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I’m attempting to edit a library in hex editor, insert mode. The main point is to rename a few entries in it. If I make it in “Otherwrite” mode, everything works fine, but every time I try to add a few symbols to the end of string in “Insert” mode, the library fails to load. Anything I’m missing here?
Yes, you’re missing plenty. A library follows the PE/COFF format, which is quite heavy on pointers throughout the file. (Eg, towards the beginning of the file is a table which points to the locations of each section in the file).
In the case that you are editing resources, there’s the potential to do it without breaking things if you make sure you correct any pointers and sizes for anything pointing to after your edits, but I doubt it’ll be easy. In the case that you are editing the .text section (ie, the code), then I doubt you’ll get it done, since the operands of function calls and jumps are relative locations to their position in code – you would need to update the entire code to account for edits.
One technique to overcome this is a “code cave”, where you replace a piece of the existing code with an explicit JMP instruction to some empty location (You can do this at runtime, where you have the ability to create new memory) – where you define some new code which can be of arbitrary length – then you explicitly JMP back to where you called from (+5 bytes say for the JMP opcode + operand).