Linux Sandbox: add Credentials::SupportsNewUserNS()

sandbox/linux/services/credentials.h

 // Copyright (c) 2013 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_LINUX_SERVICES_CREDENTIALS_H_
 #define SANDBOX_LINUX_SERVICES_CREDENTIALS_H_
 
 #include "build/build_config.h"
 // Link errors are tedious to track, raise a compile-time error instead.
 #if defined(OS_ANDROID)
@@ skipping 33 matching lines @@
   // the current process.
   bool DropAllCapabilities();
   // Return true iff there is any capability in any of the capabilities sets
   // of the current process.
   bool HasAnyCapability() const;
   // Returns the capabilities of the current process in textual form, as
   // documented in libcap2's cap_to_text(3). This is mostly useful for
   // debugging and tests.
   scoped_ptr<std::string> GetCurrentCapString() const;
 
+  // Returns whether the kernel supports CLONE_NEWUSER and whether it would be
+  // possible to immediately move to a new user namespace. There is no point
+  // in using this method right before calling MoveToNewUserNS(), simply call
+  // MoveToNewUserNS() immediately. This method is only useful to test kernel
+  // support ahead of time.
+  static bool SupportsNewUserNS();
+
   // Move the current process to a new "user namespace" as supported by Linux
   // 3.8+ (CLONE_NEWUSER).
   // The uid map will be set-up so that the perceived uid and gid will not
   // change.
   // If this call succeeds, the current process will be granted a full set of
   // capabilities in the new namespace.
   bool MoveToNewUserNS();
 
   // Remove the ability of the process to access the file system. File
   // descriptors which are already open prior to calling this API remain
   // available.
   // The implementation currently uses chroot(2) and requires CAP_SYS_CHROOT.
   // CAP_SYS_CHROOT can be acquired by using the MoveToNewUserNS() API.
   // Make sure to call DropAllCapabilities() after this call to prevent
   // escapes.
   // To be secure, it's very important for this API to not be called while the
   // process has any directory file descriptor open.
   bool DropFileSystemAccess();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Credentials);
 };
 
 }  // namespace sandbox.
 
 #endif  // SANDBOX_LINUX_SERVICES_CREDENTIALS_H_