C++ Testing using spies and fakes for isolation and simulation

Last update: Dec 9, 2022

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Overview

ELFspy is a library for linux for writing tests using fakes and spies in C++. For very large call graphs, the testing of the higher nodes can be difficult due to the sum of dependencies of lower nodes e.g. databases, network connections etc. Adding fakes and spies to the callees of a function allows it to be tested in isolation without the inherited dependencies.

ELFspy works by allowing code to externally modify an existing implementation/library of a program. This is achieved by changing function addresses in the global offset tables of the running program, so that these are redirected to a different function created by ELFspy. This allows new functions to be interjected between the caller and the callee to spy on the arguments and return value. In addition the original function can be replaced with a simulation (fake) of the original. The code under test needs no changes and is unaware of the additional calls that are being made in between a caller and a callee.

Call graph before change

External code:

auto add_spy = SPY(&add);

Call graph after change(the code does not change, but the runtime execution does)

This technique relies on code being compiled as position independent code and uses the Executable and Linking Format(ELF) information found in the executable and its shared libraries to find the global offset table entries and modify them.

To see how ELFspy can be used start by reading this example

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Comments

Faking read, write and open

Hi, I've made some examples that fake unix functions successfully (i.e. socket, close or pipe). However, when I try to fake the read() and write() functions included in <unistd.h>, the fake functions are never executed, but just the original ones.

Regarding open(), the problem arises at compilation-time because of the signature of the function:

int open(const char *, int flags, ...);

It seems that the variable arguments in functions are not allowed in this versión of elfspy. Definitely it would be nice to support this kind of signatures.

Best regards and thanks for this fantastic library, Jesus

opened by jmcruz-uma 1
Fakes with virtual inheritance do not work

example7.cpp contains code to test for this. Currently the virtual methods from the virtual base class cannot be found and also it looks like the wrong virtual methods are found for the remaining methods of a class with a virtual base class.

opened by mollismerx 0