unc_vm_engine/trap/

frame_info.rs

//! This module is used for having backtraces in the Wasm runtime.
//! Once the Compiler has compiled the ModuleInfo, and we have a set of
//! compiled functions (addresses and function index) and a module,
//! then we can use this to set a backtrace for that module.
//!
//! # Example
//! ```ignore
//! use unc_vm_vm::{FRAME_INFO};
//! use unc_vm_types::ModuleInfo;
//!
//! let module: ModuleInfo = ...;
//! FRAME_INFO.register(module, compiled_functions);
//! ```
use std::collections::BTreeMap;
use std::sync::{Arc, RwLock};
use unc_vm_compiler::{CompiledFunctionFrameInfo, SourceLoc, TrapInformation};
use unc_vm_types::entity::{EntityRef, PrimaryMap};
use unc_vm_types::{LocalFunctionIndex, ModuleInfo};

lazy_static::lazy_static! {
    /// This is a global cache of backtrace frame information for all active
    ///
    /// This global cache is used during `Trap` creation to symbolicate frames.
    /// This is populated on module compilation, and it is cleared out whenever
    /// all references to a module are dropped.
    pub static ref FRAME_INFO: RwLock<GlobalFrameInfo> = Default::default();
}

#[derive(Default)]
pub struct GlobalFrameInfo {
    /// An internal map that keeps track of backtrace frame information for
    /// each module.
    ///
    /// This map is morally a map of ranges to a map of information for that
    /// module. Each module is expected to reside in a disjoint section of
    /// contiguous memory. No modules can overlap.
    ///
    /// The key of this map is the highest address in the module and the value
    /// is the module's information, which also contains the start address.
    ranges: BTreeMap<usize, ModuleInfoFrameInfo>,
}

/// An RAII structure used to unregister a module's frame information when the
/// module is destroyed.
pub struct GlobalFrameInfoRegistration {
    /// The key that will be removed from the global `ranges` map when this is
    /// dropped.
    key: usize,
}

#[derive(Debug)]
struct ModuleInfoFrameInfo {
    start: usize,
    functions: BTreeMap<usize, FunctionInfo>,
    module: Arc<ModuleInfo>,
    frame_infos: PrimaryMap<LocalFunctionIndex, CompiledFunctionFrameInfo>,
}

impl ModuleInfoFrameInfo {
    fn function_debug_info(&self, local_index: LocalFunctionIndex) -> &CompiledFunctionFrameInfo {
        &self.frame_infos.get(local_index).unwrap()
    }

    /// Gets a function given a pc
    fn function_info(&self, pc: usize) -> Option<&FunctionInfo> {
        let (end, func) = self.functions.range(pc..).next()?;
        if func.start <= pc && pc <= *end {
            return Some(func);
        } else {
            None
        }
    }
}

#[derive(Debug)]
struct FunctionInfo {
    start: usize,
    local_index: LocalFunctionIndex,
}

impl GlobalFrameInfo {
    /// Fetches frame information about a program counter in a backtrace.
    ///
    /// Returns an object if this `pc` is known to some previously registered
    /// module, or returns `None` if no information can be found.
    pub fn lookup_frame_info(&self, pc: usize) -> Option<FrameInfo> {
        let module = self.module_info(pc)?;
        let func = module.function_info(pc)?;

        // Use our relative position from the start of the function to find the
        // machine instruction that corresponds to `pc`, which then allows us to
        // map that to a wasm original source location.
        let rel_pos = pc - func.start;
        let instr_map = &module.function_debug_info(func.local_index).address_map;
        let pos = match instr_map.instructions.binary_search_by_key(&rel_pos, |map| map.code_offset)
        {
            // Exact hit!
            Ok(pos) => Some(pos),

            // This *would* be at the first slot in the array, so no
            // instructions cover `pc`.
            Err(0) => None,

