Auto merge of #69999 - RalfJung:miri-unwind, r=oli-obk

adjust Miri to needs of changed unwinding strategy

As expected, #67502 broke unwinding in Miri. To fix it we have to adjust parts of the engine and the panic runtime, which this PR does. The Miri-side changes are in rust-lang/miri#1227.

Cc @oli-obk @Aaron1011 @Mark-Simulacrum @Amanieu

src/libcore/intrinsics.rs

@@ -1892,10 +1892,12 @@ extern "rust-intrinsic" {
     pub fn ptr_offset_from<T>(ptr: *const T, base: *const T) -> isize;
 
     /// Internal hook used by Miri to implement unwinding.
-    /// Compiles to a NOP during non-Miri codegen.
+    /// ICEs when encountered during non-Miri codegen.
     ///
-    /// Perma-unstable: do not use
-    pub fn miri_start_panic(data: *mut (dyn crate::any::Any + crate::marker::Send)) -> ();
+    /// The `payload` ptr here will be exactly the one `do_catch` gets passed by `try`.
+    ///
+    /// Perma-unstable: do not use.
+    pub fn miri_start_panic(payload: *mut u8) -> !;
 }
 
 // Some functions are defined here because they accidentally got made

src/libpanic_unwind/lib.rs

@@ -30,6 +30,8 @@
 #![feature(raw)]
 #![panic_runtime]
 #![feature(panic_runtime)]
+// `real_imp` is unused with Miri, so silence warnings.
+#![cfg_attr(miri, allow(dead_code))]
 
 use alloc::boxed::Box;
 use core::any::Any;
@@ -38,25 +40,38 @@ use core::panic::BoxMeUp;
 cfg_if::cfg_if! {
     if #[cfg(target_os = "emscripten")] {
         #[path = "emcc.rs"]
-        mod imp;
+        mod real_imp;
     } else if #[cfg(target_arch = "wasm32")] {
         #[path = "dummy.rs"]
-        mod imp;
+        mod real_imp;
     } else if #[cfg(target_os = "hermit")] {
         #[path = "hermit.rs"]
-        mod imp;
+        mod real_imp;
     } else if #[cfg(all(target_env = "msvc", target_arch = "aarch64"))] {
         #[path = "dummy.rs"]
-        mod imp;
+        mod real_imp;
     } else if #[cfg(target_env = "msvc")] {
         #[path = "seh.rs"]
-        mod imp;
+        mod real_imp;
     } else {
         // Rust runtime's startup objects depend on these symbols, so make them public.
         #[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
-        pub use imp::eh_frame_registry::*;
+        pub use real_imp::eh_frame_registry::*;
         #[path = "gcc.rs"]
+        mod real_imp;
+    }
+}
+
+cfg_if::cfg_if! {
+    if #[cfg(miri)] {
+        // Use the Miri runtime.
+        // We still need to also load the normal runtime above, as rustc expects certain lang
+        // items from there to be defined.
+        #[path = "miri.rs"]
         mod imp;
+    } else {
+        // Use the real runtime.
+        use real_imp as imp;
     }
 }
 
@@ -81,12 +96,5 @@ pub unsafe extern "C" fn __rust_start_panic(payload: usize) -> u32 {
     let payload = payload as *mut &mut dyn BoxMeUp;
     let payload = (*payload).take_box();
 
-    // Miri panic support: cfg'd out of normal builds just to be sure.
-    // When going through normal codegen, `miri_start_panic` is a NOP, so the
-    // Miri-enabled sysroot still supports normal unwinding. But when executed in
-    // Miri, this line initiates unwinding.
-    #[cfg(miri)]
-    core::intrinsics::miri_start_panic(payload);
-
     imp::panic(Box::from_raw(payload))
 }

src/libpanic_unwind/miri.rs

@@ -0,0 +1,20 @@
+//! Unwinding panics for Miri.
+use alloc::boxed::Box;
+use core::any::Any;
+
+// The type of the payload that the Miri engine propagates through unwinding for us.
+// Must be pointer-sized.
+type Payload = Box<Box<dyn Any + Send>>;
+
+pub unsafe fn panic(payload: Box<dyn Any + Send>) -> u32 {
+    // The payload we pass to `miri_start_panic` will be exactly the argument we get
+    // in `cleanup` below. So we just box it up once, to get something pointer-sized.
+    let payload_box: Payload = Box::new(payload);
+    core::intrinsics::miri_start_panic(Box::into_raw(payload_box) as *mut u8)
+}
+
+pub unsafe fn cleanup(payload_box: *mut u8) -> Box<dyn Any + Send> {
+    // Recover the underlying `Box`.
+    let payload_box: Payload = Box::from_raw(payload_box as *mut _);
+    *payload_box
+}

