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