            // This would be at the `nth` slot, so check `n-1` to see if we're
            // part of that instruction. This happens due to the minus one when
            // this function is called form trap symbolication, where we don't
            // always get called with a `pc` that's an exact instruction
            // boundary.
            Err(n) => {
                let instr = &instr_map.instructions[n - 1];
                if instr.code_offset <= rel_pos && rel_pos < instr.code_offset + instr.code_len {
                    Some(n - 1)
                } else {
                    None
                }
            }
        };

        let instr = match pos {
            Some(pos) => instr_map.instructions[pos].srcloc,
            // Some compilers don't emit yet the full trap information for each of
            // the instructions (such as LLVM).
            // In case no specific instruction is found, we return by default the
            // start offset of the function.
            None => instr_map.start_srcloc,
        };
        let func_index = module.module.func_index(func.local_index);
        Some(FrameInfo {
            module_name: module.module.name(),
            func_index: func_index.index() as u32,
            function_name: module.module.function_names.get(&func_index).cloned(),
            instr,
            func_start: instr_map.start_srcloc,
        })
    }

    /// Fetches trap information about a program counter in a backtrace.
    pub fn lookup_trap_info(&self, pc: usize) -> Option<&TrapInformation> {
        let module = self.module_info(pc)?;
        let func = module.function_info(pc)?;
        let traps = &module.function_debug_info(func.local_index).traps;
        let idx = traps
            .binary_search_by_key(&((pc - func.start) as u32), |info| info.code_offset)
            .ok()?;
        Some(&traps[idx])
    }

    /// Gets a module given a pc
    fn module_info(&self, pc: usize) -> Option<&ModuleInfoFrameInfo> {
        let (end, module_info) = self.ranges.range(pc..).next()?;
        if module_info.start <= pc && pc <= *end {
            Some(module_info)
        } else {
            None
        }
    }
}

impl Drop for GlobalFrameInfoRegistration {
    fn drop(&mut self) {
        if let Ok(mut info) = FRAME_INFO.write() {
            info.ranges.remove(&self.key);
        }
    }
}

/// Description of a frame in a backtrace for a [`RuntimeError::trace`](crate::RuntimeError::trace).
///
/// Whenever a WebAssembly trap occurs an instance of [`RuntimeError`]
/// is created. Each [`RuntimeError`] has a backtrace of the
/// WebAssembly frames that led to the trap, and each frame is
/// described by this structure.
///
/// [`RuntimeError`]: crate::RuntimeError
#[derive(Debug, Clone)]
pub struct FrameInfo {
    module_name: String,
    func_index: u32,
    function_name: Option<String>,
    func_start: SourceLoc,
    instr: SourceLoc,
}

impl FrameInfo {
    /// Returns the WebAssembly function index for this frame.
    ///
    /// This function index is the index in the function index space of the
    /// WebAssembly module that this frame comes from.
    pub fn func_index(&self) -> u32 {
        self.func_index
    }

    /// Returns the identifer of the module that this frame is for.
    ///
    /// ModuleInfo identifiers are present in the `name` section of a WebAssembly
    /// binary, but this may not return the exact item in the `name` section.
    /// ModuleInfo names can be overwritten at construction time or perhaps inferred
    /// from file names. The primary purpose of this function is to assist in
    /// debugging and therefore may be tweaked over time.
    ///
    /// This function returns `None` when no name can be found or inferred.
    pub fn module_name(&self) -> &str {
        &self.module_name
    }

    /// Returns a descriptive name of the function for this frame, if one is
    /// available.
    ///
    /// The name of this function may come from the `name` section of the
    /// WebAssembly binary, or unc_vm may try to infer a better name for it if
    /// not available, for example the name of the export if it's exported.
    ///
    /// This return value is primarily used for debugging and human-readable
    /// purposes for things like traps. Note that the exact return value may be
    /// tweaked over time here and isn't guaranteed to be something in
    /// particular about a wasm module due to its primary purpose of assisting
    /// in debugging.
    ///
    /// This function returns `None` when no name could be inferred.
    pub fn function_name(&self) -> Option<&str> {
        self.function_name.as_deref()
    }

    /// Returns the offset within the original wasm module this frame's program
    /// counter was at.
    ///
    /// The offset here is the offset from the beginning of the original wasm
    /// module to the instruction that this frame points to.
    pub fn module_offset(&self) -> usize {
        self.instr.bits() as usize
    }

    /// Returns the offset from the original wasm module's function to this
    /// frame's program counter.
    ///
    /// The offset here is the offset from the beginning of the defining
    /// function of this frame (within the wasm module) to the instruction this
    /// frame points to.
    pub fn func_offset(&self) -> usize {
        (self.instr.bits() - self.func_start.bits()) as usize
    }
}