How to set up struct pt_regs in assembly?

How to set up struct pt_regs in assembly?

[dante]

Hi everyone,

I'm having some problems writing the assembly code to set up the eax, 
ebx, ecx and edx registers for a int 0x80 call to sys_execve on 2.4 
kernels.  In C, the function is defined as int sys_execve(struct pt_regs 
regs) where struct pt_regs is defined in <asm/ptrace.h>.  I don't know 
what to put into eax-edx to get (struct pt_regs regs) right.

In 2.2 it was easy, you just did (in nasm):


; int sys_execve(const char *filename, char const argv[], char const envp[]
%macro execve 3     
        mov ebx,%1  ; ptr to null terminated string containing program
        mov ecx,%2  ; ptr to null terminated string of arguments
        mov edx,%3  ; ptr to null terminated string of environment
        mov eax,11
        int 0x80
%endmacro

global _start
section .data
        shell   db "/bin/sh",0
        nothing db 0

section .text
_start: execve shell, nothing, nothing



What is the equivalent for 2.4 kernels?

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Anthony G. Basile, Ph.D.
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Re: How to set up struct pt_regs in assembly?

[linuxassembly]

> section .data
> shell   db "/bin/sh",0
> nothing db 0
>
> section .text
> _start: execve shell, nothing, nothing

Remember that the argv and envp parameters are pointers to arrays of 
pointers, not pointers to strings.  If you want empty arrays, your 
'nothing' variable needs to be 'dd 0', not 'db 0'.

For a longer example, if you wanted some environment variables and a proper 
argv, the code would look like this:

mov eax, 11  ; ( I'm assuming that's the correct call number )
mov ebx, pathname
mov ecx, argv
mov edx, envp
int 0x80

...and then later...

pathname db "/bin/cat", 0
argv dd arg0, arg1, 0
envp dd var1, var2, 0

arg0 db "cat", 0
arg1 db "/dev/urandom", 0

var1 db "THIS=that", 0
var2 db "THESE=those", 0