ELF file headers

Question

Hey!

A quick question about elf file headers, I can't seem to find anything useful on how to add/change fields in the elf header. I'd like to be able to change the magic numbers and to add a build date to the header, and probably a few other things.

As I understand it the linker creates the header information, but I don't see anything in the LD script that refers to it (though i'm new to ld scripts).

I'm using gcc and building for ARM.

thanks!

Updates:

• ok maybe my first question should be: is it possible to create/edit the header file at link time?

Answer 1

You might be able to use libmelf, a dead project on freshmeat, but available from LOPI - http://www.ipd.bth.se/ska/lopi.html

Otherwise, you can get the spec and (over)write the header yourself.

Answer 2

I haven't done this in awhile, but can't you just append arbitrary data to an executable. If you always append fixed-size data it would be trivial to recover anything you append. Variable size wouldn't be much harder. Probably easier than messing w/ elf headers and potentially ruining you executables.

Answer 3

I don't know of linker script commands that can do this, but you can do it post-link using the objcopy command. The --add-section option can be used to add a section containing arbitrary data to the ELF file. If the ELF header doesn't contain the fields you want, just make a new section and add them there.

Answer 4

I'm fairly sure that a sufficiently complex ld script can do what you want. However, I have no idea how.

On the other hand, elfsh can easily do all sorts of manipulations to elf objects, so give it a whirl.

Answer 5

In Solaris you can use elfedit but I think you are really asking solutions for Linux. Linux Is Not UniX :P

Answer 6

I didn't finish the book but iirc Linkers and Loaders by John Levine had the gory details that you would need to be able to do this.

Answer 7

This link (teensy elf binary) was someone's answer to another question, but it goes into the intricacies of an ELF header in some detail.

Answer 8

In Linux Console:

$ man ld

$ ld --verbose

HTH