1// SPDX-License-Identifier: GPL-2.0
2#include <linux/kernel.h>
3#include <linux/export.h>
4#include <linux/uaccess.h>
5
6#include <asm/word-at-a-time.h>
7
8/* Set bits in the first 'n' bytes when loaded from memory */
9#ifdef __LITTLE_ENDIAN
10# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
11#else
12# define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
13#endif
14
15/*
16 * Do a strnlen, return length of string *with* final '\0'.
17 * 'count' is the user-supplied count, while 'max' is the
18 * address space maximum.
19 *
20 * Return 0 for exceptions (which includes hitting the address
21 * space maximum), or 'count+1' if hitting the user-supplied
22 * maximum count.
23 *
24 * NOTE! We can sometimes overshoot the user-supplied maximum
25 * if it fits in a aligned 'long'. The caller needs to check
26 * the return value against "> max".
27 */
28static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
29{
30 const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
31 long align, res = 0;
32 unsigned long c;
33
34 /*
35 * Truncate 'max' to the user-specified limit, so that
36 * we only have one limit we need to check in the loop
37 */
38 if (max > count)
39 max = count;
40
41 /*
42 * Do everything aligned. But that means that we
43 * need to also expand the maximum..
44 */
45 align = (sizeof(long) - 1) & (unsigned long)src;
46 src -= align;
47 max += align;
48
49 unsafe_get_user(c, (unsigned long __user *)src, efault);
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 /* We already handled 'unsigned long' bytes. Did we do it all ? */
61 if (unlikely(max <= sizeof(unsigned long)))
62 break;
63 max -= sizeof(unsigned long);
64 unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
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 */
79efault:
80 return 0;
81}
82
83/**
84 * strnlen_user: - Get the size of a user string INCLUDING final NUL.
85 * @str: The string to measure.
86 * @count: Maximum count (including NUL character)
87 *
88 * Context: User context only. This function may sleep if pagefaults are
89 * enabled.
90 *
91 * Get the size of a NUL-terminated string in user space.
92 *
93 * Returns the size of the string INCLUDING the terminating NUL.
94 * If the string is too long, returns a number larger than @count. User
95 * has to check the return value against "> count".
96 * On exception (or invalid count), returns 0.
97 *
98 * NOTE! You should basically never use this function. There is
99 * almost never any valid case for using the length of a user space
100 * string, since the string can be changed at any time by other
101 * threads. Use "strncpy_from_user()" instead to get a stable copy
102 * of the string.
103 */
104long strnlen_user(const char __user *str, long count)
105{
106 unsigned long max_addr, src_addr;
107
108 if (unlikely(count <= 0))
109 return 0;
110
111 max_addr = user_addr_max();
112 src_addr = (unsigned long)str;
113 if (likely(src_addr < max_addr)) {
114 unsigned long max = max_addr - src_addr;
115 long retval;
116
117 if (user_access_begin(str, max)) {
118 retval = do_strnlen_user(str, count, max);
119 user_access_end();
120 return retval;
121 }
122 }
123 return 0;
124}
125EXPORT_SYMBOL(strnlen_user);
126