syscall: fix NaCl Link syscall error handling

The existing NaCl filesystem Link system call erroneously allowed
a caller to call Link on an existing target which violates the POSIX
standard and effectively corrupted the internal filesystem
representation.

Fixes #22383

Change-Id: I77b16c37af9bf00a1799fa84277f066180edac47
Reviewed-on: https://go-review.googlesource.com/76110Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -721,6 +721,27 @@ func TestHardLink(t *testing.T) {
 	if !SameFile(tostat, fromstat) {
 		t.Errorf("link %q, %q did not create hard link", to, from)
 	}
+	// We should not be able to perform the same Link() a second time
+	err = Link(to, from)
+	switch err := err.(type) {
+	case *LinkError:
+		if err.Op != "link" {
+			t.Errorf("Link(%q, %q) err.Op = %q; want %q", to, from, err.Op, "link")
+		}
+		if err.Old != to {
+			t.Errorf("Link(%q, %q) err.Old = %q; want %q", to, from, err.Old, to)
+		}
+		if err.New != from {
+			t.Errorf("Link(%q, %q) err.New = %q; want %q", to, from, err.New, from)
+		}
+		if !IsExist(err.Err) {
+			t.Errorf("Link(%q, %q) err.Err = %q; want %q", to, from, err.Err, "file exists error")
+		}
+	case nil:
+		t.Errorf("link %q, %q: expected error, got nil", from, to)
+	default:
+		t.Errorf("link %q, %q: expected %T, got %T %v", from, to, new(LinkError), err, err)
+	}
 }
 
 // chtmpdir changes the working directory to a new temporary directory and

src/syscall/fs_nacl.go

@@ -636,6 +636,10 @@ func Link(path, link string) error {
 	if ip.Mode&S_IFMT == S_IFDIR {
 		return EPERM
 	}
+	_, _, err = fs.dirlookup(dp, elem)
+	if err == nil {
+		return EEXIST
+	}
 	fs.dirlink(dp, elem, ip)
 	return nil
 }