Auto merge of #884 - Aaron1011:fix/linux-getrandom, r=RalfJung

Allowing passing a null pointer to getrandom() when length is 0

The Linux kernel will handle a null pointer passed to 'getrandom'
without error, as long as the length is also 0. The `getrandom` crate
relies on this behavior: ab44edf3c7/src/linux_android.rs (L26)

Since it works fine on the actual kernel (and should continue to, due to
the kernel's backwards-compatibility guarantees), Miri should support it
as well.

src/helpers.rs

@@ -81,12 +81,21 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         ptr: Scalar<Tag>,
         len: usize,
     ) -> InterpResult<'tcx>  {
+        // Some programs pass in a null pointer and a length of 0
+        // to their platform's random-generation function (e.g. getrandom())
+        // on Linux. For compatibility with these programs, we don't perform
+        // any additional checks - it's okay if the pointer is invalid,
+        // since we wouldn't actually be writing to it.
+        if len == 0 {
+            return Ok(());
+        }
         let this = self.eval_context_mut();
 
-        let ptr = match this.memory().check_ptr_access(ptr, Size::from_bytes(len as u64), Align::from_bytes(1).unwrap())? {
-            Some(ptr) => ptr,
-            None => return Ok(()), // zero-sized access
-        };
+        let ptr = this.memory().check_ptr_access(
+            ptr,
+            Size::from_bytes(len as u64),
+            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];