I am somewhat reluctantly changing the API so that setProcFD is no longer

sandbox.cc

 // 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.
 
 #include "sandbox_impl.h"
 
 #include "library.h"
 #include "syscall_entrypoint.h"
 #include "system_call_table.h"
 
 namespace playground {
 
 // Global variables
-int                                 Sandbox::proc_ = -1;
 int                                 Sandbox::proc_self_maps_ = -1;
 enum Sandbox::SandboxStatus         Sandbox::status_ = STATUS_UNKNOWN;
 int                                 Sandbox::pid_;
 int                                 Sandbox::processFdPub_;
 int                                 Sandbox::cloneFdPub_
   // This is necessary to locate the symbol from assembly code on
   // x86-64 (with %rip-relative addressing) in order for this to work
   // in relocatable code (a .so or a PIE).  On i386 this is not
   // necessary but it does not hurt.
   __attribute__((visibility("internal")));
@@ skipping 220 matching lines @@
         status_ = STATUS_AVAILABLE;
       }
       int rc;
       (void)NOINTR_SYS(sys.waitpid(pid, &rc, 0));
       (void)NOINTR_SYS(sys.close(fds[0]));
       return status_ != STATUS_UNSUPPORTED;
   }
 }
 
 void Sandbox::setProcFd(int proc) {
-  proc_ = proc;
+  if (proc >= 0) {
+    SysCalls sys;
+    proc_self_maps_ = sys.openat(proc, "self/maps", O_RDONLY, 0);
+    if (NOINTR_SYS(sys.close(proc))) {
+      die("Failed to close file descriptor pointing to /proc");
+    }
+  }
 }
 
 void Sandbox::startSandbox() {
   if (status_ == STATUS_UNSUPPORTED) {
     die("The seccomp sandbox is not supported on this computer");
   } else if (status_ == STATUS_ENABLED) {
     return;
   }
 
   SysCalls sys;
-  if (proc_ >= 0) {
-    proc_self_maps_ = sys.openat(proc_, "self/maps", O_RDONLY, 0);
-    if (NOINTR_SYS(sys.close(proc_))) {
-      die("Failed to close file descriptor pointing to /proc");
-    }
-    proc_ = -1;
-  }
   if (proc_self_maps_ < 0) {
     proc_self_maps_        = sys.open("/proc/self/maps", O_RDONLY, 0);
     if (proc_self_maps_ < 0) {
       die("Cannot access \"/proc/self/maps\"");
     }
   }
 
   // The pid is unchanged for the entire program, so we can retrieve it once
   // and store it in a global variable.
   pid_                     = sys.getpid();
@@ skipping 103 matching lines @@
   } entrypoint;
   *entrypoint.get_syscall_entrypoint() = syscallEntryPointNoFrame;
 
   // We can no longer check for sandboxing support at this point, but we also
   // know for a fact that it is available (as we just turned it on). So update
   // the status to reflect this information.
   status_ = STATUS_ENABLED;
 }
 
 } // namespace