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compiler-constructionlinkerbinaryelf

How to extend a ELF binary

I am writing a small instrumentation tool. I must insert the instrumentation routine within the binary file. A good approach should be to insert those routines in a separate code segment and a separate data segment, could you explain how to accomplish this? Furthemore how can I modify the size of the code segment in the original file?

Best,

Solution

  • I must insert the instrumentation routine within the binary file. A good approach should be to insert those routines in a separate code segment and a separate data segment

    What is a binary file? There is a big difference between doing this for a relocable (ET_REL) object file, vs. doing this for a fully linked executable (ET_EXEC)or shared library (ET_DYN).

    could you explain how to accomplish this?

    For an ET_REL, it should be fairly straight-forward: you read the file header, which points to section headers, which tells you where .data and .text sections are. You then write a new file, extending the sections you want, copying everything else, and adjusting the section headers to reflect new section offsets and sizes.

    For an ET_DYN or ET_EXEC, the problem is very likely too hard: you'll need to adjust relocation tables, hash tables, program headers; keeping all the structures self-consistent and properly aligned.