stackwalk.rs

//! Stack-walking of a Wasm stack.
//!
//! A stack walk requires a first and last frame pointer (FP), and it
//! only works on code that has been compiled with frame pointers
//! enabled (`preserve_frame_pointers` Cranelift option enabled). The
//! stack walk follows the singly-linked list of saved frame pointer
//! and return address pairs on the stack that is naturally built by
//! function prologues.
//!
//! This crate makes use of the fact that Wasmtime surrounds Wasm
//! frames by trampolines both at entry and exit, and is "up the
//! stack" from the point doing the unwinding: in other words, host
//! code invokes Wasm code via an entry trampoline, that code may call
//! other Wasm code, and ultimately it calls back to host code via an
//! exit trampoline. That exit trampoline is able to provide the
//! "start FP" (FP at exit trampoline) and "end FP" (FP at entry
//! trampoline) and this stack-walker can visit all Wasm frames
//! active on the stack between those two.
//!
//! This module provides a visitor interface to frames, but is
//! agnostic to the desired use-case or consumer of the frames, and to
//! the overall runtime structure.

use core::ops::ControlFlow;

/// Implementation necessary to unwind the stack, used by `Backtrace`.
///
/// # Safety
///
/// This trait is `unsafe` because the return values of each function are
/// required to be semantically correct when connected to the `visit_frames`
/// function below. Incorrect and/or arbitrary values in this trait will cause
/// unwinding to segfault or otherwise result in UB.
pub unsafe trait Unwind {
    /// Returns the offset, from the current frame pointer, of where to get to
    /// the previous frame pointer on the stack.
    fn next_older_fp_from_fp_offset(&self) -> usize;

    /// Returns the offset, from the current frame pointer, of the
    /// stack pointer of the next older frame.
    fn next_older_sp_from_fp_offset(&self) -> usize;

    /// Load the return address of a frame given the frame pointer for that
    /// frame.
    ///
    /// # Safety
    ///
    /// This function is expected to read raw memory from `fp` and thus is not
    /// safe to operate on any value of `fp` passed in, instead it must be a
    /// trusted Cranelift-defined frame pointer.
    unsafe fn get_next_older_pc_from_fp(&self, fp: usize) -> usize;

    /// Debug assertion that the frame pointer is aligned.
    fn assert_fp_is_aligned(&self, fp: usize);
}

/// A stack frame within a Wasm stack trace.
#[derive(Debug)]
pub struct Frame {
    /// The program counter in this frame. Because every frame in the
    /// stack-walk is paused at a call (as we are in host code called
    /// by Wasm code below these frames), the PC is at the return
    /// address, i.e., points to the instruction after the call
    /// instruction.
    pc: usize,
    /// The frame pointer value corresponding to this frame.
    fp: usize,
}

impl Frame {
    /// Get this frame's program counter.
    pub fn pc(&self) -> usize {
        self.pc
    }

    /// Get this frame's frame pointer.
    pub fn fp(&self) -> usize {
        self.fp
    }

    /// Read out a machine-word-sized value at the given offset from
    /// FP in this frame.
    ///
    /// # Safety
    ///
    /// Requires that this frame is a valid, active frame. A `Frame`
    /// provided by `visit_frames()` will be valid for the duration of
    /// the invoked closure.
    ///
    /// Requires that `offset` falls within the size of this
    /// frame. This ordinarily requires knowledge passed from the
    /// compiler that produced the running function, e.g., Cranelift.
    pub unsafe fn read_slot_from_fp(&self, offset: isize) -> usize {
        // SAFETY: we required that this is a valid frame, and that
        // `offset` is a valid offset within that frame.
        unsafe { *(self.fp.wrapping_add_signed(offset) as *mut usize) }
    }
}

/// Walk through a contiguous sequence of Wasm frames starting with
/// the frame at the given PC and FP and ending at
/// `trampoline_fp`. This FP should correspond to that of a trampoline
/// that was used to enter the Wasm code.
///
/// We require that the initial PC, FP, and `trampoline_fp` values are
/// non-null (non-zero).
///
/// # Safety
///
/// This function is not safe as `unwind`, `pc`, `fp`, and `trampoline_fp` must
/// all be "correct" in that if they're wrong or mistakenly have the wrong value
/// then this method may segfault. These values must point to valid Wasmtime
/// compiled code which respects the frame pointers that Wasmtime currently
/// requires.
pub unsafe fn visit_frames<R>(
    unwind: &dyn Unwind,
    mut pc: usize,
    mut fp: usize,
    trampoline_fp: usize,
    mut f: impl FnMut(Frame) -> ControlFlow<R>,
) -> ControlFlow<R> {
    log::trace!("=== Tracing through contiguous sequence of Wasm frames ===");
    log::trace!("trampoline_fp = 0x{trampoline_fp:016x}");
    log::trace!("   initial pc = 0x{pc:016x}");
    log::trace!("   initial fp = 0x{fp:016x}");

