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src/routes/blog/rust_stack_jmp.mdx

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+---
+title: A Rusty Stack Jump
+description: Jumping into a new stack with Rust
+date: 2025-02-27
+featuredImage:
+featuredImageDesc:
+tags:
+  - rust
+  - asm
+  - systems
+  - operating systems
+  - async
+---
+
+import { Notes, PostImage } from "~/components/Markdown";
+import { Tree } from "~/components/Tree";
+
+In my quest to learn to build an async runtime in Rust, I have to learn about CPU context switching. In order to switch from one async task to another, our async runtime has to perform a context switch. This means saving the current CPU registers marked as `callee saved` by the System V ABI manual and loading the CPU registers with our new async stack.
+
+In this article, I will show you what I have learned about jumping onto a new stack in a x86_64 CPU.
+
+<Notes>
+I'm learning about async runtimes in Rust based on the amazing book [Asynchronous Programming in Rust: Learn asynchronous programming by building working examples of futures, green threads, and runtimes](https://www.packtpub.com/en-mt/product/asynchronous-programming-in-rust-9781805128137)
+
+It's an amazing book, don't get me wrong, but I feel like the explanation can be hand-wavy sometimes. Thus, I write this to archive my own explanation and potentially help other people who also struggle with the subject.
+
+</Notes>
+
+<Notes>
+  Most async runtimes in Rust do not use stackful coroutines (which are used by
+  Go's `gochannel`, Erlang's `processes`) and instead, use state machines to
+  manage async tasks.
+</Notes>
+
+## Contents
+
+<hr />
+
+## Setting the stage
+
+Why do we need to swap the stack of async tasks in a runtime with stackful coroutines ?
+
+Async tasks, by nature, are paused and resumed. Everytime a task is paused to move into a new task, we would have to save the current context of the task that is running and load the context of the upcoming task.
+
+## Jumping into the new stack
+
+Here is the code in its entirely, I'd recommend you run this on the [Rust Playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024). I have left comments through out the code so you can get the general idea.
+
+Note that you have to manually stop the process.
+
+```rust lang="rust" file="stack_swap.rs"
+use core::arch::asm;
+
+// stack size of 48 bytes so its easy to print the stack before we switch contexts
+const SSIZE: isize = 48;
+
+// a struct that represents our CPU state
+//
+// This struct will stores the stack pointer
+#[derive(Debug, Default)]
+#[repr(C)]
+struct ThreadContext {
+    rsp: u64,
+}
+
+// Returning ! means
+// it will panic OR runs forever
+fn hello() -> ! {
+    println!("I LOVE WAKING UP ON A NEW STACK!");
+    loop {}
+}
+
+// new is a pointer to a ThreadContext
+unsafe fn gt_switch(new: *const ThreadContext) {
+    // inline assembly
+    asm!(
+        "mov rsp, [{0} + 0x00]", // move the content of where the new pointer is pointing to, into the rsp register
+        "ret", // ret pops the return address from our custom stack—in our example, the address of hello.
+        in(reg) new,
+    );
+}
+
+fn main() {
+    // initialize
+    let mut ctx = ThreadContext::default();
+
+    // stack initialize
+    // ie. 0x10
+    let mut stack = vec![0_u8; SSIZE as usize];
+
+    unsafe {
+        // we get the bottom of the stack
+        // remember that the stack grows downward from high memory address to low memory address
+        // i.e 0x40 -> because 0x30 = 0x40 - 0x10 and 0x30 = SSIZE in decimal
+        // NOTE: offset() is applied in units of the size of the type that the pointer points to
+        // in our case, stack is a pointer to u8 (a byte) so offset(SSIZE) == offset(48 bytes) == offset(0x30)
+        let stack_bottom = stack.as_mut_ptr().offset(SSIZE);
+
+        // we align the bottom of the stack to be 16-byte-aligned
+        // this is for performance reasons as some CPU instructions (SSE and SIMD)
+
+        // The technicality: 15 is b1111 so if we do (stack_bottom AND !15) we will zero out the bottom 4 bits
+        //
+        // we also want the bottom of the stack pointer to point to a byte (8bit or u8)
+        let sb_aligned = (stack_bottom as usize & !15) as *mut u8;
+
+        // Here, we write the address of the hello function as 64 bits(8 bytes)
+        // Remember that 16 bytes = 0x10 in hex
+        // So we go DOWN 10 memory addresses, i.e from 0x40 to 0x30
+        // NOTE: 16 bytes down (0x10) even though, the hello function pointer is ONLY 8 bytes
+        // This is because the System V ABI requires the stack pointer to be always be 16-byte aligned
+        std::ptr::write(sb_aligned.offset(-16) as *mut u64, hello as u64);
+
+        // we write the stack pointer into the rsp inside context
+        ctx.rsp = sb_aligned.offset(-16) as u64;
+
+        for i in 0..SSIZE {
+            println!("mem: {}, val: {}",
+            sb_aligned.offset(-i as isize) as usize,
+            *sb_aligned.offset(-i as isize))
+        };
+
+        // we go into the function
+        // we will write our stack pointer to the cpu stack pointer
+        // and `ret` will pop that stack pointer
+        gt_switch(&mut ctx);
+    }
+}
+```