rust/src/shims/panic.rs

//! Panic runtime for Miri.
//!
//! The core pieces of the runtime are:
//! - An implementation of `__rust_maybe_catch_panic` that pushes the invoked stack frame with
//!   some extra metadata derived from the panic-catching arguments of `__rust_maybe_catch_panic`.
//! - A hack in `libpanic_unwind` that calls the `miri_start_panic` intrinsic instead of the
//!   target-native panic runtime. (This lives in the rustc repo.)
//! - An implementation of `miri_start_panic` that stores its argument (the panic payload), and then
//!   immediately returns, but on the *unwind* edge (not the normal return edge), thus initiating unwinding.
//! - A hook executed each time a frame is popped, such that if the frame pushed by `__rust_maybe_catch_panic`
//!   gets popped *during unwinding*, we take the panic payload and store it according to the extra
//!   metadata we remembered when pushing said frame.

use rustc::mir;
use rustc::ty::{self, layout::LayoutOf};
use rustc_target::spec::PanicStrategy;
use rustc_span::source_map::Span;

use crate::*;

/// Holds all of the relevant data for a call to
/// `__rust_maybe_catch_panic`.
///
/// If a panic occurs, we update this data with
/// the information from the panic site.
#[derive(Debug)]
pub struct CatchUnwindData<'tcx> {
    /// The dereferenced `data_ptr` argument passed to `__rust_maybe_catch_panic`.
    pub data_place: MPlaceTy<'tcx, Tag>,
    /// The dereferenced `vtable_ptr` argument passed to `__rust_maybe_catch_panic`.
    pub vtable_place: MPlaceTy<'tcx, Tag>,
    /// The `dest` from the original call to `__rust_maybe_catch_panic`.
    pub dest: PlaceTy<'tcx, Tag>,
}

impl<'mir, 'tcx> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
    /// Handles the special "miri_start_panic" intrinsic, which is called
    /// by libpanic_unwind to delegate the actual unwinding process to Miri.
    fn handle_miri_start_panic(
        &mut self,
        args: &[OpTy<'tcx, Tag>],
        unwind: Option<mir::BasicBlock>,
    ) -> InterpResult<'tcx> {
        let this = self.eval_context_mut();

        trace!("miri_start_panic: {:?}", this.frame().span);

        // Get the raw pointer stored in arg[0] (the panic payload).
        let scalar = this.read_immediate(args[0])?;
        assert!(
            this.machine.panic_payload.is_none(),
            "the panic runtime should avoid double-panics"
        );
        this.machine.panic_payload = Some(scalar);

        // Jump to the unwind block to begin unwinding.
        this.unwind_to_block(unwind);
        return Ok(());
    }

    fn handle_catch_panic(
        &mut self,
        args: &[OpTy<'tcx, Tag>],
        dest: PlaceTy<'tcx, Tag>,
        ret: mir::BasicBlock,
    ) -> InterpResult<'tcx> {
        let this = self.eval_context_mut();
        let tcx = &{ this.tcx.tcx };

        // fn __rust_maybe_catch_panic(
        //     f: fn(*mut u8),
        //     data: *mut u8,
        //     data_ptr: *mut usize,
        //     vtable_ptr: *mut usize,
        // ) -> u32

        // Get all the arguments.
        let f = this.read_scalar(args[0])?.not_undef()?;
        let f_arg = this.read_scalar(args[1])?.not_undef()?;
        let data_place = this.deref_operand(args[2])?;
        let vtable_place = this.deref_operand(args[3])?;

        // Now we make a function call, and pass `f_arg` as first and only argument.
        let f_instance = this.memory.get_fn(f)?.as_instance()?;
        trace!("__rust_maybe_catch_panic: {:?}", f_instance);
        let ret_place = MPlaceTy::dangling(this.layout_of(tcx.mk_unit())?, this).into();
        this.call_function(
            f_instance,
            &[f_arg.into()],
            Some(ret_place),
            // Directly return to caller.
            StackPopCleanup::Goto { ret: Some(ret), unwind: None },
        )?;