src/librustc/mir/interpret/value.rs

@@ -272,11 +272,13 @@ impl<'tcx, Tag> Scalar<Tag> {
 
     #[inline]
     pub fn from_bool(b: bool) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: b as u128, size: 1 }
     }
 
     #[inline]
     pub fn from_char(c: char) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: c as u128, size: 4 }
     }
 
@@ -299,21 +301,25 @@ impl<'tcx, Tag> Scalar<Tag> {
 
     #[inline]
     pub fn from_u8(i: u8) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 1 }
     }
 
     #[inline]
     pub fn from_u16(i: u16) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 2 }
     }
 
     #[inline]
     pub fn from_u32(i: u32) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 4 }
     }
 
     #[inline]
     pub fn from_u64(i: u64) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 8 }
     }
 
@@ -341,6 +347,26 @@ impl<'tcx, Tag> Scalar<Tag> {
             .unwrap_or_else(|| bug!("Signed value {:#x} does not fit in {} bits", i, size.bits()))
     }
 
+    #[inline]
+    pub fn from_i8(i: i8) -> Self {
+        Self::from_int(i, Size::from_bits(8))
+    }
+
+    #[inline]
+    pub fn from_i16(i: i16) -> Self {
+        Self::from_int(i, Size::from_bits(16))
+    }
+
+    #[inline]
+    pub fn from_i32(i: i32) -> Self {
+        Self::from_int(i, Size::from_bits(32))
+    }
+
+    #[inline]
+    pub fn from_i64(i: i64) -> Self {
+        Self::from_int(i, Size::from_bits(64))
+    }
+
     #[inline]
     pub fn from_machine_isize(i: i64, cx: &impl HasDataLayout) -> Self {
         Self::from_int(i, cx.data_layout().pointer_size)

src/librustc_mir/interpret/eval_context.rs

@@ -21,7 +21,7 @@ use rustc_span::source_map::{self, Span, DUMMY_SP};
 
 use super::{
     Immediate, MPlaceTy, Machine, MemPlace, MemPlaceMeta, Memory, OpTy, Operand, Place, PlaceTy,
-    ScalarMaybeUndef, StackPopInfo,
+    ScalarMaybeUndef, StackPopJump,
 };
 
 pub struct InterpCx<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
@@ -623,31 +623,24 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         ::log_settings::settings().indentation -= 1;
         let frame = self.stack.pop().expect("tried to pop a stack frame, but there were none");
-        let stack_pop_info = M::stack_pop(self, frame.extra, unwinding)?;
-        if let (false, StackPopInfo::StopUnwinding) = (unwinding, stack_pop_info) {
-            bug!("Attempted to stop unwinding while there is no unwinding!");
-        }
 
         // Now where do we jump next?
 
-        // Determine if we leave this function normally or via unwinding.
-        let cur_unwinding =
-            if let StackPopInfo::StopUnwinding = stack_pop_info { false } else { unwinding };
-
         // Usually we want to clean up (deallocate locals), but in a few rare cases we don't.
         // In that case, we return early. We also avoid validation in that case,
         // because this is CTFE and the final value will be thoroughly validated anyway.
         let (cleanup, next_block) = match frame.return_to_block {
             StackPopCleanup::Goto { ret, unwind } => {
-                (true, Some(if cur_unwinding { unwind } else { ret }))
+                (true, Some(if unwinding { unwind } else { ret }))
             }
             StackPopCleanup::None { cleanup, .. } => (cleanup, None),
         };
 
         if !cleanup {
             assert!(self.stack.is_empty(), "only the topmost frame should ever be leaked");
             assert!(next_block.is_none(), "tried to skip cleanup when we have a next block!");
-            // Leak the locals, skip validation.
+            assert!(!unwinding, "tried to skip cleanup during unwinding");
+            // Leak the locals, skip validation, skip machine hook.
             return Ok(());
         }
 
