Miri: replace canonical_alloc_id mechanism by extern_static_alloc_id which is called only when a pointer is 'imported' into the machine

Author: RalfJung

src/librustc_middle/mir/interpret/error.rs

@@ -502,8 +502,6 @@ impl fmt::Display for UndefinedBehaviorInfo<'_> {
 pub enum UnsupportedOpInfo {
     /// Free-form case. Only for errors that are never caught!
     Unsupported(String),
-    /// Accessing an unsupported foreign static.
-    ReadForeignStatic(DefId),
     /// Could not find MIR for a function.
     NoMirFor(DefId),
     /// Encountered a pointer where we needed raw bytes.
@@ -515,15 +513,17 @@ pub enum UnsupportedOpInfo {
     ReadBytesAsPointer,
     /// Accessing thread local statics
     ThreadLocalStatic(DefId),
+    /// Accessing an unsupported extern static.
+    ReadExternStatic(DefId),
 }
 
 impl fmt::Display for UnsupportedOpInfo {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         use UnsupportedOpInfo::*;
         match self {
             Unsupported(ref msg) => write!(f, "{}", msg),
-            ReadForeignStatic(did) => {
-                write!(f, "cannot read from foreign (extern) static ({:?})", did)
+            ReadExternStatic(did) => {
+                write!(f, "cannot read from extern static ({:?})", did)
             }
             NoMirFor(did) => write!(f, "no MIR body is available for {:?}", did),
             ReadPointerAsBytes => write!(f, "unable to turn pointer into raw bytes",),

src/librustc_mir/interpret/eval_context.rs

@@ -323,14 +323,17 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     }
 
     /// Call this to turn untagged "global" pointers (obtained via `tcx`) into
-    /// the *canonical* machine pointer to the allocation.  Must never be used
-    /// for any other pointers!
+    /// the machine pointer to the allocation.  Must never be used
+    /// for any other pointers, nor for TLS statics.
     ///
-    /// This represents a *direct* access to that memory, as opposed to access
-    /// through a pointer that was created by the program.
+    /// Using the resulting pointer represents a *direct* access to that memory
+    /// (e.g. by directly using a `static`),
+    /// as opposed to access through a pointer that was created by the program.
+    ///
+    /// This function can fail only if `ptr` points to an `extern static`.
     #[inline(always)]
-    pub fn tag_global_base_pointer(&self, ptr: Pointer) -> Pointer<M::PointerTag> {
-        self.memory.tag_global_base_pointer(ptr)
+    pub fn global_base_pointer(&self, ptr: Pointer) -> InterpResult<'tcx, Pointer<M::PointerTag>> {
+        self.memory.global_base_pointer(ptr)
     }
 
     #[inline(always)]

src/librustc_mir/interpret/machine.rs

@@ -238,45 +238,30 @@ pub trait Machine<'mir, 'tcx>: Sized {
         Ok(())
     }
 
-    /// Called for *every* memory access to determine the real ID of the given allocation.
-    /// This provides a way for the machine to "redirect" certain allocations as it sees fit.
-    ///
-    /// This is used by Miri to redirect extern statics to real allocations.
-    ///
-    /// This function must be idempotent.
-    #[inline]
-    fn canonical_alloc_id(_mem: &Memory<'mir, 'tcx, Self>, id: AllocId) -> AllocId {
-        id
+    /// Return the `AllocId` for the given thread-local static in the current thread.
+    fn thread_local_static_alloc_id(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        def_id: DefId,
+    ) -> InterpResult<'tcx, AllocId> {
+        throw_unsup!(ThreadLocalStatic(def_id))
     }
 
