Assembly on Lloyd Rochester's Geek Blog

Stack Protection with a Canary

Fri, 11 Jun 2021 00:00:00 +0000

Stack Protection with a Canary

Buffer overflows can be detected by inserting a canary into a function. These canaries are inserted when a function’s stack frame is created. Before a function’s return we check for the canary on the stack frame. If the canary isn’t found then buffer overflow or stack smashing is detected.

Example

Let’s first create a deliberate buffer overflow. Then we can see how it behaves with and without stack protection.

Disassembly of Recursion in C

Fri, 25 Sep 2020 00:00:00 +0000

Disassembly of Recursion in C

Let’s disassemble a recursive function in C to ARM assembly. We can use the textbook example of a recursive factorial function. We’ll play around with optimization levels and touch on Tail Recursion or Tail Calls at the end of the blog post.

C Code

Below is the C code we’ll use to disassemble. This code is a text book function that implements a factorial using recursion.

The Stack of Frames in C with ARM Assembly Example

Fri, 03 Apr 2020 08:34:05 -0600

Function calls in the C programming language make heavy use of the stack, also called the call stack. When functions are called they create the so-called stack of frames. Each function call creates a frame, and these frames are allocated on the stack. This stack frame is used to allocate memory for local variables and intermediate values. The stack frame also contains the previous frame pointer and program counter’s value to execute from once the frame is popped off the stack. We will disassemble C function calls to understand the stack of frames in ARM assembly.

Converting C Conditionals to ARM Assembly

Fri, 27 Mar 2020 00:00:00 +0000

Converting C Conditionals to ARM Assembly

In this post we can explore how to the gcc compiler converts to C to ARM assembly language. We’ll specifically look at conditional logic. We’ll look at examples of if statements, if statements with else if and else, then end with a switch statement. To convert C to ARM assembly we will use the -S flag on GCC. All these examples were done on a Raspberry Pi running Raspbian.