Adding section to ELF file

Question

I need to be able to add an arbitrary section to an ELF file. I cannot use GPL code in this program, so BFD is out of the question. I can use libelf/gelf to read sections, but the documentation is fairly sparse for these, and I cannot figure out how to add a section. Does anybody know how to do this? I would rather not write my own ELF code.

Solution 1

There's a few (possibly) related answers in this question about ELF file headers. The accepted answer mentioned using objcopy to add sections to an ELF file, and the BSD bintools claims to have a BSD-licensed implementation of objcopy that might suit your needs.

Solution 2

I know this is an old question but i found a working example that helped me to apply it to my project. (In case anyone stumbles upon this question)

taken from Sourceware Mail Archiv

$ echo 'int main() { puts ("Hello world"); }' | gcc -x c - -c -o hello.o

$ echo "this is my special data" >mydata

$ objcopy --add-section .mydata=mydata \
          --set-section-flags .mydata=noload,readonly hello.o hello2.o

$ gcc hello2.o -o hello

$ ./hello
Hello world

$ objdump -sj .mydata hello

Solution 3

The following links could be useful:

• The tutorial "libelf by Example" has a chapter on creating new ELF objects.
• The elftoolchain project has manual pages describing libelf's API in detail.

Solution 4

have a look at ELFsh, which is now part of the ERESI project http://www.eresi-project.org/ They have a lot of documentation and cool uses of their tools.

Solution 5

Haven't tried it (yet) myself but you might look at the function elf_newscn in libelf

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Comments

[Zibri]

and how do you access it from the program? by re-readin all the exe? naaaa there is a better way.

[Ben]

thats a silly comment. The question was not how to access anything at runtime. The question is, how to add a section to ELF fil. Or i misunderstand the comment.

[Zibri]

your is a silly comment and a silly answer, if he needs to add a section I imagine he wants also to access it. It's just applied logic.

[Zibri]

Access it from the program, not with objdump! I did it in a program of mine... I asked that because if needed I can post the method.

[Ben]

If you read the question, you have to acknowledge that it wasn’t even a question how to access it. “Adding section to ELF file” I don’t even know what we are arguing about.

[Edd Barrett]

Well, he didn't ask how to read the section, but if you want you can either read the relevant section of the binary from disk, or remove the noload flag when making the section and use dl_iterate_phdr(3) to find it in memory.