Ask any systems programmer what the NX (No-eXecute) bit does and you'll get a confident, one-sentence answer: it marks memory pages as non-executable so an attacker can't jump into a buffer they've filled with shellcode. Case closed, DEP/W^X, move on. This post — hosted on purplesyringa's guest blog, a domain that consistently publishes some of the sharpest low-level writing on the internet — argues that this framing sells the NX bit dramatically short.

The title is a promise: there's a second life for this humble page-table bit that has nothing to do with stopping stack smashing. Based on the venue and framing, the likely territory includes:

• JIT compilation guards — using NX transitions (mprotect flips between W and X) as a way to detect and instrument code generation, not just to satisfy W^X hygiene.
• Lazy loading and demand paging tricks — trapping on execute-faults to materialize code on first use, similar to how the kernel already uses NX-adjacent faults for copy-on-write and demand-zero pages.
• Profiling and coverage — marking pages NX to fault the first time each function executes, giving you a zero-instrumentation code-coverage or cold-code detector.
• Garbage collection barriers — some managed runtimes use page protection bits as a cheap way to intercept accesses without emitting inline checks; NX gives an execute-side equivalent.
• Debugging and reverse engineering — forcing execution faults on specific regions to build hardware-assisted tracers without touching the code itself.

What makes this worth the click is that the NX bit sits at a beautiful abstraction seam. It's one bit in a PTE, backed by silicon since the mid-2000s, and its intended use has become so canonical that most engineers never think past it. But every page-protection primitive the MMU exposes is really a general-purpose fault-on-access mechanism, and clever runtimes have been quietly repurposing them for decades — mprotect for GC write barriers, PROT_NONE for guard pages and stack overflow detection, and now, apparently, NX for something more than defense.

The kind of post that makes you look at a familiar mechanism sideways and see a whole new set of uses.