Linux Sandbox: Add support for SECCOMP_RET_TRACE.

sandbox/linux/seccomp-bpf/errorcode.h

 // Copyright (c) 2012 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_SECCOMP_BPF_ERRORCODE_H__
 #define SANDBOX_LINUX_SECCOMP_BPF_ERRORCODE_H__
 
 #include "sandbox/linux/sandbox_export.h"
 #include "sandbox/linux/seccomp-bpf/linux_seccomp.h"
 #include "sandbox/linux/seccomp-bpf/trap.h"
@@ skipping 12 matching lines @@
 // field.
 class SANDBOX_EXPORT ErrorCode {
  public:
   enum {
     // Allow this system call. The value of ERR_ALLOWED is pretty much
     // completely arbitrary. But we want to pick it so that is is unlikely
     // to be passed in accidentally, when the user intended to return an
     // "errno" (see below) value instead.
     ERR_ALLOWED = 0x04000000,
 
+    // If the progress is being ptraced with PTRACE_O_TRACESECCOMP, then then

[jln (very slow on Chromium), 2014/05/20 03:02:10]

Nit: one "then"

[rickyz (Google), 2014/05/20 22:34:01]

Done.

+    // the tracer will be notified of a PTRACE_EVENT_SECCOMP and allowed to
+    // change or skip the system call.  The lower 16 bits of err will be
+    // available to the tracer via PTRACE_GETEVENTMSG.
+    ERR_TRACE   = 0x08000000,
+
     // Deny the system call with a particular "errno" value.
     // N.B.: It is also possible to return "0" here. That would normally
     //       indicate success, but it won't actually run the system call.
     //       This is very different from return ERR_ALLOWED.
     ERR_MIN_ERRNO = 0,
     // TODO(markus): Android only supports errno up to 255
     // (crbug.com/181647).
     ERR_MAX_ERRNO = 4095,
   };
 
@@ skipping 146 matching lines @@
 
   // 32bit field used for all possible types of ErrorCode values. This is
   // the value that uniquely identifies any ErrorCode and it (typically) can
   // be emitted directly into a BPF filter program.
   uint32_t err_;
 };
 
 }  // namespace sandbox
 
 #endif  // SANDBOX_LINUX_SECCOMP_BPF_ERRORCODE_H__