Use the libc clone wrapper in sys_clone.

sandbox/linux/services/syscall_wrappers.h

 // Copyright 2014 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_SYSCALL_WRAPPERS_H_
 #define SANDBOX_LINUX_SERVICES_SYSCALL_WRAPPERS_H_
 
 #include <sys/types.h>
 
 #include "sandbox/sandbox_export.h"
 
 struct sock_fprog;
 struct rlimit64;
 
 namespace sandbox {
 
 // Provide direct system call wrappers for a few common system calls.
 // These are guaranteed to perform a system call and do not rely on things such
-// as caching the current pid (c.f. getpid()).
+// as caching the current pid (c.f. getpid()) unless otherwise specified.
 
 SANDBOX_EXPORT pid_t sys_getpid(void);
 
 SANDBOX_EXPORT pid_t sys_gettid(void);
 
 SANDBOX_EXPORT long sys_clone(unsigned long flags);
 
-// |regs| is not supported and must be passed as nullptr.
+// |regs| is not supported and must be passed as nullptr. |child_stack| must be
+// nullptr, since otherwise this function cannot safely return.  As a
+// consequence, this function does not support CLONE_VM.
+//
+// This function uses the libc clone wrapper (which updates libc's pid cache)
+// internally, so callers may expect things like getpid() to work correctly
+// after in both the child and parent.
 SANDBOX_EXPORT long sys_clone(unsigned long flags,
-                              void* child_stack,
+                              decltype(nullptr) child_stack,
                               pid_t* ptid,
                               pid_t* ctid,
                               decltype(nullptr) regs);
 
 SANDBOX_EXPORT void sys_exit_group(int status);
 
 // The official system call takes |args| as void*  (in order to be extensible),
 // but add more typing for the cases that are currently used.
 SANDBOX_EXPORT int sys_seccomp(unsigned int operation,
                                unsigned int flags,
                                const struct sock_fprog* args);
 
 // Some libcs do not expose a prlimit64 wrapper.
 SANDBOX_EXPORT int sys_prlimit64(pid_t pid,
                                  int resource,
                                  const struct rlimit64* new_limit,
                                  struct rlimit64* old_limit);
 
 }  // namespace sandbox
 
 #endif  // SANDBOX_LINUX_SERVICES_SYSCALL_WRAPPERS_H_