Auto merge of #914 - christianpoveda:use-host-rng, r=oli-obk

Use host's rng when communication is enabled

This uses the host's randomness when the communication enabled flag is used. I am not sure about the error handling. I was thinking about fallbacking to `rand` if `getrandom` fails and also print something so the user knows miri is not using the host's rng because it failed. Let me know what you think.

Related issue: #800.

r? @RalfJung @oli-obk

Author: bors

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"

src/helpers.rs

@@ -97,9 +97,18 @@ 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| {
+                InterpError::Unsupported(UnsupportedOpInfo::Unsupported(err.to_string()))
+            })?;
+        }
+        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)