In this article I am going to cover such a trivial (as it may seem) subject as DLL injection. For some reason, most of the tutorials on the web only give us a brief coverage of the topic, mostly limited to invocation of LoadLibraryA/W Windows API function in the address space of another process. While this is not bad at all, it gives us the least flexible solution. Meaning that all the logic MUST be hardcoded in the DLL we want to inject. On the other hand, we may incorporate all the configuration management (loading config files, parsing thereof, etc) into our DLL. This is better, but still fills it with code which is only going to run once.

This is the last part of the Hijack Linux System Calls series. By now, we have created a simple loadable kernel module which registers a miscellaneous character device. This means, that we have everything we need in order to patch the system call table. Almost everything, to be honest. We still have to fill the our_ioctl function and add a couple of declarations to our source file. By the end of this article we will be able to intercept any system call in our system should there be a need for that.

There are hundreds, if not thousands, of posts regarding this problem. Most of them are outdated, as they refer to older kernels (those, that still exported sys_call_table), others are about adding custom system call and recompiling the kernel. There are a few covering modern kernels, but those are brief and, mostly, only give you a general idea of how it works. I decided to make an in-depth description of the procedure and provide a working example.