[uml-devel] How to intercept int 0x40 in UML

[Dimitry Golubovsky <golubovsky@gmail.com>]

Hi,

I am trying to implement some ideas of Glendix [1], but without
patching the underlying Linux kernel, staying completely within UML.

I am experimenting with kernel 2.6.31.6 sources (the same version for
which Glendix patch is available)

I got stuck trying to intercept the "int 0x40" instruction which is
used for syscalls in Plan 9. Example code below (NASM syntax, example
borrowed from Glendix):

======================
section .data
        hello:     db 'Hello world!',10    ; 'Hello world!' plus a
linefeed character
        helloLen:  equ $-hello             ; Length of the 'Hello world!' string

section .text
        global _start

_start:
        mov eax,8
        mov ebx,hello
        int 40h
======================

I am trying to find a place within arch/um/os-Linux/skas/process.c
where to my understanding the code responsible for intercepting
syscalls is located - the place where I could insert my own code to
intercept 0x40 interrupt and translate it to some kernel action.

When I run this program compiled I get "Segmentation fault" message.

I tried to insert printk call in the "userspace" function in
arch/um/os-Linux/skas/process.c in various places to see if the "int
0x40" instruction is trapped by the UML kernel, unsuccessfully so far.

In particular, I tried:
  * right after the if(WIFSTOPPED(status)) statement
(arch/um/os-Linux/skas/process.c, line 406)
  * right before the same check (like this: if (!WIFSTOPPED(status)) printk ...)
  * inside the "fatal_sigsegv" function in arch/um/kernel/trap.c

None caused any output when running the test program

Any suggestions where can I intercept it? Or does this signal
completely bypass UML?

Thanks for any ideas.

----------------
[1] http://www.glendix.org - Linux kernel patch to run Plan9 binaries
on Linux kernel

-- 
Dimitry Golubovsky

Anywhere on the Web

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