Root/lib/strnlen_user.c

1#include <linux/kernel.h>
2#include <linux/export.h>
3#include <linux/uaccess.h>
4
5#include <asm/word-at-a-time.h>
6
7/* Set bits in the first 'n' bytes when loaded from memory */
8#ifdef __LITTLE_ENDIAN
9# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
10#else
11# define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
12#endif
13
14/*
15 * Do a strnlen, return length of string *with* final '\0'.
16 * 'count' is the user-supplied count, while 'max' is the
17 * address space maximum.
18 *
19 * Return 0 for exceptions (which includes hitting the address
20 * space maximum), or 'count+1' if hitting the user-supplied
21 * maximum count.
22 *
23 * NOTE! We can sometimes overshoot the user-supplied maximum
24 * if it fits in a aligned 'long'. The caller needs to check
25 * the return value against "> max".
26 */
27static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
28{
29    const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
30    long align, res = 0;
31    unsigned long c;
32
33    /*
34     * Truncate 'max' to the user-specified limit, so that
35     * we only have one limit we need to check in the loop
36     */
37    if (max > count)
38        max = count;
39
40    /*
41     * Do everything aligned. But that means that we
42     * need to also expand the maximum..
43     */
44    align = (sizeof(long) - 1) & (unsigned long)src;
45    src -= align;
46    max += align;
47
48    if (unlikely(__get_user(c,(unsigned long __user *)src)))
49        return 0;
50    c |= aligned_byte_mask(align);
51
52    for (;;) {
53        unsigned long data;
54        if (has_zero(c, &data, &constants)) {
55            data = prep_zero_mask(c, data, &constants);
56            data = create_zero_mask(data);
57            return res + find_zero(data) + 1 - align;
58        }
59        res += sizeof(unsigned long);
60        if (unlikely(max < sizeof(unsigned long)))
61            break;
62        max -= sizeof(unsigned long);
63        if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
64            return 0;
65    }
66    res -= align;
67
68    /*
69     * Uhhuh. We hit 'max'. But was that the user-specified maximum
70     * too? If so, return the marker for "too long".
71     */
72    if (res >= count)
73        return count+1;
74
75    /*
76     * Nope: we hit the address space limit, and we still had more
77     * characters the caller would have wanted. That's 0.
78     */
79    return 0;
80}
81
82/**
83 * strnlen_user: - Get the size of a user string INCLUDING final NUL.
84 * @str: The string to measure.
85 * @count: Maximum count (including NUL character)
86 *
87 * Context: User context only. This function may sleep.
88 *
89 * Get the size of a NUL-terminated string in user space.
90 *
91 * Returns the size of the string INCLUDING the terminating NUL.
92 * If the string is too long, returns 'count+1'.
93 * On exception (or invalid count), returns 0.
94 */
95long strnlen_user(const char __user *str, long count)
96{
97    unsigned long max_addr, src_addr;
98
99    if (unlikely(count <= 0))
100        return 0;
101
102    max_addr = user_addr_max();
103    src_addr = (unsigned long)str;
104    if (likely(src_addr < max_addr)) {
105        unsigned long max = max_addr - src_addr;
106        return do_strnlen_user(str, count, max);
107    }
108    return 0;
109}
110EXPORT_SYMBOL(strnlen_user);
111
112/**
113 * strlen_user: - Get the size of a user string INCLUDING final NUL.
114 * @str: The string to measure.
115 *
116 * Context: User context only. This function may sleep.
117 *
118 * Get the size of a NUL-terminated string in user space.
119 *
120 * Returns the size of the string INCLUDING the terminating NUL.
121 * On exception, returns 0.
122 *
123 * If there is a limit on the length of a valid string, you may wish to
124 * consider using strnlen_user() instead.
125 */
126long strlen_user(const char __user *str)
127{
128    unsigned long max_addr, src_addr;
129
130    max_addr = user_addr_max();
131    src_addr = (unsigned long)str;
132    if (likely(src_addr < max_addr)) {
133        unsigned long max = max_addr - src_addr;
134        return do_strnlen_user(str, ~0ul, max);
135    }
136    return 0;
137}
138EXPORT_SYMBOL(strlen_user);
139

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