Linux:Building Kernel and installation process

When the user types 'make zImage' or 'make bzImage' the resulting bootable kernel image is stored as arch/i386/boot/zImage or arch/i386/boot/bzImage respectively. Here is how the image is built:

1. C and assembly source files are compiled into ELF relocatable object format (.o) and some of them are grouped logically into archives (.a) using ar(1).To know more about ELF go through http://subramanyaachar.blogspot.com/2011/05/understanding-elf-1.html
2. Using ld(1), the above .o and .a are linked into vmlinux which is a statically linked, non-stripped ELF 32-bit LSB 80386 executable file.
3. System.map is produced by nm vmlinux, irrelevant or uninteresting symbols are grepped out.
4. Enter directory arch/i386/boot.
5. Bootsector asm code bootsect.S is preprocessed either with or without -D__BIG_KERNEL__, depending on whether the target is bzImage or zImage, into bbootsect.s or bootsect.srespectively.
6. bbootsect.s is assembled and then converted into 'raw binary' form called bbootsect (or bootsect.s assembled and raw-converted into bootsect for zImage).
7. Setup code setup.S (setup.S includes video.S) is preprocessed into bsetup.s for bzImage or setup.s for zImage. In the same way as the bootsector code, the difference is marked by -D__BIG_KERNEL__ present for bzImage. The result is then converted into 'raw binary' form called bsetup.
8. Enter directory arch/i386/boot/compressed and convert /usr/src/linux/vmlinux to $tmppiggy (tmp filename) in raw binary format, removing .note and .comment ELF sections.
9. gzip -9 < $tmppiggy > $tmppiggy.gz
10. Link $tmppiggy.gz into ELF relocatable (ld -r) piggy.o.
11. Compile compression routines head.S and misc.c (still in arch/i386/boot/compressed directory) into ELF objects head.o and misc.o.
12. Link together head.o, misc.o and piggy.o into bvmlinux (or vmlinux for zImage, don't mistake this for /usr/src/linux/vmlinux!). Note the difference between -Ttext 0x1000 used forvmlinux and -Ttext 0x100000 for bvmlinux, i.e. for bzImage compression loader is high-loaded.
13. Convert bvmlinux to 'raw binary' bvmlinux.out removing .note and .comment ELF sections.
14. Go back to arch/i386/boot directory and, using the program tools/build, cat together bbootsect, bsetup and compressed/bvmlinux.out into bzImage (delete extra 'b' above for zImage). This writes important variables like setup_sects and root_dev at the end of the bootsector.

The size of the bootsector is always 512 bytes. The size of the setup must be greater than 4 sectors but is limited above by about 12K - the rule is:0x4000 bytes >= 512 + setup_sects * 512 + room for stack while running bootsector/setup
We will see later where this limitation comes from.
The upper limit on the bzImage size produced at this step is about 2.5M for booting with LILO and 0xFFFF paragraphs (0xFFFF0 = 1048560 bytes) for booting raw image, e.g. from floppy disk or CD-ROM (El-Torito emulation mode).
Note that while tools/build does validate the size of boot sector, kernel image and lower bound of setup size, it does not check the *upper* bound of said setup size. Therefore it is easy to build a broken kernel by just adding some large ".space" at the end of setup.S.


To Be continued