-    /// Called when converting a `ty::Const` to an operand (in
-    /// `eval_const_to_op`).
-    ///
-    /// Miri uses this callback for creating per thread allocations for thread
-    /// locals. In Rust, one way of creating a thread local is by marking a
-    /// static with `#[thread_local]`. On supported platforms this gets
-    /// translated to a LLVM thread local for which LLVM automatically ensures
-    /// that each thread gets its own copy. Since LLVM automatically handles
-    /// thread locals, the Rust compiler just treats thread local statics as
-    /// regular statics even though accessing a thread local static should be an
-    /// effectful computation that depends on the current thread. The long term
-    /// plan is to change MIR to make accesses to thread locals explicit
-    /// (https://github.com/rust-lang/rust/issues/70685). While the issue 70685
-    /// is not fixed, our current workaround in Miri is to use this function to
-    /// make per-thread copies of thread locals. Please note that we cannot make
-    /// these copies in `canonical_alloc_id` because that is too late: for
-    /// example, if one created a pointer in thread `t1` to a thread local and
-    /// sent it to another thread `t2`, resolving the access in
-    /// `canonical_alloc_id` would result in pointer pointing to `t2`'s thread
-    /// local and not `t1` as it should.
-    #[inline]
-    fn adjust_global_const(
-        _ecx: &InterpCx<'mir, 'tcx, Self>,
-        val: mir::interpret::ConstValue<'tcx>,
-    ) -> InterpResult<'tcx, mir::interpret::ConstValue<'tcx>> {
-        Ok(val)
+    /// Return the `AllocId` backing the given `extern static`.
+    fn extern_static_alloc_id(
+        mem: &Memory<'mir, 'tcx, Self>,
+        def_id: DefId,
+    ) -> InterpResult<'tcx, AllocId> {
+        // Use the `AllocId` associated with the `DefId`. Any actual *access* will fail.
+        Ok(mem.tcx.create_static_alloc(def_id))
     }
 
+    /// Return the "base" tag for the given *global* allocation: the one that is used for direct
+    /// accesses to this static/const/fn allocation. If `id` is not a global allocation,
+    /// this will return an unusable tag (i.e., accesses will be UB)!
+    ///
+    /// Called on the id returned by `thread_local_static_alloc_id` and `extern_static_alloc_id`, if needed.
+    fn tag_global_base_pointer(memory_extra: &Self::MemoryExtra, id: AllocId) -> Self::PointerTag;
+
     /// Called to initialize the "extra" state of an allocation and make the pointers
     /// it contains (in relocations) tagged.  The way we construct allocations is
     /// to always first construct it without extra and then add the extra.
@@ -309,13 +294,6 @@ pub trait Machine<'mir, 'tcx>: Sized {
         Ok(())
     }
 
-    /// Return the "base" tag for the given *global* allocation: the one that is used for direct
-    /// accesses to this static/const/fn allocation. If `id` is not a global allocation,
-    /// this will return an unusable tag (i.e., accesses will be UB)!
-    ///
-    /// Expects `id` to be already canonical, if needed.
-    fn tag_global_base_pointer(memory_extra: &Self::MemoryExtra, id: AllocId) -> Self::PointerTag;
-
     /// Executes a retagging operation
     #[inline]
     fn retag(
@@ -375,13 +353,6 @@ pub trait Machine<'mir, 'tcx>: Sized {
         _mem: &Memory<'mir, 'tcx, Self>,
         _ptr: Pointer<Self::PointerTag>,
     ) -> InterpResult<'tcx, u64>;
-
-    fn thread_local_alloc_id(
-        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        did: DefId,
-    ) -> InterpResult<'tcx, AllocId> {
-        throw_unsup!(ThreadLocalStatic(did))
-    }
 }
 
 // A lot of the flexibility above is just needed for `Miri`, but all "compile-time" machines

src/librustc_mir/interpret/memory.rs

@@ -137,15 +137,33 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
     }
 
     /// Call this to turn untagged "global" pointers (obtained via `tcx`) into
-    /// the *canonical* machine pointer to the allocation.  Must never be used
-    /// for any other pointers!
+    /// the machine pointer to the allocation.  Must never be used
+    /// for any other pointers, nor for TLS statics.
     ///
-    /// This represents a *direct* access to that memory, as opposed to access
-    /// through a pointer that was created by the program.
+    /// Using the resulting pointer represents a *direct* access to that memory
+    /// (e.g. by directly using a `static`),
+    /// as opposed to access through a pointer that was created by the program.
+    ///
+    /// This function can fail only if `ptr` points to an `extern static`.
     #[inline]
-    pub fn tag_global_base_pointer(&self, ptr: Pointer) -> Pointer<M::PointerTag> {
-        let id = M::canonical_alloc_id(self, ptr.alloc_id);
-        ptr.with_tag(M::tag_global_base_pointer(&self.extra, id))
+    pub fn global_base_pointer(&self, mut ptr: Pointer) -> InterpResult<'tcx, Pointer<M::PointerTag>> {
+        // We need to handle `extern static`.
+        let ptr = match self.tcx.get_global_alloc(ptr.alloc_id) {
+            Some(GlobalAlloc::Static(def_id)) if self.tcx.is_thread_local_static(def_id) => {
+                bug!("global memory cannot point to thread-local static")
+            }
+            Some(GlobalAlloc::Static(def_id)) if self.tcx.is_foreign_item(def_id) => {
+                ptr.alloc_id = M::extern_static_alloc_id(self, def_id)?;
+                ptr
+            }
+            _ => {
+                // No need to change the `AllocId`.
+                ptr
+            }
+        };
+        // And we need to get the tag.
+        let tag = M::tag_global_base_pointer(&self.extra, ptr.alloc_id);
+        Ok(ptr.with_tag(tag))
     }
 
