[fasm2] Selective modification of headers

First, if you include "pe.inc" directly instead of using the "format PE" wrapper, you can set up some of the header fields with "PE.Settings" symbols, like in the fasmg's examples:

Code:

PE.Settings.Machine = IMAGE_FILE_MACHINE_AMD64
PE.Settings.Stub = 'nul'
PE.Settings.Magic = 0x20B
PE.Settings.ImageBase = 0x140000000
PE.Settings.Characteristics = IMAGE_FILE_EXECUTABLE_IMAGE + IMAGE_FILE_LARGE_ADDRESS_AWARE
PE.Settings.DllCharacteristics = IMAGE_DLLCHARACTERISTICS_NX_COMPAT + IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE
PE.Settings.Subsystem = IMAGE_SUBSYSTEM_WINDOWS_GUI
include 'format/pe.inc'

Second, if you need to alter header fields not covered by "PE.Settings", you can still rewrite any of the output values with STORE directive. There is a STORE variant that allow to write at any offset in the output:

Code:

store "ZM":word at :0 ; change the "MZ" signature to "ZM" (a trick to force stub execution)

But there is also a cleaner way. The global "PE" symbol, which is the base for the namespace, is defined as "PE::", allowing to access the headers:

Code:

store -1 at PE : PE.OptionalHeader.MajorLinkerVersion

These labels are correctly sized, that's why the size setting can be omitted. Of course you need to place STORE in the source after the headers have already been generated. I would even recommend to put any patching in a "postpone ?" block.

The STORE method works when you use "format PE" wrapper, too.