linux: turn on -Wextra

sandbox/linux/seccomp/sandbox_impl.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SANDBOX_IMPL_H__
 #define SANDBOX_IMPL_H__
 
 #include <asm/ldt.h>
 #include <errno.h>
 #include <fcntl.h>
@@ skipping 250 matching lines @@
     }
     for (;;) {
       sys.exit_group(1);
       sys._exit(1);
     }
   }
 
   // Wrapper around "read()" that can deal with partial and interrupted reads
   // and that does not modify the global errno variable.
   static ssize_t read(SysCalls& sys, int fd, void* buf, size_t len) {
-    if (len < 0) {
+    if (static_cast<ssize_t>(len) < 0) {
       sys.my_errno = EINVAL;
       return -1;
     }
     size_t offset = 0;
     while (offset < len) {
       ssize_t partial =
           NOINTR_SYS(sys.read(fd, reinterpret_cast<char*>(buf) + offset,
                               len - offset));
       if (partial < 0) {
         return partial;
@@ skipping 374 matching lines @@
   static ProtectedMap                  protectedMap_;
   static std::vector<SecureMem::Args*> secureMemPool_;
 };
 
 } // namespace
 
 using playground::Sandbox;
 #endif // __cplusplus
 
 #endif // SANDBOX_IMPL_H__