@@ -656,13 +649,13 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             self.deallocate_local(local.value)?;
         }
 
-        trace!(
-            "StackPopCleanup: {:?} StackPopInfo: {:?} cur_unwinding = {:?}",
-            frame.return_to_block,
-            stack_pop_info,
-            cur_unwinding
-        );
-        if cur_unwinding {
+        if M::stack_pop(self, frame.extra, unwinding)? == StackPopJump::NoJump {
+            // The hook already did everything.
+            // We want to skip the `info!` below, hence early return.
+            return Ok(());
+        }
+        // Normal return.
+        if unwinding {
             // Follow the unwind edge.
             let unwind = next_block.expect("Encountered StackPopCleanup::None when unwinding!");
             self.unwind_to_block(unwind);
@@ -697,7 +690,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 "CONTINUING({}) {} (unwinding = {})",
                 self.cur_frame(),
                 self.frame().instance,
-                cur_unwinding
+                unwinding
             );
         }
 

src/librustc_mir/interpret/machine.rs

@@ -17,16 +17,16 @@ use super::{
 /// Data returned by Machine::stack_pop,
 /// to provide further control over the popping of the stack frame
 #[derive(Eq, PartialEq, Debug, Copy, Clone)]
-pub enum StackPopInfo {
+pub enum StackPopJump {
     /// Indicates that no special handling should be
     /// done - we'll either return normally or unwind
     /// based on the terminator for the function
     /// we're leaving.
     Normal,
 
-    /// Indicates that we should stop unwinding,
-    /// as we've reached a catch frame
-    StopUnwinding,
+    /// Indicates that we should *not* jump to the return/unwind address, as the callback already
+    /// took care of everything.
+    NoJump,
 }
 
 /// Whether this kind of memory is allowed to leak
@@ -276,9 +276,9 @@ pub trait Machine<'mir, 'tcx>: Sized {
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _extra: Self::FrameExtra,
         _unwinding: bool,
-    ) -> InterpResult<'tcx, StackPopInfo> {
+    ) -> InterpResult<'tcx, StackPopJump> {
         // By default, we do not support unwinding from panics
-        Ok(StackPopInfo::Normal)
+        Ok(StackPopJump::Normal)
     }
 
     fn int_to_ptr(

src/librustc_mir/interpret/mod.rs

@@ -24,7 +24,7 @@ pub use self::place::{MPlaceTy, MemPlace, MemPlaceMeta, Place, PlaceTy};
 
 pub use self::memory::{AllocCheck, FnVal, Memory, MemoryKind};
 
-pub use self::machine::{AllocMap, Machine, MayLeak, StackPopInfo};
+pub use self::machine::{AllocMap, Machine, MayLeak, StackPopJump};
 
 pub use self::operand::{ImmTy, Immediate, OpTy, Operand, ScalarMaybeUndef};
 

src/libstd/panicking.rs

@@ -251,21 +251,20 @@ pub unsafe fn r#try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<dyn Any + Send>>
     //
     // We go through a transition where:
     //
-    // * First, we set the data to be the closure that we're going to call.
+    // * First, we set the data field `f` to be the argumentless closure that we're going to call.
     // * When we make the function call, the `do_call` function below, we take
-    //   ownership of the function pointer. At this point the `Data` union is
+    //   ownership of the function pointer. At this point the `data` union is
     //   entirely uninitialized.
     // * If the closure successfully returns, we write the return value into the
-    //   data's return slot. Note that `ptr::write` is used as it's overwriting
-    //   uninitialized data.
+    //   data's return slot (field `r`).
+    // * If the closure panics (`do_catch` below), we write the panic payload into field `p`.
     // * Finally, when we come back out of the `try` intrinsic we're
     //   in one of two states:
     //
     //      1. The closure didn't panic, in which case the return value was
-    //         filled in. We move it out of `data` and return it.
-    //      2. The closure panicked, in which case the return value wasn't
-    //         filled in. In this case the entire `data` union is invalid, so
-    //         there is no need to drop anything.
+    //         filled in. We move it out of `data.r` and return it.
+    //      2. The closure panicked, in which case the panic payload was
+    //         filled in. We move it out of `data.p` and return it.
     //
     // Once we stack all that together we should have the "most efficient'
     // method of calling a catch panic whilst juggling ownership.