        // We ourselves will return `0`, eventually (will be overwritten if we catch a panic).
        this.write_null(dest)?;

        // In unwind mode, we tag this frame with some extra data.
        // This lets `handle_stack_pop` (below) know that we should stop unwinding
        // when we pop this frame.
        if this.tcx.tcx.sess.panic_strategy() == PanicStrategy::Unwind {
            this.frame_mut().extra.catch_panic =
                Some(CatchUnwindData { data_place, vtable_place, dest })
        }

        return Ok(());
    }

    fn handle_stack_pop(
        &mut self,
        mut extra: FrameData<'tcx>,
        unwinding: bool,
    ) -> InterpResult<'tcx, StackPopInfo> {
        let this = self.eval_context_mut();

        trace!("handle_stack_pop(extra = {:?}, unwinding = {})", extra, unwinding);

        // We only care about `catch_panic` if we're unwinding - if we're doing a normal
        // return, then we don't need to do anything special.
        let res = if let (true, Some(unwind_data)) = (unwinding, extra.catch_panic.take()) {
            // We've just popped a frame that was pushed by `__rust_maybe_catch_panic`,
            // and we are unwinding, so we should catch that.
            trace!("unwinding: found catch_panic frame during unwinding: {:?}", this.frame().span);

            // `panic_payload` now holds a `*mut (dyn Any + Send)`,
            // provided by the `miri_start_panic` intrinsic.
            // We want to split this into its consituient parts -
            // the data and vtable pointers - and store them according to
            // `unwind_data`, i.e., we store them where `__rust_maybe_catch_panic`
            // was told to put them.
            let payload = this.machine.panic_payload.take().unwrap();
            let payload = this.ref_to_mplace(payload)?;
            let payload_data_place = payload.ptr;
            let payload_vtable_place = payload.meta.unwrap_meta();

            this.write_scalar(payload_data_place, unwind_data.data_place.into())?;
            this.write_scalar(payload_vtable_place, unwind_data.vtable_place.into())?;

            // We set the return value of `__rust_maybe_catch_panic` to 1,
            // since there was a panic.
            let dest = unwind_data.dest;
            this.write_scalar(Scalar::from_int(1, dest.layout.size), dest)?;

            StackPopInfo::StopUnwinding
        } else {
            StackPopInfo::Normal
        };
        this.memory.extra.stacked_borrows.borrow_mut().end_call(extra.call_id);
        Ok(res)
    }

    fn assert_panic(
        &mut self,
        span: Span,
        msg: &AssertMessage<'tcx>,
        unwind: Option<mir::BasicBlock>,
    ) -> InterpResult<'tcx> {
        use rustc::mir::interpret::PanicInfo::*;
        let this = self.eval_context_mut();

        match msg {
            BoundsCheck { ref index, ref len } => {
                // Forward to `panic_bounds_check` lang item.

                // First arg: Caller location.
                let location = this.alloc_caller_location_for_span(span);
                // Second arg: index.
                let index = this.read_scalar(this.eval_operand(index, None)?)?;
                // Third arg: len.
                let len = this.read_scalar(this.eval_operand(len, None)?)?;

                // Call the lang item.
                let panic_bounds_check = this.tcx.lang_items().panic_bounds_check_fn().unwrap();
                let panic_bounds_check = ty::Instance::mono(this.tcx.tcx, panic_bounds_check);
                this.call_function(
                    panic_bounds_check,
                    &[location.ptr.into(), index.into(), len.into()],
                    None,
                    StackPopCleanup::Goto { ret: None, unwind },
                )?;
            }
            _ => {
                // Forward everything else to `panic` lang item.

                // First arg: Message.
                let msg = msg.description();
                let msg = this.allocate_str(msg, MiriMemoryKind::Env.into());

                // Call the lang item.
                let panic = this.tcx.lang_items().panic_fn().unwrap();
                let panic = ty::Instance::mono(this.tcx.tcx, panic);
                this.call_function(
                    panic,
                    &[msg.to_ref()],
                    None,
                    StackPopCleanup::Goto { ret: None, unwind },
                )?;
            }
        }
        Ok(())
    }
}