strncpy_from_user.c

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#include <linux/module.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/errno.h>

#include <asm/byteorder.h>
#include <asm/word-at-a-time.h>

#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
#define IS_UNALIGNED(src, dst)  0
#else
#define IS_UNALIGNED(src, dst)  \
    (((long) dst | (long) src) & (sizeof(long) - 1))
#endif

/*
 * Do a strncpy, return length of string without final '\0'.
 * 'count' is the user-supplied count (return 'count' if we
 * hit it), 'max' is the address space maximum (and we return
 * -EFAULT if we hit it).
 */
static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
{
    const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
    long res = 0;

    /*
     * Truncate 'max' to the user-specified limit, so that
     * we only have one limit we need to check in the loop
     */
    if (max > count)
        max = count;

    if (IS_UNALIGNED(src, dst))
        goto byte_at_a_time;

    while (max >= sizeof(unsigned long)) {
        unsigned long c, data;

        /* Fall back to byte-at-a-time if we get a page fault */
        if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
            break;
        *(unsigned long *)(dst+res) = c;
        if (has_zero(c, &data, &constants)) {
            data = prep_zero_mask(c, data, &constants);
            data = create_zero_mask(data);
            return res + find_zero(data);
        }
        res += sizeof(unsigned long);
        max -= sizeof(unsigned long);
    }

byte_at_a_time:
    while (max) {
        char c;

        if (unlikely(__get_user(c,src+res)))
            return -EFAULT;
        dst[res] = c;
        if (!c)
            return res;
        res++;
        max--;
    }

    /*
     * Uhhuh. We hit 'max'. But was that the user-specified maximum
     * too? If so, that's ok - we got as much as the user asked for.
     */
    if (res >= count)
        return res;

    /*
     * Nope: we hit the address space limit, and we still had more
     * characters the caller would have wanted. That's an EFAULT.
     */
    return -EFAULT;
}

long strncpy_from_user(char *dst, const char __user *src, long count)
{
    unsigned long max_addr, src_addr;

    if (unlikely(count <= 0))
        return 0;

    max_addr = user_addr_max();
    src_addr = (unsigned long)src;
    if (likely(src_addr < max_addr)) {
        unsigned long max = max_addr - src_addr;
        return do_strncpy_from_user(dst, src, count, max);
    }
    return -EFAULT;
}
EXPORT_SYMBOL(strncpy_from_user);