for those paranoid that the kernel clobbers userspace
The x86-64 System V ABI guarantees that the 128 bytes below the stack pointer will always be preserved by the operating system. This 128-byte area is known as the "red zone". Unfortunately, the ABI does not explicitly say what happens to memory beyond the red zone. In particular, interrupt service routines (ISRs) may dump process state onto the userspace stack just beyond the red zone, before switching to a different stack to handle the interrupt (e.g, preemption by the kernel).
My understanding is that some older kernels, like Linux 2.6.x, would write beyond the red zone.
Test the theory that your favorite operating system does not dump anything beyond the red zone, I've written this little tool.
- run
make - run
./redzone - after about 20-30 seconds, you will see the result message.
If you see a happy message, the kernel did not write a non-zero value to the red zone or any part of at least 1-2kb beyond the red zone. Otherwise, we detected a discrepancy.