fix readlink/readlinkat to return too long only when it is long

2 files changed, 18 insertions, 5 deletions

diff --git a/src/fcntl.rs b/src/fcntl.rs
index be6ee0f7..f590e361 100644
--- a/src/fcntl.rs
+++ b/src/fcntl.rs
@@ -181,7 +181,9 @@ fn wrap_readlink_result(buffer: &mut[u8], res: ssize_t) -> Result<&OsStr> {
     match Errno::result(res) {
         Err(err) => Err(err),
         Ok(len) => {
-            if (len as usize) >= buffer.len() {
+            if len < 0 {
+                Err(Error::Sys(Errno::EINVAL))
+            } else if (len as usize) > buffer.len() {
                 Err(Error::Sys(Errno::ENAMETOOLONG))
             } else {
                 Ok(OsStr::from_bytes(&buffer[..(len as usize)]))
diff --git a/test/test_fcntl.rs b/test/test_fcntl.rs
index 6b2bbd67..c8773219 100644
--- a/test/test_fcntl.rs
+++ b/test/test_fcntl.rs
@@ -56,12 +56,23 @@ fn test_readlink() {
     let dirfd = open(tempdir.path(),
                      OFlag::empty(),
                      Mode::empty()).unwrap();
+    let expected_dir = src.to_str().unwrap();
 
+    // When the size of the buffer is bigger than the expected directory length
     let mut buf = vec![0; src.to_str().unwrap().len() + 1];
-    assert_eq!(readlink(&dst, &mut buf).unwrap().to_str().unwrap(),
-               src.to_str().unwrap());
-    assert_eq!(readlinkat(dirfd, "b", &mut buf).unwrap().to_str().unwrap(),
-               src.to_str().unwrap());
+    assert_eq!(readlink(&dst, &mut buf).unwrap().to_str().unwrap(), expected_dir);
+    assert_eq!(readlinkat(dirfd, "b", &mut buf).unwrap().to_str().unwrap(), expected_dir);
+
+    // When the size of the buffer is equal to the expected directory length
+    let mut exact_buf = vec![0; src.to_str().unwrap().len()];
+    assert_eq!(readlink(&dst, &mut exact_buf).unwrap().to_str().unwrap(), expected_dir);
+    assert_eq!(readlinkat(dirfd, "b", &mut exact_buf).unwrap().to_str().unwrap(), expected_dir);
+
+    // When the size of the buffer is smaller than the expected directory length
+    let mut small_buf = vec![0;0];
+    assert_eq!(readlink(&dst, &mut small_buf).unwrap().to_str().unwrap(), "");
+    assert_eq!(readlinkat(dirfd, "b", &mut small_buf).unwrap().to_str().unwrap(), "");
+
 }
 
 #[cfg(any(target_os = "linux", target_os = "android"))]