strnlen_user.c

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#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/uaccess.h>

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

/* Set bits in the first 'n' bytes when loaded from memory */
#ifdef __LITTLE_ENDIAN
#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
#else
#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
#endif

/*
 * Do a strnlen, return length of string *with* final '\0'.
 * 'count' is the user-supplied count, while 'max' is the
 * address space maximum.
 *
 * Return 0 for exceptions (which includes hitting the address
 * space maximum), or 'count+1' if hitting the user-supplied
 * maximum count.
 *
 * NOTE! We can sometimes overshoot the user-supplied maximum
 * if it fits in a aligned 'long'. The caller needs to check
 * the return value against "> max".
 */
static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
{
    const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
    long align, res = 0;
    unsigned long c;

    /*
     * 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;

    /*
     * Do everything aligned. But that means that we
     * need to also expand the maximum..
     */
    align = (sizeof(long) - 1) & (unsigned long)src;
    src -= align;
    max += align;

    if (unlikely(__get_user(c,(unsigned long __user *)src)))
        return 0;
    c |= aligned_byte_mask(align);

    for (;;) {
        unsigned long data;
        if (has_zero(c, &data, &constants)) {
            data = prep_zero_mask(c, data, &constants);
            data = create_zero_mask(data);
            return res + find_zero(data) + 1 - align;
        }
        res += sizeof(unsigned long);
        if (unlikely(max < sizeof(unsigned long)))
            break;
        max -= sizeof(unsigned long);
        if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
            return 0;
    }
    res -= align;

    /*
     * Uhhuh. We hit 'max'. But was that the user-specified maximum
     * too? If so, return the marker for "too long".
     */
    if (res >= count)
        return count+1;

    /*
     * Nope: we hit the address space limit, and we still had more
     * characters the caller would have wanted. That's 0.
     */
    return 0;
}

long strnlen_user(const char __user *str, long count)
{
    unsigned long max_addr, src_addr;

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

    max_addr = user_addr_max();
    src_addr = (unsigned long)str;
    if (likely(src_addr < max_addr)) {
        unsigned long max = max_addr - src_addr;
        return do_strnlen_user(str, count, max);
    }
    return 0;
}
EXPORT_SYMBOL(strnlen_user);

long strlen_user(const char __user *str)
{
    unsigned long max_addr, src_addr;

    max_addr = user_addr_max();
    src_addr = (unsigned long)str;
    if (likely(src_addr < max_addr)) {
        unsigned long max = max_addr - src_addr;
        return do_strnlen_user(str, ~0ul, max);
    }
    return 0;
}
EXPORT_SYMBOL(strlen_user);