    // Safety requirements documented above.
    assert_ne!(pc, 0);
    assert_ne!(fp, 0);
    assert_ne!(trampoline_fp, 0);

    // This loop will walk the linked list of frame pointers starting
    // at `fp` and going up until `trampoline_fp`. We know that both
    // `fp` and `trampoline_fp` are "trusted values" aka generated and
    // maintained by Wasmtime. This means that it should be safe to
    // walk the linked list of pointers and inspect Wasm frames.
    //
    // Note, though, that any frames outside of this range are not
    // guaranteed to have valid frame pointers. For example native code
    // might be using the frame pointer as a general purpose register. Thus
    // we need to be careful to only walk frame pointers in this one
    // contiguous linked list.
    //
    // To know when to stop iteration all architectures' stacks currently
    // look something like this:
    //
    //     | ...               |
    //     | Native Frames     |
    //     | ...               |
    //     |-------------------|
    //     | ...               | <-- Trampoline FP            |
    //     | Trampoline Frame  |                              |
    //     | ...               | <-- Trampoline SP            |
    //     |-------------------|                            Stack
    //     | Return Address    |                            Grows
    //     | Previous FP       | <-- Wasm FP                Down
    //     | ...               |                              |
    //     | Cranelift Frames  |                              |
    //     | ...               |                              V
    //
    // The trampoline records its own frame pointer (`trampoline_fp`),
    // which is guaranteed to be above all Wasm code. To check when

    // to check when the next frame pointer is equal to
    // `trampoline_fp`. Once that's hit then we know that the entire
    // linked list has been traversed.
    //
    // Note that it might be possible that this loop doesn't execute
    // at all.  For example if the entry trampoline called Wasm code
    // which `return_call`'d an exit trampoline, then `fp ==
    // trampoline_fp` on the entry of this function, meaning the loop
    // won't actually execute anything.
    while fp != trampoline_fp {
        // At the start of each iteration of the loop, we know that
        // `fp` is a frame pointer from Wasm code. Therefore, we know
        // it is not being used as an extra general-purpose register,
        // and it is safe dereference to get the PC and the next older
        // frame pointer.
        //
        // The stack also grows down, and therefore any frame pointer
        // we are dealing with should be less than the frame pointer
        // on entry to Wasm code. Finally also assert that it's
        // aligned correctly as an additional sanity check.
        assert!(trampoline_fp > fp, "{trampoline_fp:#x} > {fp:#x}");
        unwind.assert_fp_is_aligned(fp);

        log::trace!("--- Tracing through one Wasm frame ---");
        log::trace!("pc = {:p}", pc as *const ());
        log::trace!("fp = {:p}", fp as *const ());

        f(Frame { pc, fp })?;

        // SAFETY: this unsafe traversal of the linked list on the stack is
        // reflected in the contract of this function where `pc`, `fp`,
        // `trampoline_fp`, and `unwind` must all be trusted/correct values.
        unsafe {
            pc = unwind.get_next_older_pc_from_fp(fp);

            // We rely on this offset being zero for all supported
            // architectures in
            // `crates/cranelift/src/component/compiler.s`r when we set
            // the Wasm exit FP. If this ever changes, we will need to
            // update that code as well!
            assert_eq!(unwind.next_older_fp_from_fp_offset(), 0);

            // Get the next older frame pointer from the current Wasm
            // frame pointer.
            let next_older_fp = *(fp as *mut usize).add(unwind.next_older_fp_from_fp_offset());

            // Because the stack always grows down, the older FP must be greater
            // than the current FP.
            assert!(next_older_fp > fp, "{next_older_fp:#x} > {fp:#x}");
            fp = next_older_fp;
        }
    }

    log::trace!("=== Done tracing contiguous sequence of Wasm frames ===");
    ControlFlow::Continue(())
}