This is just a simple PAM module and test code for it. There really isn't much to it, but it does make a good example of how to get started with a PAM module.
To build, either use the build scripts or use these commands:
Build the PAM module
gcc -fPIC -fno-stack-protector -c src/mypam.c
sudo ld -x --shared -o /lib/security/mypam.so mypam.o
The first command builds the object file in the current directory and the second links it with PAM. Since it's a shared library, PAM can use it on the fly without having to restart.
Build Test
g++ -o pam_test src/test.c -lpam -lpam_misc
OR
gcc -o pam_test src/test.c -lpam -lpam_misc
The test program is valid C, so it could be compiled using gcc or g++. I like g++ better because I'll probably want to extend it and I like C++ better.
The build scripts will take care of putting your module where it needs to be, /lib/security, so the next thing to do is edit config files.
The config files are located in /etc/pam.d/ and the one I edited was /etc/pam.d/common-auth.
The test application tests auth and account functionality (although account isn't very interesting). At the top of the pam file (or anywhere), put these lines:
auth sufficient mypam.so
account sufficient mypam.so
I think the account part should technically go in /etc/pam.d/common-account, but I put mine in the same place so I'd remember to take them out later.
To run the test program, just do: pam_test backdoor and you should get some messages saying that you're authenticated! Maybe this is how Sam Flynn 'hacked' his father's computer in TRON Legacy =D.
I found these resources especially helpful:
These guides give brief overviews about PAM and how to write modules. This is useful if you already have a little knowledge.
Good example for simple authentication. I adapted this one in my simple PAM module.
2-factor authentication & writing PAM modules
Gives an example program that uses PAM. I adapted this for testing my PAM module.