     pub fn create_fn_alloc(
@@ -162,7 +180,9 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
                 id
             }
         };
-        self.tag_global_base_pointer(Pointer::from(id))
+        // Functions are global allocations, so make sure we get the right base pointer.
+        // We know this is not an `extern static` so this cannmot fail.
+        self.global_base_pointer(Pointer::from(id)).unwrap()
     }
 
     pub fn allocate(
@@ -195,6 +215,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             M::GLOBAL_KIND.map(MemoryKind::Machine),
             "dynamically allocating global memory"
         );
+        // This is a new allocation, not a new global one, so no `global_base_ptr`.
         let (alloc, tag) = M::init_allocation_extra(&self.extra, id, Cow::Owned(alloc), Some(kind));
         self.alloc_map.insert(id, (kind, alloc.into_owned()));
         Pointer::from(id).with_tag(tag)
@@ -437,6 +458,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             Some(GlobalAlloc::Function(..)) => throw_ub!(DerefFunctionPointer(id)),
             None => throw_ub!(PointerUseAfterFree(id)),
             Some(GlobalAlloc::Static(def_id)) => {
+                assert!(tcx.is_static(def_id));
                 assert!(!tcx.is_thread_local_static(def_id));
                 // Notice that every static has two `AllocId` that will resolve to the same
                 // thing here: one maps to `GlobalAlloc::Static`, this is the "lazy" ID,
@@ -448,24 +470,15 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
                 // The `GlobalAlloc::Memory` branch here is still reachable though; when a static
                 // contains a reference to memory that was created during its evaluation (i.e., not
                 // to another static), those inner references only exist in "resolved" form.
-                //
-                // Assumes `id` is already canonical.
                 if tcx.is_foreign_item(def_id) {
-                    trace!("get_global_alloc: foreign item {:?}", def_id);
-                    throw_unsup!(ReadForeignStatic(def_id))
+                    throw_unsup!(ReadExternStatic(def_id));
                 }
                 trace!("get_global_alloc: Need to compute {:?}", def_id);
                 let instance = Instance::mono(tcx, def_id);
                 let gid = GlobalId { instance, promoted: None };
                 // Use the raw query here to break validation cycles. Later uses of the static
                 // will call the full query anyway.
-                let raw_const =
-                    tcx.const_eval_raw(ty::ParamEnv::reveal_all().and(gid)).map_err(|err| {
-                        // no need to report anything, the const_eval call takes care of that
-                        // for statics
-                        assert!(tcx.is_static(def_id));
-                        err
-                    })?;
+                let raw_const = tcx.const_eval_raw(ty::ParamEnv::reveal_all().and(gid))?;
                 // Make sure we use the ID of the resolved memory, not the lazy one!
                 let id = raw_const.alloc_id;
                 let allocation = tcx.global_alloc(id).unwrap_memory();
@@ -482,6 +495,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             alloc,
             M::GLOBAL_KIND.map(MemoryKind::Machine),
         );
+        // Sanity check that this is the same pointer we would have gotten via `global_base_pointer`.
         debug_assert_eq!(tag, M::tag_global_base_pointer(memory_extra, id));
         Ok(alloc)
     }
@@ -492,7 +506,6 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         &self,
         id: AllocId,
     ) -> InterpResult<'tcx, &Allocation<M::PointerTag, M::AllocExtra>> {
-        let id = M::canonical_alloc_id(self, id);
         // The error type of the inner closure here is somewhat funny.  We have two
         // ways of "erroring": An actual error, or because we got a reference from
         // `get_global_alloc` that we can actually use directly without inserting anything anywhere.
@@ -529,7 +542,6 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         &mut self,
         id: AllocId,
     ) -> InterpResult<'tcx, &mut Allocation<M::PointerTag, M::AllocExtra>> {
-        let id = M::canonical_alloc_id(self, id);
         let tcx = self.tcx;
         let memory_extra = &self.extra;
         let a = self.alloc_map.get_mut_or(id, || {
@@ -568,7 +580,6 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         id: AllocId,
         liveness: AllocCheck,
     ) -> InterpResult<'static, (Size, Align)> {
-        let id = M::canonical_alloc_id(self, id);
         // # Regular allocations
         // Don't use `self.get_raw` here as that will
         // a) cause cycles in case `id` refers to a static
@@ -621,7 +632,6 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         }
     }
 
