Use host's rng when communication is enabled

Cargo.toml

@@ -35,6 +35,7 @@ byteorder = { version = "1.1", features = ["i128"]}
 cargo_metadata = { version = "0.8", optional = true }
 directories = { version = "2.0", optional = true }
 rustc_version = { version = "0.2.3", optional = true }
+getrandom = "0.1.10"
 env_logger = "0.6"
 log = "0.4"
 shell-escape = "0.1.4"

README.md

@@ -158,8 +158,8 @@ Several `-Z` flags are relevant for Miri:
   will miss bugs in your program.  However, this can also help to make Miri run
   faster.
 * `-Zmiri-enable-communication` enables communication between the host
-  environment and Miri, i.e., all the host environment variables are available
-  during Miri runtime.
+  environment and Miri, i.e., Miri uses the host's random number generator and 
+  all the host environment variables are available during runtime.
 * `-Zmir-opt-level` controls how many MIR optimizations are performed.  Miri
   overrides the default to be `0`; be advised that using any higher level can
   make Miri miss bugs in your program because they got optimized away.

src/helpers.rs

@@ -97,9 +97,17 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             Align::from_bytes(1).unwrap()
         )?.expect("we already checked for size 0");
 
-        let rng = this.memory_mut().extra.rng.get_mut();
         let mut data = vec![0; len];
-        rng.fill_bytes(&mut data);
+
+        if this.machine.communicate {
+            // Fill the buffer using the host's rng.
+            getrandom::getrandom(&mut data)
+                .map_err(|err| err_unsup_format!("getrandom failed: {}", err))?;
+        }
+        else {
+            let rng = this.memory_mut().extra.rng.get_mut();
+            rng.fill_bytes(&mut data);
+        }
 
         let tcx = &{this.tcx.tcx};
         this.memory_mut().get_mut(ptr.alloc_id)?.write_bytes(tcx, ptr, &data)

src/machine.rs

@@ -93,7 +93,8 @@ pub struct Evaluator<'tcx> {
     /// TLS state.
     pub(crate) tls: TlsData<'tcx>,
 
-    /// If enabled, the `env_vars` field is populated with the host env vars during initialization.
+    /// If enabled, the `env_vars` field is populated with the host env vars during initialization
+    /// and random number generation is delegated to the host.
     pub(crate) communicate: bool,
 }