Booting Linux on x86 with FIT
Background
Generally Linux x86 uses its own very complex booting method. There is a setup binary which contains all sorts of parameters and a compressed self-extracting binary for the kernel itself, often with a small built-in serial driver to display decompression progress.
The x86 CPU has various processor modes. I am no expert on these, but my understanding is that an x86 CPU (even a really new one) starts up in a 16-bit ‘real’ mode where only 1MB of memory is visible, moves to 32-bit ‘protected’ mode where 4GB is visible (or more with special memory access techniques) and then to 64-bit ‘long’ mode if 64-bit execution is required.
Partly the self-extracting nature of Linux was introduced to cope with boot loaders that were barely capable of loading anything. Even changing to 32-bit mode was something of a challenge, so putting this logic in the kernel seemed to make sense.
Bit by bit more and more logic has been added to this post-boot pre-Linux wrapper:
Changing to 32-bit mode
Decompression
Serial output (with drivers for various chips)
Load address randomisation
Elf loader complete with relocation (for the above)
Random number generator via 3 methods (again for the above)
Some sort of EFI mini-loader (1000+ glorious lines of code)
Locating and tacking on a device tree and ramdisk
To my mind, if you sit back and look at things from first principles, this doesn’t make a huge amount of sense. Any boot loader worth its salts already has most of the above features and more besides. The boot loader already knows the layout of memory, has a serial driver, can decompress things, includes an ELF loader and supports device tree and ramdisks. The decision to duplicate all these features in a Linux wrapper caters for the lowest common denominator: a boot loader which consists of a BIOS call to load something off disk, followed by a jmp instruction.
(Aside: On ARM systems, we worry that the boot loader won’t know where to load the kernel. It might be easier to just provide that information in the image, or in the boot loader rather than adding a self-relocator to put it in the right place. Or just use ELF?
As a result, the x86 kernel boot process is needlessly complex. The file format is also complex, and obfuscates the contents to a degree that it is quite a challenge to extract anything from it. This bzImage format has become so prevalent that is actually isn’t possible to produce the ‘raw’ kernel build outputs with the standard Makefile (as it is on ARM for example, at least at the time of writing).
This document describes an alternative boot process which uses simple raw images which are loaded into the right place by the boot loader and then executed.
Build the kernel
Note: these instructions assume a 32-bit kernel. U-Boot also supports directly booting a 64-bit kernel by jumping into 64-bit mode first (see below).
You can build the kernel as normal with ‘make’. This will create a file called ‘vmlinux’. This is a standard ELF file and you can look at it if you like:
$ objdump -h vmlinux
vmlinux: file format elf32-i386
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00416850 81000000 01000000 00001000 2**5
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .notes 00000024 81416850 01416850 00417850 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
2 __ex_table 00000c50 81416880 01416880 00417880 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
3 .rodata 00154b9e 81418000 01418000 00419000 2**5
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
4 __bug_table 0000597c 8156cba0 0156cba0 0056dba0 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
5 .pci_fixup 00001b80 8157251c 0157251c 0057351c 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
6 .tracedata 00000024 8157409c 0157409c 0057509c 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
7 __ksymtab 00007ec0 815740c0 015740c0 005750c0 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
8 __ksymtab_gpl 00004a28 8157bf80 0157bf80 0057cf80 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
9 __ksymtab_strings 0001d6fc 815809a8 015809a8 005819a8 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 __init_rodata 00001c3c 8159e0a4 0159e0a4 0059f0a4 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
11 __param 00000ff0 8159fce0 0159fce0 005a0ce0 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
12 __modver 00000330 815a0cd0 015a0cd0 005a1cd0 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
13 .data 00063000 815a1000 015a1000 005a2000 2**12
CONTENTS, ALLOC, LOAD, RELOC, DATA
14 .init.text 0002f104 81604000 01604000 00605000 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
15 .init.data 00040cdc 81634000 01634000 00635000 2**12
CONTENTS, ALLOC, LOAD, RELOC, DATA
16 .x86_cpu_dev.init 0000001c 81674cdc 01674cdc 00675cdc 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
17 .altinstructions 0000267c 81674cf8 01674cf8 00675cf8 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
18 .altinstr_replacement 00000942 81677374 01677374 00678374 2**0
CONTENTS, ALLOC, LOAD, READONLY, CODE
19 .iommu_table 00000014 81677cb8 01677cb8 00678cb8 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
20 .apicdrivers 00000004 81677cd0 01677cd0 00678cd0 2**2
CONTENTS, ALLOC, LOAD, RELOC, DATA
21 .exit.text 00001a80 81677cd8 01677cd8 00678cd8 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
22 .data..percpu 00007880 8167a000 0167a000 0067b000 2**12
CONTENTS, ALLOC, LOAD, RELOC, DATA
23 .smp_locks 00003000 81682000 01682000 00683000 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
24 .bss 000a1000 81685000 01685000 00686000 2**12
ALLOC
25 .brk 00424000 81726000 01726000 00686000 2**0
ALLOC
26 .comment 00000049 00000000 00000000 00686000 2**0
CONTENTS, READONLY
27 .GCC.command.line 0003e055 00000000 00000000 00686049 2**0
CONTENTS, READONLY
28 .debug_aranges 0000f4c8 00000000 00000000 006c40a0 2**3
CONTENTS, RELOC, READONLY, DEBUGGING
29 .debug_info 0440b0df 00000000 00000000 006d3568 2**0
CONTENTS, RELOC, READONLY, DEBUGGING
30 .debug_abbrev 0022a83b 00000000 00000000 04ade647 2**0
CONTENTS, READONLY, DEBUGGING
31 .debug_line 004ead0d 00000000 00000000 04d08e82 2**0
CONTENTS, RELOC, READONLY, DEBUGGING
32 .debug_frame 0010a960 00000000 00000000 051f3b90 2**2
CONTENTS, RELOC, READONLY, DEBUGGING
33 .debug_str 001b442d 00000000 00000000 052fe4f0 2**0
CONTENTS, READONLY, DEBUGGING
34 .debug_loc 007c7fa9 00000000 00000000 054b291d 2**0
CONTENTS, RELOC, READONLY, DEBUGGING
35 .debug_ranges 00098828 00000000 00000000 05c7a8c8 2**3
CONTENTS, RELOC, READONLY, DEBUGGING
There is also the setup binary mentioned earlier. This is at arch/x86/boot/setup.bin and is about 12KB in size. It includes the command line and various settings need by the kernel. Arguably the boot loader should provide all of this also, but setting it up is some complex that the kernel helps by providing a head start.
