0003.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x7 (NonVolatile,BootServ,RunTime).
  Active: Yes
  Info: ubuntu
  Path: \HARDDRIVE(1,22,9897,0f52a6e132775546,ab,f6)\FILE('\EFI\ubuntu\grubx64.efi').

Name: Boot0004.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x7 (NonVolatile,BootServ,RunTime).
  Active: Yes
  Info: EFI DVD/CDROM
  Path: \ACPI(0xa0341d0,0x0)\PCI(0x2,0x1f)\ATAPI(0x0,0x1,0x0).

Name: BootOptionSupport.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x6 (BootServ,RunTime).
  BootOptionSupport: 0x0303.

Name: BootOrder.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x7 (NonVolatile,BootServ,RunTime).
  Boot Order: 0x0003,0x0000,0x0001,0x0004,0x0005,0x0006.

Name: ConIn.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x7 (NonVolatile,BootServ,RunTime).
  Device Path: \ACPI(0xa0341d0,0x0)\PCI(0x0,0x1f)\ACPI(0x50141d0,0x0)\UART(115200 baud,8,1,1)\VENDOR(11d2f9be-0c9a-9000-273f-c14d7f010400)\USBCLASS(0xffff,0xffff,0x3,0x1,0x1).

Name: ConInDev.
  GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c
  Attr: 0x6 (BootServ,RunTime).
  Device Path: \ACPI(0xa0341d0,0x0)\PCI(0x0,0x1f)\ACPI(0x50141d0,0x0)\UART(115200 baud,8,1,1)\VENDOR(11d2f9be-0c9a-9000-273f-c14d7fff0400).

Name: Setup.
  GUID: 038bcef0-21e2-49d1-a47c-b7257296b980
  Attr: 0x7 (NonVolatile,BootServ,RunTime).
  Size: 114 bytes of data.
  Data: 0000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  Data: 0010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  Data: 0020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  Data: 0030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  Data: 0040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  Data: 0050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  Data: 0060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  Data: 0070: 01 00   
..

The tool will try to decode the binary data, however, if it cannot identify the variable type it will resort to doing a hex dump of the data instead.