Write to own executable in Linux C program, error "Text file busy"

For a security class I am supposed to write self-modifying code for a program which finds its own executable on the disk, reads the binary data, and encrypts part of it before writing it back out to the disk. This is supposed to be like a polymorphic virus which changes itself to fool antivirus scanners which detect known signatures.

I have all the pieces pretty much in place:

• I am finding the executable using /proc/self/exe.
• I am using a simple AES implementation to encrypt 16 byte string in some dummy code in the executable.
• I am able to read the binary data in and locate the part I need to encrypt.

My problem is that the only way I have been able to open the executable is in read-only mode "rb". If I try to open the file for writing in mode "wb" or "r+b" I get back the error "Text file busy". Is there anyway for me to write to a process's own executable in C? Can I do this by changing the permissions somehow?

EDIT: What I am trying to accomplish is to have an executable which will encrypt part of itself each time that it runs so that it will have a new checksum after every time it runs.

After reading data from the executable binary, how can I either write back to it or remove it and replace it with a new file with the same filename?

You cannot write to a file that is currently mapped as an executable. However, you can write to a file that has the same path as the current executable, so long as it isn't actually the same file — try unlinking the file you're being executed from and creating a new file in its place, for instance.