As you can see the code loads to address 0x01000000 and everything else follows after that. We could load this image using the ‘bootelf’ command but we would still need to provide the setup binary. This is not supported by U-Boot although I suppose you could mostly script it. This would permit the use of a relocatable kernel.
All we need to boot is the vmlinux file and the setup.bin file.
Create a FIT
To create a FIT you will need a source file describing what should go in the FIT. See kernel.its for an example for x86 and also instructions on setting the ‘arch’ value for booting 64-bit kernels if desired. Put this into a file called image.its.
Note that setup is loaded to the special address of 0x90000 (a special address you just have to know) and the kernel is loaded to 0x01000000 (the address you saw above). This means that you will need to load your FIT to a different address so that U-Boot doesn’t overwrite it when decompressing. Something like 0x02000000 will do so you can set CONFIG_SYS_LOAD_ADDR to that.
In that example the kernel is compressed with lzo. Also we need to provide a flat binary, not an ELF. So the steps needed to set things are are:
# Create a flat binary
objcopy -O binary vmlinux vmlinux.bin
# Compress it into LZO format
lzop vmlinux.bin
# Build a FIT image
mkimage -f image.its image.fit
(be careful to run the mkimage from your U-Boot tools directory since it will have x86_setup support.)
You can take a look at the resulting fit file if you like:
$ dumpimage -l image.fit
FIT description: Simple image with single Linux kernel on x86
Created: Tue Oct 7 10:57:24 2014
Image 0 (kernel)
Description: Vanilla Linux kernel
Created: Tue Oct 7 10:57:24 2014
Type: Kernel Image
Compression: lzo compressed
Data Size: 4591767 Bytes = 4484.15 kB = 4.38 MB
Architecture: Intel x86
OS: Linux
Load Address: 0x01000000
Entry Point: 0x00000000
Hash algo: sha256
Hash value: 4bbf49981ade163ed089f8525236fedfe44508e9b02a21a48294a96a1518107b
Image 1 (setup)
Description: Linux setup.bin
Created: Tue Oct 7 10:57:24 2014
Type: x86 setup.bin
Compression: uncompressed
Data Size: 12912 Bytes = 12.61 kB = 0.01 MB
Hash algo: sha256
Hash value: 6aa50c2e0392cb119cdf0971dce8339f100608ed3757c8200b0e39e889e432d2
Default Configuration: 'config-1'
Configuration 0 (config-1)
Description: Boot Linux kernel
Kernel: kernel
Booting the FIT
To make it boot you need to load it and then use ‘bootm’ to boot it. A suitable script to do this from a network server is:
bootp
tftp image.fit
bootm
This will load the image from the network and boot it. The command line (from the ‘bootargs’ environment variable) will be passed to the kernel.
If you want a ramdisk you can add it as normal with FIT. If you want a device tree then x86 doesn’t normally use those - it has ACPI instead.
Why Bother?
It demystifies the process of booting an x86 kernel
It allows use of the standard U-Boot boot file format
It allows U-Boot to perform decompression - problems will provide an error message and you are still in the boot loader. It is possible to investigate.
It avoids all the pre-loader code in the kernel which is quite complex to follow
You can use verified/secure boot and other features which haven’t yet been added to the pre-Linux
It makes x86 more like other architectures in the way it boots a kernel. You can potentially use the same file format for the kernel, and the same procedure for building and packaging it.
References
In the Linux kernel, Documentation/x86/boot.txt defines the boot protocol for the kernel including the setup.bin format. This is handled in U-Boot in arch/x86/lib/zimage.c and arch/x86/lib/bootm.c.
Various files in the same directory as this file describe the FIT format.