System.map purpose?

Scot Mc Pherson scot at linuxfromscratch.org
Fri Nov 1 10:52:58 PST 2002


On Fri, 01 Nov 2002 18:39:38 -0500, David C wrote:


> What is the System.map file in the /boot directory for? The reason I ask
> is because I have been using my hosts' LILO so far, simply because I
> like the nice graphical menu RH uses for it. Now I'm going to use the
> 'LILO Beautified' hint to attempt to mimic this and hopefully improve
> upon it within LFS. This will mean switching over to booting from LFS's
> LILO and /boot directory, which has it's own System.map file.
> 
> In the past, I'm pretty sure I've used LFS' System.map for both LFS and
> RH with no ill effects apparent, but I am wondering if they truly are
> interchangeable, and just what exactly they are for anyway.
> 
> I know they are not identical because the file size is different by a
> sizeable amount.
> 
> I assume they must be at least somewhat interchangeable, since when
> booting multiple kernels I've never seen mention of requiring multiple
> System.maps...
> 
> Thanks,
 
Unless you compile all the kernel's yourself, do NOT assume the
System.map files are interchangeable. The System.map maps the offsets of
the kernel so that kernel modules are loaded properly. IF and only IF
both kernels were compiled with all options enabled in "Loadable Module
Support" will modules be "almost guarranteed" to load with modules from
an older kernel.

The MOST correct way to boot a system is to load the System.map that was
built along with the kernel. For the most part I don't use modules at all
so it effects me very little, but people who use modules heavily may find
they have problems if the System.map doesn't match their kernel. RH very
likely uses the "loadable module support" options and so that's likely
why you haven't had an issues.

Now having said all this, I haven't experimented with TRYING to break a
system by having bad System.map files, so perhaps what is "documented"
isn't entirely as important as it sounds.

Scot Mc Pherson <scot at linuxfromscratch>
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