-    /// Assumes `id` is already canonical.
     fn get_fn_alloc(&self, id: AllocId) -> Option<FnVal<'tcx, M::ExtraFnVal>> {
         trace!("reading fn ptr: {}", id);
         if let Some(extra) = self.extra_fn_ptr_map.get(&id) {
@@ -642,8 +652,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         if ptr.offset.bytes() != 0 {
             throw_ub!(InvalidFunctionPointer(ptr.erase_tag()))
         }
-        let id = M::canonical_alloc_id(self, ptr.alloc_id);
-        self.get_fn_alloc(id).ok_or_else(|| err_ub!(InvalidFunctionPointer(ptr.erase_tag())).into())
+        self.get_fn_alloc(ptr.alloc_id).ok_or_else(|| err_ub!(InvalidFunctionPointer(ptr.erase_tag())).into())
     }
 
     pub fn mark_immutable(&mut self, id: AllocId) -> InterpResult<'tcx> {

src/librustc_mir/interpret/operand.rs

@@ -541,9 +541,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         val: &ty::Const<'tcx>,
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
-        let tag_scalar = |scalar| match scalar {
-            Scalar::Ptr(ptr) => Scalar::Ptr(self.tag_global_base_pointer(ptr)),
-            Scalar::Raw { data, size } => Scalar::Raw { data, size },
+        let tag_scalar = |scalar| -> InterpResult<'tcx, _> {
+            Ok(match scalar {
+                Scalar::Ptr(ptr) => Scalar::Ptr(self.global_base_pointer(ptr)?),
+                Scalar::Raw { data, size } => Scalar::Raw { data, size },
+            })
         };
         // Early-return cases.
         let val_val = match val.val {
@@ -570,10 +572,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
             ty::ConstKind::Value(val_val) => val_val,
         };
-        // This call allows the machine to create fresh allocation ids for
-        // thread-local statics (see the `adjust_global_const` function
-        // documentation).
-        let val_val = M::adjust_global_const(self, val_val)?;
         // Other cases need layout.
         let layout =
             from_known_layout(self.tcx, self.param_env, layout, || self.layout_of(val.ty))?;
@@ -582,10 +580,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let id = self.tcx.create_memory_alloc(alloc);
                 // We rely on mutability being set correctly in that allocation to prevent writes
                 // where none should happen.
-                let ptr = self.tag_global_base_pointer(Pointer::new(id, offset));
+                let ptr = self.global_base_pointer(Pointer::new(id, offset))?;
                 Operand::Indirect(MemPlace::from_ptr(ptr, layout.align.abi))
             }
-            ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x).into()),
+            ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x)?.into()),
             ConstValue::Slice { data, start, end } => {
                 // We rely on mutability being set correctly in `data` to prevent writes
                 // where none should happen.
@@ -594,7 +592,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     Size::from_bytes(start), // offset: `start`
                 );
                 Operand::Immediate(Immediate::new_slice(
-                    self.tag_global_base_pointer(ptr).into(),
+                    self.global_base_pointer(ptr)?.into(),
                     u64::try_from(end.checked_sub(start).unwrap()).unwrap(), // len: `end - start`
                     self,
                 ))

src/librustc_mir/interpret/place.rs

@@ -1126,7 +1126,7 @@ where
     ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
         // This must be an allocation in `tcx`
         let _ = self.tcx.global_alloc(raw.alloc_id);
-        let ptr = self.tag_global_base_pointer(Pointer::from(raw.alloc_id));
+        let ptr = self.global_base_pointer(Pointer::from(raw.alloc_id))?;
         let layout = self.layout_of(raw.ty)?;
         Ok(MPlaceTy::from_aligned_ptr(ptr, layout))
     }

src/librustc_mir/interpret/step.rs

@@ -141,8 +141,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         use rustc_middle::mir::Rvalue::*;
         match *rvalue {
             ThreadLocalRef(did) => {
-                let id = M::thread_local_alloc_id(self, did)?;
-                let val = Scalar::Ptr(self.tag_global_base_pointer(id.into()));
+                let id = M::thread_local_static_alloc_id(self, did)?;
+                let val = self.global_base_pointer(id.into())?;
                 self.write_scalar(val, dest)?;
             }