Dear Fawaad Thank you very much for sending me the document from Mr Rubini. Here is the sentences that convince me that I am correct: "Before calling the function, however, a preparing step must be performed. Like any other function that transfers data to/from user space using a user-provided pointer, the system call checks whether the provided buffer is a valid address or not. During normal operation, an address that lies in the user address range (0-3GB for standard kernel configuration) is considered valid, and an address that lies in kernel address space (3GB-4GB) is not. If the system call is invoked from kernel space, though, we must prevent the usual check to fail, because the virtual address of our destination buffer will be in kernel space, above the 3GB mark. " I hope it also helps other to shed some light for this kind of issue. regards Mulyadi -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
