Changing program behavior at runtime

A method implemented at a computer system for overriding an original behavior of a function at runtime without changing source code of the function, the method comprising:

• identifying an override behavior for overriding the original behavior of the function, the override behavior including at least one of (1) overriding a return value of the function with a predetermined return value, or (2) overriding the function with a second function;
• encoding replacement code based on the override behavior;
• identifying a size of the replacement code;
• identifying a first memory address of the function where code of the function is stored;
• copying a block of code having the size of the replacement code from the first memory address to a second memory address;
• copying the replacement code to the first memory address;
• and
• after the replacement code is executed, copying the block of code stored at the second memory address back to the first memory address, restoring the code of the function, wherein:
• when the override behavior includes overriding the return value of the function with the predetermined return value, encoding the replacement code comprises:
• identifying a particular register that stores a return value of the function;
• and
• encoding the replacement code configured to move the predetermined return value into the particular register, and
• when the override behavior includes overriding the function with the second function, encoding the replacement code includes:
• identifying a third memory address of the second function where code of the second function is stored;
• and
• encoding the third memory address into the replacement code.

A computer program product comprising one or more hardware storage devices having stored thereon computer-executable instructions that are structured such that, when the computer-executable instructions are executed by one or more processors of a computer system, the computer system is configured to perform:

• identify an override behavior for overriding an original behavior of a function, the override behavior including at least one of (1) overriding a return value of the function with a predetermined return value, or (2) overriding the function with a second function;
• encode replacement code based on the override behavior;
• identify a size of the replacement code;
• identify a first memory address of the function where code of the function is stored;
• copy a block of code having the size of the replacement code from the first memory address to a second memory address;
• copy the replacement code to the first memory address;
• and
• after the replacement code is executed, copy the block of code stored at the second memory address back to the first memory address, restoring the code of the function, wherein:
• when the override behavior includes overriding the return value of the function with the predetermined return value, encoding the replacement code comprises:
• identifying a particular register that stores a return value of the function;
• and
• encoding the replacement code configured to move the predetermined return value into the particular register, and
• when the override behavior includes overriding the function with the second function, encoding the replacement code includes:
• identifying a third memory address of the second function where code of the second function is stored;
• and
• encoding the third memory address into the replacement code.