flip-link adds zero-cost stack overflow protection to your embedded programs.

The problem

Bare metal Rust programs may not be memory safe in presence of stack overflows.
For example, this is the case for Rust programs based on v0.6.x of the
cortex-m-rt crate.

The following program, which contains no unsafe code block, can run into
undefined behavior if it reaches a stack overflow condition.

// static variables placed in the .bss / .data sections
static FLAG1: AtomicBool = AtomicU32::new(false); // .bss
static FLAG2: AtomicBool = AtomicU32::new(true);  // .data

fn main() {
    let _x = fib(100);
}

#[inline(never)]
fn fib(n: u32) -> u32 {
    // allocate and initialize 4 kilobytes of stack memory
    let _use_stack = [0xAA; 1024];

    if n < 2 {
        1
    } else {
        fib(n - 1) + fib(n - 2) // recursion
    }
}

#[interrupt]
fn interrupt_handler() {
    // does some operation with `FLAG1` and `FLAG2`
}

The function call stack, also known as the "stack", grows downwards on function
calls and when local variables (e.g. let x) are created (these variables are
also placed on the stack).

If the stack grows too large it collides with the .bss + .data region, which
contains all the program's static variables. The collision results in the
static variables being overwritten with unrelated data. This can result in
the program observing the static variables in an invalid state: for example
an AtomicBool may hold the value 3 -- this is undefined behavior because the
Rust ABI expects this single-byte variable to be either 0 or 1.

The solution

One potential solution is to change the memory layout of the program and place
the stack below the .bss+.data region.

With this flipped memory layout the stack cannot collide with the static
variables. Instead it will collide with the boundary of the physical RAM
memory region. In the ARM Cortex-M architecture, trying to read or write past
the boundaries of the RAM region produces a "hardware exception".
The cortex-m-rt crate provides an API to handle this condition: a HardFault
exception handler can be defined; this "handler" (function) will be executed
when the invalid memory operation is attempted.

flip-link implements this stack overflow solution. Linking your program with
flip-link produces the flipped memory layout, which is memory safe in presence
of stack overflows.

Architecture support

flip-link is known to work with ARM Cortex-M programs that link to version
0.6.x of the cortex-m-rt crate and are linked using the linker shipped with the
Rust toolchain (LLD). At this time, it hasn't been tested with other
architectures or runtime crates.