Use the libc clone wrapper in sys_clone.

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..318d701d52bc42931d7e761479527cb80dac71c4 100644
--- a/sandbox/linux/services/syscall_wrappers.h
+++ b/sandbox/linux/services/syscall_wrappers.h
@@ -24,9 +24,11 @@ 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.

[jln (very slow on Chromium), 2014/12/15 23:31:35]

Please, mention that under the hood, this is using GLIBC's clone and that
callers can expect getpid() etc to work as expected.

[rickyz (no longer on Chrome), 2014/12/16 00:35:09]

Done.

 SANDBOX_EXPORT long sys_clone(unsigned long flags,
-                              void* child_stack,
+                              decltype(nullptr) child_stack,
                               pid_t* ptid,
                               pid_t* ctid,
                               decltype(nullptr) regs);