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