UNIX3.51m Kernel object files [...]

[Lenny Tropiano]

In article <1135 at icus.ICUS.COM> lenny at icus.ICUS.COM (Lenny Tropiano) writes:
|>I was able to get a copy of the kern.o.cpio.Z file before Brant had
|>a chance to release it to the osu-cis Internet archives.  According
[...]

Gil and I were doing some hacking/brainstorming this weekend, we
decided to do some major peeking at these object files.  Some interesting
things were found ...

The archive library lib4 which contained ...

|>rw-r--r--     0/   100   1500 Mar 15 09:00 1990 main.o
|>rw-r--r--     0/   100    807 Mar 15 09:00 1990 trap.o
|>rw-r--r--     0/   100   2662 Mar 15 09:00 1990 getcmd.o
|>rw-r--r--     0/   100   1374 Mar 15 09:00 1990 memory.o
|>rw-r--r--     0/   100   1054 Mar 15 09:00 1990 regs.o
|>rw-r--r--     0/   100    248 Mar 15 09:00 1990 dload.o
|>rw-r--r--     0/   100    614 Mar 15 09:00 1990 io.o
|>rw-r--r--     0/   100    661 Mar 15 09:01 1990 atoh.o
|>rw-r--r--     0/   100   1475 Mar 15 09:01 1990 8251.o
|>rw-r--r--     0/   100   1821 Mar 15 09:01 1990 break.o
|>rw-r--r--     0/   100    676 Mar 15 09:01 1990 dispreg.o
|>rw-r--r--     0/   100   1030 Mar 15 09:01 1990 modbyte.o
|>rw-r--r--     0/   100    762 Mar 15 09:01 1990 disass.o
|>rw-r--r--     0/   100    898 Mar 15 09:01 1990 dispblk.o
|>rw-r--r--     0/   100  12000 Mar 15 09:02 1990 text68.o
|>rw-r--r--     0/   100   1148 Mar 15 09:03 1990 pnt.o
|>rw-r--r--     0/   100   2727 Mar 15 09:03 1990 btrace.o
|>rw-r--r--     0/   100    350 Mar 15 09:03 1990 getpid.o
|>rw-r--r--     0/   100    306 Mar 15 09:03 1990 misc.o
|>rw-r--r--     0/   100   1594 Mar 15 09:04 1990 dbstr.o
|>

Is basically all the pieces for the kernel debugger.  What's interesting
is that this (lib4) can be easily removed from the link (ld) stage.  I
suppose this is here for the use of the db.o (kernel debugger).  References
to an 8251 chip are interesting, at least to hardware hackers like Gil -- he
mentioned we don't even have one of those in the machine.  Hmmm.  

|>Now what?  Most people would like to hack their kernel, and without source
|>that becomes a bit of a challenge.  Since none of us are going to get 3.51m
|>source (or it seems quite unlikely in the future), we need to go to other
|>extremes.  Thanks to Alex Crain's work on unc (a good disassembler), it
|>makes ripping apart binaries to assembly code a bit easier.  For those
|>real-hardcore hackers, adb will do in a lot of cases ;-).  

Well we did have a bunch of problems with unc (you listening Alex?).  
Mostly it would bomb out in 50% of the cases with "invalid text address"
messages.  It also doesn't show initialization of various spaces in
the kernel.  Specifically in conf.o, the v-block (see <sys/var.h>)
should be initialized to the values in (<sys/space.h>).  This is where
the infamous NMOUNT kernel variables are kept (v.v_mount).  adb'ing
the kernel now shows it's 8 (as suspected).

So what does this all say... well it's not easy to change NMOUNT without
the conf.c source code, or a fix to unc the disassembler.  

|>Ok, now looking in (lib1)sys3.o there is the system call mount().  Assembly
|>shows that the "8" is compared to bdevcnt, and then if not it returns 
|>errno=6.  (#define	ENXIO	6	/* No such device or address */)
|>

Mistake here.  Gil and I looked further and it looks like the mount()
system call just goes through the mount[] structure for an available slot.
If there are no available slots, after reaching the end, it then returns
the appropriate errno. 

So... one should only have to increase the space for mount[] structure in
the kernel, and modify the v-block NMOUNT parameters.  Unfortunately
both the space declaration and v-block initialization are in conf.o.
Alex! :-)

|>You'll all be happy hacking soon!
|>

Either that or we'll all be up late pulling our hair out more often...

Back to the laboratory...

-Lenny
-- 
| Lenny Tropiano           ICUS Software Systems        lenny at icus.ICUS.COM |
| {ames,pacbell,decuac,sbcs,hombre,rayssd}!icus!lenny   attmail!icus!lenny  |
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