Add a ForkWithFlags wrapper using the libc clone wrapper.

sandbox/linux/services/syscall_wrappers.h

Index: sandbox/linux/services/syscall_wrappers.h
diff --git a/sandbox/linux/services/syscall_wrappers.h b/sandbox/linux/services/syscall_wrappers.h
index 64028f7d0f6d61316566ad20b1f8a260128f3f8b..2001f38f40b4f8d6dd1babc3e0bdca7cfc01f31e 100644
--- a/sandbox/linux/services/syscall_wrappers.h
+++ b/sandbox/linux/services/syscall_wrappers.h
@@ -16,7 +16,7 @@ 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);
 
@@ -24,13 +24,27 @@ 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.
 SANDBOX_EXPORT long sys_clone(unsigned long flags,
-                              void* child_stack,
+                              decltype(nullptr) child_stack,
                               pid_t* ptid,
                               pid_t* ctid,
                               decltype(nullptr) regs);
 
+// A wrapper for clone with fork-like behavior, meaning that it returns the
+// child's pid in the parent and 0 in the child. |flags|, |ptid|, and |ctid| are
+// as in the clone system call (the CLONE_VM flag is not supported).
+//
+// 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. An exception is when this code is run
+// under Valgrind. Valgrind does not support the libc clone wrapper, so the libc
+// pid cache may be incorrect after this function is called under Valgrind.
+SANDBOX_EXPORT pid_t
+ForkWithFlags(unsigned long flags, pid_t* ptid, pid_t* ctid);
+
 SANDBOX_EXPORT void sys_exit_group(int status);
 
 // The official system call takes |args| as void*  (in order to be extensible),