linux: turn on -Wextra

sandbox/linux/suid/linux_util.c

 // Copyright (c) 2009 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.
 
 // The following is duplicated from base/linux_utils.cc.
 // We shouldn't link against C++ code in a setuid binary.
 
 #include "linux_util.h"
 
 #include <dirent.h>
@@ skipping 65 matching lines @@
         continue;
     }
 
     char buf[256];
     snprintf(buf, sizeof(buf), "/proc/%lu/fd", pid_ul);
     DIR* fd = opendir(buf);
     if (!fd)
       continue;
 
     while ((dent = readdir(fd))) {
-      if (snprintf(buf, sizeof(buf), "/proc/%lu/fd/%s", pid_ul,
-                   dent->d_name) >= sizeof(buf) - 1) {
+      int printed = snprintf(buf, sizeof(buf), "/proc/%lu/fd/%s", pid_ul,
+                             dent->d_name);
+      if (printed < 0 || printed >= (int)(sizeof(buf) - 1)) {
         continue;
       }
 
       ino_t fd_inode;
       if (ProcPathGetInode(&fd_inode, buf)) {
         if (fd_inode == socket_inode) {
           if (already_found) {
             closedir(fd);
             closedir(proc);
             return false;
           }
 
           already_found = true;
           *pid_out = pid_ul;
           break;
         }
       }
     }
     closedir(fd);
   }
   closedir(proc);
 
   return already_found;
 }