How does __do_clear_user() works?

["Prasad Joshi" <prasadjoshi124@xxxxxxxxx>]

Hi All,

Here is the code for __do_clear_user, I am not getting how does it work. Can any one please explain?

#define __do_clear_user(addr,size)                                      \
do {                                                                    \
        int __d0;                                                       \
        might_sleep();                                                  \
        __asm__ __volatile__(                                           \
                "0:     rep; stosl\n"                                   \
                "       movl %2,%0\n"                                   \
                "1:     rep; stosb\n"                                   \
                "2:\n"                                                  \
                ".section .fixup,\"ax\"\n"                              \
                "3:     lea 0(%2,%0,4),%0\n"                            \
                "       jmp 2b\n"                                       \
                ".previous\n"                                           \
                _ASM_EXTABLE(0b,3b)                                     \
                _ASM_EXTABLE(1b,2b)                                     \
                : "=&c"(size), "=&D" (__d0)                             \
                : "r"(size & 3), "0"(size / 4), "1"(addr), "a"(0));     \
} while (0)

/**
 * clear_user: - Zero a block of memory in user space.
 * @to:   Destination address, in user space.
 * @n:    Number of bytes to zero.
 *
 * Zero a block of memory in user space.
 *
 * Returns number of bytes that could not be cleared.
 * On success, this will be zero.
 */
unsigned long
clear_user(void __user *to, unsigned long n)
{
        might_sleep();
        if (access_ok(VERIFY_WRITE, to, n))
                __do_clear_user(to, n);
        return n;
}
EXPORT_SYMBOL(clear_user);


clear_user() is the function which is calling __do_clear_user(), from the context I can understand it is clearing the user space buffer. But how does it work?

Thanks and Regards,
Prasad.