centos man page for __copy_from_user

__COPY_FROM_USER(9)					    Memory Management in Linux					       __COPY_FROM_USER(9)

NAME
       __copy_from_user - Copy a block of data from user space, with less checking.

SYNOPSIS
       unsigned long __copy_from_user(void * to, const void __user * from, unsigned long n);

ARGUMENTS
       to
	   Destination address, in kernel space.

       from
	   Source address, in user space.

       n
	   Number of bytes to copy.

CONTEXT
       User context only. This function may sleep.

DESCRIPTION
       Copy data from user space to kernel space. Caller must check the specified block with access_ok before calling this function.

       Returns number of bytes that could not be copied. On success, this will be zero.

       If some data could not be copied, this function will pad the copied data to the requested size using zero bytes.

       An alternate version - __copy_from_user_inatomic - may be called from atomic context and will fail rather than sleep. In this case the
       uncopied bytes will *NOT* be padded with zeros. See fs/filemap.h for explanation of why this is needed.

COPYRIGHT
Kernel Hackers Manual 3.10					     June 2014						       __COPY_FROM_USER(9)