problems with getting eip in an exception

[Poseidon]

I'm currently working on an exception handler which gives some more info than only the name of the exception ;D. My only problem is that I can't get the eip value from the stack. This is what I have currently (I've tried loads of other ways before):

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   movl %esp, %ebp
   addl $0x8, %ebp
   movl (%ebp), %eax
   movl %eax, (eip) // eip is here no register, but a double word from my c code.

but this crashes my whole system. Anyone ideas?

Thanks.

[Pype.Clicker]

does this appear right in your ASM stub ?

maybe you'd like to ensure DS register has a valid value before you blindly mov things to memory

[Poseidon]

it works now... i forgot to pop ebp from the stack :-[
i didn't really understand you pype. what value should ds have then?

[AR]

He's probably referring to making sure DS has a valid Kernel space segment so you aren't using the user space segment in the kernel (if that's even allowed).

[Pype.Clicker]

it works now... i forgot to pop ebp from the stack :-[

hehe ... stack garbage ... the usual assembly error.

that being said, keep in mind that some exceptions (notably GPF) push an error code on the stack, and that some other don't.

i didn't really understand you pype. what value should ds have then?

AR got me right. If the exception comes from loading an invalid DS segment, or if it comes from userland, you'll be in trouble when trying to use DS. so it'd be wise to have something like

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exception__x:
    push ebp
    mov ebp, esp
    pushad  ;; just to make sure
    push ds
    push es
    mov ax, KERNEL_SEGMENT
    mov ds,ax
    mov es,ax
    __handle your exception here__
    pop es
    pop ds
    popad
    pop ebp
    __add esp,4 if some error code was pushed__
    iret