Avoid writing more bytes than necessary

Author: pvdrz

src/shims/env.rs

@@ -120,25 +120,29 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, Scalar<Tag>> {
         let this = self.eval_context_mut();
 
-        if this.machine.communicate {
-            let tcx = &{this.tcx.tcx};
+        if !this.machine.communicate {
+            throw_unsup_format!("Function not available when isolation is enabled")
+        }
 
-            let buf = this.force_ptr(this.read_scalar(buf_op)?.not_undef()?)?;
-            let size = this.read_scalar(size_op)?.to_usize(&*this.tcx)?;
-            // If we cannot get the current directory, we return null
-            if let Ok(cwd) = env::current_dir() {
-                // It is not clear what happens with non-utf8 paths here
-                let mut bytes = cwd.display().to_string().into_bytes();
-                // If the buffer is smaller than the path, we return null
-                if bytes.len() as u64 <= size {
-                    // We need `size` bytes exactly
-                    bytes.resize(size as usize, 0);
-                    this.memory_mut().get_mut(buf.alloc_id)?.write_bytes(tcx, buf, &bytes)?;
-                    return Ok(Scalar::Ptr(buf))
-                }
+        let tcx = &{this.tcx.tcx};
+
+        let buf = this.force_ptr(this.read_scalar(buf_op)?.not_undef()?)?;
+        let size = this.read_scalar(size_op)?.to_usize(&*this.tcx)?;
+        // If we cannot get the current directory, we return null
+        // FIXME: Technically we have to set the `errno` global too
+        if let Ok(cwd) = env::current_dir() {
+            // It is not clear what happens with non-utf8 paths here
+            let mut bytes = cwd.display().to_string().into_bytes();
+            // If the buffer is smaller or equal than the path, we return null.
+            // FIXME: Technically we have to set the `errno` global too
+            if (bytes.len() as u64) < size {
+                // We add a `/0` terminator
+                bytes.push(0);
+                // This is ok because the buffer is larger than the path with the null terminator.
+                this.memory_mut().get_mut(buf.alloc_id)?.write_bytes(tcx, buf, &bytes)?;
+                return Ok(Scalar::Ptr(buf))
             }
-            return Ok(Scalar::ptr_null(&*this.tcx));
         }
-        throw_unsup_format!("Function not available when isolation is enabled")
+        Ok(Scalar::ptr_null(&*this.tcx))
     }
 }