trust

First, compile compiler-clean.c normally (cc compiler-clean.c -o compiler-clean)
./compiler-clean input_file out_file takes in the input_file, "compiles it", and gives the binary out_file (actually just passes it to gcc)
This is now our main compiler and is innocent.
Now, compile auth.c (./compiler-clean auth.c -o auth)
./auth only authorizes if you run as root
Compile compiler-trojan.c using the compiler (./compiler-clean compiler-trojan.c compiler-trojan)
Now if we use it to compile auth.c (./compiler-trojan auth.c auth), it will give us access even if we are not root.

Now,

Compile compiler-clean.c using the malicious compiler (compiler-trojan compiler-clean.c compiler-clean)
We can throw away compiler-trojan.c and compiler-trojan and pretend like nothing ever happened.
Now, if we use our new compiler-clean (which is actually not-so-clean) to compile auth.c (compiler-clean auth.c -o auth), we see that it gives us access even if we are not root.

Moral: We can't trust our compiler.

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