Question

I am trying to set SEH without using try except
(This is for my own personal knowledge to get a better idea on how SEH works)

The following code doesn’t work

type
    TSeh = packed record
    OldSeh:DWORD;
    NewSeh:DWORD;
    end;


procedure test;
begin
WriteLn('Hello from seh');
end;


var
    eu:TSeh;
    old_seh:DWORD;
begin
    asm
    mov eax,fs:[0]
    mov old_seh,eax
    end;
    eu.OldSeh := old_seh;
    eu.NewSeh := DWORD(@test);
    asm
        mov eax,offset eu
        mov fs:[0],eax
        ret //This will cause an exception because jumps on an invalid memory address
    end;
end.

But this does

procedure test;
begin
WriteLn('Hello from seh');
end;



begin
    asm
    push offset test
    push fs:[0]
    mov fs:[0],esp
    ret //This will cause an exception because jumps on an invalid memory address
    end;
end.

What am I doing wrong? What is the difference between the first code and the second one?

Answer 1

Windows requires all stack frames to be inside the stack allocated by the system. It also requires the stack frames to be in sequential order on the stack. Furthermore, for exception handling, it requires all ‘exception records’ to be on the stack, and for them to chain in a
sequential order through stack memory.

I figured this out/read this somewhere years ago while writing a micro-thread library (http://www.eternallines.com/microthreads).