src/shims/panic.rs

   1 //! Panic runtime for Miri.
   2 //!
   3 //! The core pieces of the runtime are:
   4 //! - An implementation of `__rust_maybe_catch_panic` that pushes the invoked stack frame with
   5 //!   some extra metadata derived from the panic-catching arguments of `__rust_maybe_catch_panic`.
   6 //! - A hack in `libpanic_unwind` that calls the `miri_start_panic` intrinsic instead of the
   7 //!   target-native panic runtime. (This lives in the rustc repo.)
   8 //! - An implementation of `miri_start_panic` that stores its argument (the panic payload), and then
   9 //!   immediately returns, but on the *unwind* edge (not the normal return edge), thus initiating unwinding.
  10 //! - A hook executed each time a frame is popped, such that if the frame pushed by `__rust_maybe_catch_panic`
  11 //!   gets popped *during unwinding*, we take the panic payload and store it according to the extra
  12 //!   metadata we remembered when pushing said frame.
  13
  14 use log::trace;
  15
  16 use rustc_ast::Mutability;
  17 use rustc_middle::{mir, ty};
  18 use rustc_span::Symbol;
  19 use rustc_target::spec::abi::Abi;
  20 use rustc_target::spec::PanicStrategy;
  21
  22 use crate::*;
  23 use helpers::check_arg_count;
  24
  25 /// Holds all of the relevant data for when unwinding hits a `try` frame.
  26 #[derive(Debug)]
  27 pub struct CatchUnwindData<'tcx> {
  28     /// The `catch_fn` callback to call in case of a panic.
  29     catch_fn: Scalar<Tag>,
  30     /// The `data` argument for that callback.
  31     data: Scalar<Tag>,
  32     /// The return place from the original call to `try`.
  33     dest: PlaceTy<'tcx, Tag>,
  34     /// The return block from the original call to `try`.
  35     ret: mir::BasicBlock,
  36 }
  37
  38 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
  39 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
  40     /// Handles the special `miri_start_panic` intrinsic, which is called
  41     /// by libpanic_unwind to delegate the actual unwinding process to Miri.
  42     fn handle_miri_start_panic(
  43         &mut self,
  44         abi: Abi,
  45         link_name: Symbol,
  46         args: &[OpTy<'tcx, Tag>],
  47         unwind: StackPopUnwind,
  48     ) -> InterpResult<'tcx> {
  49         let this = self.eval_context_mut();
  50
  51         trace!("miri_start_panic: {:?}", this.frame().instance);
  52
  53         // Get the raw pointer stored in arg[0] (the panic payload).
  54         let &[ref payload] = this.check_shim(abi, Abi::Rust, link_name, args)?;
  55         let payload = this.read_scalar(payload)?.check_init()?;
  56         let thread = this.active_thread_mut();
  57         assert!(thread.panic_payload.is_none(), "the panic runtime should avoid double-panics");
  58         thread.panic_payload = Some(payload);
  59
  60         // Jump to the unwind block to begin unwinding.
  61         this.unwind_to_block(unwind)?;
  62         return Ok(());
  63     }
  64
  65     /// Handles the `try` intrinsic, the underlying implementation of `std::panicking::try`.
  66     fn handle_try(
  67         &mut self,
  68         args: &[OpTy<'tcx, Tag>],
  69         dest: &PlaceTy<'tcx, Tag>,
  70         ret: mir::BasicBlock,
  71     ) -> InterpResult<'tcx> {
  72         let this = self.eval_context_mut();
  73
  74         // Signature:
  75         //   fn r#try(try_fn: fn(*mut u8), data: *mut u8, catch_fn: fn(*mut u8, *mut u8)) -> i32
  76         // Calls `try_fn` with `data` as argument. If that executes normally, returns 0.
  77         // If that unwinds, calls `catch_fn` with the first argument being `data` and
  78         // then second argument being a target-dependent `payload` (i.e. it is up to us to define
  79         // what that is), and returns 1.
  80         // The `payload` is passed (by libstd) to `__rust_panic_cleanup`, which is then expected to
  81         // return a `Box<dyn Any + Send + 'static>`.
  82         // In Miri, `miri_start_panic` is passed exactly that type, so we make the `payload` simply
  83         // a pointer to `Box<dyn Any + Send + 'static>`.
  84
  85         // Get all the arguments.
  86         let &[ref try_fn, ref data, ref catch_fn] = check_arg_count(args)?;
  87         let try_fn = this.read_scalar(try_fn)?.check_init()?;
  88         let data = this.read_scalar(data)?.check_init()?;
  89         let catch_fn = this.read_scalar(catch_fn)?.check_init()?;
  90
  91         // Now we make a function call, and pass `data` as first and only argument.
  92         let f_instance = this.memory.get_fn(try_fn)?.as_instance()?;
  93         trace!("try_fn: {:?}", f_instance);
  94         let ret_place = MPlaceTy::dangling(this.machine.layouts.unit, this).into();
  95         this.call_function(
  96             f_instance,
  97             Abi::Rust,
  98             &[data.into()],
  99             Some(&ret_place),
 100             // Directly return to caller.
 101             StackPopCleanup::Goto { ret: Some(ret), unwind: StackPopUnwind::Skip },
 102         )?;
 103
 104         // We ourselves will return `0`, eventually (will be overwritten if we catch a panic).
 105         this.write_null(dest)?;
 106
 107         // In unwind mode, we tag this frame with the extra data needed to catch unwinding.
