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Co-authored-by: Ralf Jung <[email protected]>

Author: nia-e

src/shims/native_lib/mod.rs

@@ -349,6 +349,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
             if !tracing {
                 // Expose all provenances in this allocation, since the native code can do $whatever.
+                // Can be skipped when tracing; in that case we'll expose just the actually-read parts later.
                 for prov in alloc.provenance().provenances() {
                     this.expose_provenance(prov)?;
                 }
@@ -357,6 +358,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             // Prepare for possible write from native code if mutable.
             if info.mutbl.is_mut() {
                 let (alloc, cx) = this.get_alloc_raw_mut(alloc_id)?;
+                // These writes could initialize everything and wreck havoc with the pointers.
+                // We can skip that when tracing; in that case we'll later do that only for the memory that got actually written.
                 if !tracing {
                     alloc.process_native_write(&cx.tcx, None);
                 }

tests/native-lib/fail/trace_write.rs

@@ -16,9 +16,11 @@ fn partial_init() {
     let mut slice = std::mem::MaybeUninit::<[u8; 3]>::uninit();
     let slice_ptr = slice.as_mut_ptr().cast::<u8>();
     unsafe {
+        // Initialize the first two elements.
         init_n(2, slice_ptr);
         assert!(*slice_ptr == 0);
         assert!(*slice_ptr.offset(1) == 0);
+        // Reading the third is UB!
         let _val = *slice_ptr.offset(2); //~ ERROR: Undefined Behavior: using uninitialized data
     }
 }