Structured Exception Handler and Delphi

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?

Reorganization

+1 for trying something weird. – Dortheydorthy

+1 too for trying something weird in Delphi and with asm – Beaverette

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).

Yuzik

Thanks for your answer that shades a light on what was bothering me.Thanks a lot – Reorganization

Answer 2

You can't use test procedure as exception callback function because exception callback function have different prototype. Read Matt Pietrek article, IMO the best source of information about Win32 SEH.

---

Update

For the further investigations I would recommend the following changes in the code to make the problem a bit more clean:

function test: Integer;
begin
  WriteLn('Hello from seh');
  Result:= 0;
end;

(because exception callback should return integer value in EAX)

And for the first code snippet

begin
    asm
        mov eax,fs:[0]
        mov old_seh,eax
    end;
    eu.OldSeh := old_seh;
    eu.NewSeh := Cardinal(@test);
    asm
        lea eax, eu
        mov fs:[0],eax
        mov ds:[0],eax //This will cause an AV exception
    end;
end.

Now you see that the exception is handled correctly as:

---------------------------
Debugger Fault Notification
---------------------------
Project C:\Users\Serg\Documents\RAD Studio\Projects\Project13.exe faulted with
message: 'access violation at 0x004050f5: write of address 0x00000000'. Process
Stopped. Use Step or Run to continue.
---------------------------

but not by your exception handler. Probably OS ignores exception registration records that are not stack-based (OS can easily do it because it knows minimum and maximum stack values)

Suez

I know i've read that but what confuses me is why the second code snippet works ? – Reorganization

the exception function is stdcall defined, so it will work with a procedure defined without any parameters, like test – Lyndell

@Reorganization Interesting that the first code snippet does not handle exception. Looks like OS does not like exception registration record that is not stack-based. – Suez

Answer 3

For the first code, the TSeh is on the DATA global section of the executable, whereas the 2nd code stores it on the Stack.

This is IMHO where the difference is. The _EXCEPTION_REGISTRATION_RECORD structure should probably be on stack. Don't know why, honestly (some low-level SS register trick?).

To raise an exception, you should better try something like a division per zero or an access to a nil absolute address:

PInteger(nil)^ := 0; // will always raise an exception

asm
  xor eax,eax
  mov [eax],eax // will always raise an exception
end;

About how to intercept exceptions in Delphi, take a look at this article. In fact, Delphi add some custom layer over SEH over Windows.

And note also that the exception handling changes in Win64 mode. Worth reading when moving to up to come Delphi XE2.

Lyndell

The ret causes an exception because i can't see it in a debugger ;) i tried to trigger the exception the way you said i still get the same result...So that's not the problem thanks for your reply though – Reorganization

Did you try your code in a sub-procedure? The TSeh instance would be on stack, just like for the 2nd code. AFAIK the only diff is that the _EXCEPTION_REGISTRATION_RECORD is in the DATA block of the program for the 1st code, whereas it is on stack for the 2nd... – Lyndell