 108         // This lets `handle_stack_pop` (below) know that we should stop unwinding
 109         // when we pop this frame.
 110         if this.tcx.sess.panic_strategy() == PanicStrategy::Unwind {
 111             this.frame_mut().extra.catch_unwind =
 112                 Some(CatchUnwindData { catch_fn, data, dest: *dest, ret });
 113         }
 114
 115         return Ok(());
 116     }
 117
 118     fn handle_stack_pop(
 119         &mut self,
 120         mut extra: FrameData<'tcx>,
 121         unwinding: bool,
 122     ) -> InterpResult<'tcx, StackPopJump> {
 123         let this = self.eval_context_mut();
 124
 125         trace!("handle_stack_pop(extra = {:?}, unwinding = {})", extra, unwinding);
 126         if let Some(stacked_borrows) = &this.memory.extra.stacked_borrows {
 127             stacked_borrows.borrow_mut().end_call(extra.call_id);
 128         }
 129
 130         // We only care about `catch_panic` if we're unwinding - if we're doing a normal
 131         // return, then we don't need to do anything special.
 132         if let (true, Some(catch_unwind)) = (unwinding, extra.catch_unwind.take()) {
 133             // We've just popped a frame that was pushed by `try`,
 134             // and we are unwinding, so we should catch that.
 135             trace!(
 136                 "unwinding: found catch_panic frame during unwinding: {:?}",
 137                 this.frame().instance
 138             );
 139
 140             // We set the return value of `try` to 1, since there was a panic.
 141             this.write_scalar(Scalar::from_i32(1), &catch_unwind.dest)?;
 142
 143             // The Thread's `panic_payload` holds what was passed to `miri_start_panic`.
 144             // This is exactly the second argument we need to pass to `catch_fn`.
 145             let payload = this.active_thread_mut().panic_payload.take().unwrap();
 146
 147             // Push the `catch_fn` stackframe.
 148             let f_instance = this.memory.get_fn(catch_unwind.catch_fn)?.as_instance()?;
 149             trace!("catch_fn: {:?}", f_instance);
 150             let ret_place = MPlaceTy::dangling(this.machine.layouts.unit, this).into();
 151             this.call_function(
 152                 f_instance,
 153                 Abi::Rust,
 154                 &[catch_unwind.data.into(), payload.into()],
 155                 Some(&ret_place),
 156                 // Directly return to caller of `try`.
 157                 StackPopCleanup::Goto { ret: Some(catch_unwind.ret), unwind: StackPopUnwind::Skip },
 158             )?;
 159
 160             // We pushed a new stack frame, the engine should not do any jumping now!
 161             Ok(StackPopJump::NoJump)
 162         } else {
 163             Ok(StackPopJump::Normal)
 164         }
 165     }
 166
 167     /// Start a panic in the interpreter with the given message as payload.
 168     fn start_panic(&mut self, msg: &str, unwind: StackPopUnwind) -> InterpResult<'tcx> {
 169         let this = self.eval_context_mut();
 170
 171         // First arg: message.
 172         let msg = this.allocate_str(msg, MiriMemoryKind::Machine.into(), Mutability::Not);
 173
 174         // Call the lang item.
 175         let panic = this.tcx.lang_items().panic_fn().unwrap();
 176         let panic = ty::Instance::mono(this.tcx.tcx, panic);
 177         this.call_function(
 178             panic,
 179             Abi::Rust,
 180             &[msg.to_ref()],
 181             None,
 182             StackPopCleanup::Goto { ret: None, unwind },
 183         )
 184     }
 185
 186     fn assert_panic(
 187         &mut self,
 188         msg: &mir::AssertMessage<'tcx>,
 189         unwind: Option<mir::BasicBlock>,
 190     ) -> InterpResult<'tcx> {
 191         use rustc_middle::mir::AssertKind::*;
 192         let this = self.eval_context_mut();
 193
 194         match msg {
 195             BoundsCheck { index, len } => {
 196                 // Forward to `panic_bounds_check` lang item.
 197
 198                 // First arg: index.
 199                 let index = this.read_scalar(&this.eval_operand(index, None)?)?;
 200                 // Second arg: len.
 201                 let len = this.read_scalar(&this.eval_operand(len, None)?)?;
 202
 203                 // Call the lang item.
 204                 let panic_bounds_check = this.tcx.lang_items().panic_bounds_check_fn().unwrap();
 205                 let panic_bounds_check = ty::Instance::mono(this.tcx.tcx, panic_bounds_check);
 206                 this.call_function(
 207                     panic_bounds_check,
 208                     Abi::Rust,
 209                     &[index.into(), len.into()],
 210                     None,
 211                     StackPopCleanup::Goto {
 212                         ret: None,
 213                         unwind: match unwind {
 214                             Some(cleanup) => StackPopUnwind::Cleanup(cleanup),
 215                             None => StackPopUnwind::Skip,
 216                         },
 217                     },
 218                 )?;
 219             }
 220             _ => {
 221                 // Forward everything else to `panic` lang item.
 222                 this.start_panic(
 223                     msg.description(),
 224                     match unwind {
 225                         Some(cleanup) => StackPopUnwind::Cleanup(cleanup),
 226                         None => StackPopUnwind::Skip,
 227                     },
 228                 )?;
 229             }
 230         }
 231         Ok(())
 232     }
 233 }