Return earlier when reading/writing 0 bytes

Author: pvdrz

src/shims/io.rs

@@ -149,25 +149,24 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         let tcx = &{ this.tcx.tcx };
 
+        let count = this.read_scalar(count_op)?.to_usize(&*this.tcx)?;
+        // Reading zero bytes should not change `buf`
+        if count == 0 {
+            return Ok(0);
+        }
         let fd = this.read_scalar(fd_op)?.to_i32()?;
         let buf_scalar = this.read_scalar(buf_op)?.not_undef()?;
-        let count = this.read_scalar(count_op)?.to_usize(&*this.tcx)?;
 
         // Remove the file handle to avoid borrowing issues
         this.remove_handle_and(fd, |mut handle, this| {
             // Don't use `?` to avoid returning before reinserting the handle
-            let bytes =
-            if count == 0 {
-                Ok(handle.file.read(&mut []))
-            } else {
-                this.force_ptr(buf_scalar).and_then(|buf| this
-                    .memory_mut()
-                    .get_mut(buf.alloc_id).and_then(|alloc|
-                        alloc.get_bytes_mut(tcx, buf, Size::from_bytes(count))
+            let bytes = this.force_ptr(buf_scalar).and_then(|buf| {
+                this.memory_mut().get_mut(buf.alloc_id).and_then(|alloc| {
+                    alloc
+                        .get_bytes_mut(tcx, buf, Size::from_bytes(count))
                         .map(|buffer| handle.file.read(buffer))
-                    ))
-
-            };
+                })
+            });
             // Reinsert the file handle
             this.machine.file_handler.handles.insert(fd, handle);
             this.consume_result(bytes?.map(|bytes| bytes as i64))
@@ -188,9 +187,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         let tcx = &{ this.tcx.tcx };
 
+        let count = this.read_scalar(count_op)?.to_usize(&*this.tcx)?;
+        // Reading zero bytes should not change `buf`
+        if count == 0 {
+            return Ok(0);
+        }
         let fd = this.read_scalar(fd_op)?.to_i32()?;
         let buf = this.force_ptr(this.read_scalar(buf_op)?.not_undef()?)?;
-        let count = this.read_scalar(count_op)?.to_usize(&*this.tcx)?;
 
         this.remove_handle_and(fd, |mut handle, this| {
             let bytes = this.memory().get(buf.alloc_id).and_then(|alloc| {

tests/run-pass/file_read.rs

@@ -2,7 +2,7 @@
 // compile-flags: -Zmiri-disable-isolation
 
 use std::fs::File;
-use std::io::{ Read, Write };
+use std::io::{Read, Write};
 
 fn main() {
     // FIXME: remove the file and delete it when `rm` is implemented.