Approach for "lea reg64, [reg64 + FunctionName]"

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alorent

Joined: 05 Dec 2005
Posts: 227

alorent 08 Feb 2026, 08:00

Hi,

In my x86 code I'm using something like:

Code:

lea eax, [ebp + FunctionName]

That is the old "delta offset way" for code that is injected.

Now, I create a MS64 COFF and I put:

Code:

lea rax, [rbp + FunctionName]

but when I link that FASM OBJ with the MSCV linker I get:

Code:

MyLibrary.lib(test_coff64.obj) : error LNK2017: 'ADDR32' relocation to '.text' invalid without /LARGEADDRESSAWARE:NO

The problem should go away with /LARGEADDRESSAWARE:NO but my code goes in a DLL, so the linker
complains that DLLs cannot use the /LARGEADDRESSAWARE:NO switch Sad

Is there a way use that "lea" instruction to get the "rva" of FunctionName?

I have tried things like:

Code:

lea rax, [rbp + rva FunctionName]

; ERROR:
;     lea rax, [rbp + rva FunctionName]
; error: invalid use of symbol.


extrn __ImageBase:qword
lea rax, [rbp +  FunctionName - __ImageBase]

; ERROR:
; lea rax, [rbp +  FunctionName - __ImageBase]
; error: invalid use of symbol.

Is there a way to use that addressing approach? I know that I could use a set of instructions but I need it to be on a SINGLE lea instruction (in the form of "lea reg, [reg + xxx]).

I know that I could remove the "rbp" delta, but I need it as my parser checks for those "delta" instructions (like "[rbp + Name]) to fix them.

Thanks!

08 Feb 2026, 08:00

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20874
Location: In your JS exploiting you and your system

revolution 08 Feb 2026, 08:27

How is FunctionName defined? I had no trouble with this:

Code:

format pe64

FunctionName:
lea     rax, [rbp + rva FunctionName]

Code:

flat assembler  version 1.73.31  (16384 kilobytes memory)
1 passes, 1024 bytes.

08 Feb 2026, 08:27

alorent

Joined: 05 Dec 2005
Posts: 227

alorent 08 Feb 2026, 09:55

Thanks revolution.

Your example builds correctly.

When I use "MS64 COFF" it fails:

Code:

format MS64 COFF

FunctionName:
lea     rax, [rbp + rva FunctionName]

Not sure if it's a bug in FASM or a limitation for COFF OBJ?

Thanks!

08 Feb 2026, 09:55

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20874
Location: In your JS exploiting you and your system

revolution 08 Feb 2026, 10:18

For linkable formats use $$

Code:

format ms64 coff
org 0x123450
FunctionName:
lea     rax, [rbp + FunctionName - $$]

08 Feb 2026, 10:18

alorent

Joined: 05 Dec 2005
Posts: 227

alorent 08 Feb 2026, 16:49

Thanks revolution! That worked!! I will do some tests Smile

08 Feb 2026, 16:49

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8504
Location: Kraków, Poland

Tomasz Grysztar 09 Feb 2026, 01:15

alorent wrote:

When I use "MS64 COFF" it fails:

Code:

format MS64 COFF

FunctionName:
lea     rax, [rbp + rva FunctionName]

Not sure if it's a bug in FASM or a limitation for COFF OBJ?

It is a limitation of fasm's implementation. The same source assembles successfully with fasm2.

As for the $$, it evaluates to the base address of the section. More precisely: the section fragment which you define in your object file (the linker may combine multiple such sections into a single one).

09 Feb 2026, 01:15

alorent

Joined: 05 Dec 2005
Posts: 227

alorent 13 Feb 2026, 11:36

Thanks a lot Tomasz!

You are right, with the "$$" approach it's from the base of the OBJ and not the RVA in the finally linked module.

13 Feb 2026, 11:36

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20874
Location: In your JS exploiting you and your system

revolution 13 Feb 2026, 12:34

Any label can be used as the base.

Code:

format ms64 coff

my_base:
lea     rbp,[my_base]

lea     rax, [rbp + FunctionName1 - my_base]
lea     rbx, [rbp + FunctionName2 - my_base]
lea     rcx, [rbp + FunctionName3 - my_base]

FunctionName1:
FunctionName2:
FunctionName3:

13 Feb 2026, 12:34

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