More doubts in bzImage

Question

The description of the bzImage in Wikipedia is really confusing me.

The above picture is from Wikipedia, but the line next to it is:

The bzImage file is in a specific format: It contains concatenated bootsect.o + setup.o + misc.o + piggy.o.

I can't find the others (misc.o and piggy.o) in the image.
I would also like to get more clarity on these object files.

The info on this post about why we can't boot a vmlinux file is also really confusing me.

Another doubt is regarding the System.map. How is it linked to the bzImage? I know it contains the symbols of vmlinux before creating bzImage. But then at the time of booting, how does bzImage get attached to the System.map?

Answer 1

Till Linux 2.6.22, bzImage contained:

• bbootsect (bootsect.o):
• bsetup (setup.o)
• bvmlinux (head.o, misc.o, piggy.o)

Linux 2.6.23 merged bbootsect and bsetup into one (header.o).

At boot up, the kernel needs to initialize some sequences (see the header file above) which are only necessary to bring the system into a desired, usable state. At runtime, those sequences are not important anymore (so why include them into the running kernel?).

System.map stands in relation with vmlinux, bzImage is just the compressed container, out of which vmlinux gets extracted at boot time (=> bzImage doesn't really care about System.map).

Linux 2.5.39 intruduced CONFIG_KALLSYMS. If enabled, the kernel keeps it's own map of symbols (/proc/kallsyms).

System.map is primary used by user space programs like klogd and ksymoops for debugging purposes.

Where to put System.map depends on the user space programs which consults it.
ksymoops tries to get the symbol map either from /proc/ksyms or /usr/src/linux/System.map.
klogd searches in /boot/System.map, /System.map and /usr/src/linux/System.map.

Removing /boot/System.map generated no problems on a Linux system with kernel 2.6.27.19 .