Root/lib/strncpy_from_user.c

1#include <linux/module.h>
2#include <linux/uaccess.h>
3#include <linux/kernel.h>
4#include <linux/errno.h>
5
6#include <asm/byteorder.h>
7#include <asm/word-at-a-time.h>
8
9#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
10#define IS_UNALIGNED(src, dst) 0
11#else
12#define IS_UNALIGNED(src, dst) \
13    (((long) dst | (long) src) & (sizeof(long) - 1))
14#endif
15
16/*
17 * Do a strncpy, return length of string without final '\0'.
18 * 'count' is the user-supplied count (return 'count' if we
19 * hit it), 'max' is the address space maximum (and we return
20 * -EFAULT if we hit it).
21 */
22static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
23{
24    const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
25    long res = 0;
26
27    /*
28     * Truncate 'max' to the user-specified limit, so that
29     * we only have one limit we need to check in the loop
30     */
31    if (max > count)
32        max = count;
33
34    if (IS_UNALIGNED(src, dst))
35        goto byte_at_a_time;
36
37    while (max >= sizeof(unsigned long)) {
38        unsigned long c, data;
39
40        /* Fall back to byte-at-a-time if we get a page fault */
41        if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
42            break;
43        *(unsigned long *)(dst+res) = c;
44        if (has_zero(c, &data, &constants)) {
45            data = prep_zero_mask(c, data, &constants);
46            data = create_zero_mask(data);
47            return res + find_zero(data);
48        }
49        res += sizeof(unsigned long);
50        max -= sizeof(unsigned long);
51    }
52
53byte_at_a_time:
54    while (max) {
55        char c;
56
57        if (unlikely(__get_user(c,src+res)))
58            return -EFAULT;
59        dst[res] = c;
60        if (!c)
61            return res;
62        res++;
63        max--;
64    }
65
66    /*
67     * Uhhuh. We hit 'max'. But was that the user-specified maximum
68     * too? If so, that's ok - we got as much as the user asked for.
69     */
70    if (res >= count)
71        return res;
72
73    /*
74     * Nope: we hit the address space limit, and we still had more
75     * characters the caller would have wanted. That's an EFAULT.
76     */
77    return -EFAULT;
78}
79
80/**
81 * strncpy_from_user: - Copy a NUL terminated string from userspace.
82 * @dst: Destination address, in kernel space. This buffer must be at
83 * least @count bytes long.
84 * @src: Source address, in user space.
85 * @count: Maximum number of bytes to copy, including the trailing NUL.
86 *
87 * Copies a NUL-terminated string from userspace to kernel space.
88 *
89 * On success, returns the length of the string (not including the trailing
90 * NUL).
91 *
92 * If access to userspace fails, returns -EFAULT (some data may have been
93 * copied).
94 *
95 * If @count is smaller than the length of the string, copies @count bytes
96 * and returns @count.
97 */
98long strncpy_from_user(char *dst, const char __user *src, long count)
99{
100    unsigned long max_addr, src_addr;
101
102    if (unlikely(count <= 0))
103        return 0;
104
105    max_addr = user_addr_max();
106    src_addr = (unsigned long)src;
107    if (likely(src_addr < max_addr)) {
108        unsigned long max = max_addr - src_addr;
109        return do_strncpy_from_user(dst, src, count, max);
110    }
111    return -EFAULT;
112}
113EXPORT_SYMBOL(strncpy_from_user);
114

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