1 | // SPDX-License-Identifier: GPL-2.0 |
2 | #include <linux/kernel.h> |
3 | #include <linux/export.h> |
4 | #include <linux/uaccess.h> |
5 | #include <linux/mm.h> |
6 | #include <linux/bitops.h> |
7 | |
8 | #include <asm/word-at-a-time.h> |
9 | |
10 | /* |
11 | * Do a strnlen, return length of string *with* final '\0'. |
12 | * 'count' is the user-supplied count, while 'max' is the |
13 | * address space maximum. |
14 | * |
15 | * Return 0 for exceptions (which includes hitting the address |
16 | * space maximum), or 'count+1' if hitting the user-supplied |
17 | * maximum count. |
18 | * |
19 | * NOTE! We can sometimes overshoot the user-supplied maximum |
20 | * if it fits in a aligned 'long'. The caller needs to check |
21 | * the return value against "> max". |
22 | */ |
23 | static __always_inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max) |
24 | { |
25 | const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS; |
26 | unsigned long align, res = 0; |
27 | unsigned long c; |
28 | |
29 | /* |
30 | * Do everything aligned. But that means that we |
31 | * need to also expand the maximum.. |
32 | */ |
33 | align = (sizeof(unsigned long) - 1) & (unsigned long)src; |
34 | src -= align; |
35 | max += align; |
36 | |
37 | unsafe_get_user(c, (unsigned long __user *)src, efault); |
38 | c |= aligned_byte_mask(align); |
39 | |
40 | for (;;) { |
41 | unsigned long data; |
42 | if (has_zero(a: c, bits: &data, c: &constants)) { |
43 | data = prep_zero_mask(a: c, bits: data, c: &constants); |
44 | data = create_zero_mask(bits: data); |
45 | return res + find_zero(mask: data) + 1 - align; |
46 | } |
47 | res += sizeof(unsigned long); |
48 | /* We already handled 'unsigned long' bytes. Did we do it all ? */ |
49 | if (unlikely(max <= sizeof(unsigned long))) |
50 | break; |
51 | max -= sizeof(unsigned long); |
52 | unsafe_get_user(c, (unsigned long __user *)(src+res), efault); |
53 | } |
54 | res -= align; |
55 | |
56 | /* |
57 | * Uhhuh. We hit 'max'. But was that the user-specified maximum |
58 | * too? If so, return the marker for "too long". |
59 | */ |
60 | if (res >= count) |
61 | return count+1; |
62 | |
63 | /* |
64 | * Nope: we hit the address space limit, and we still had more |
65 | * characters the caller would have wanted. That's 0. |
66 | */ |
67 | efault: |
68 | return 0; |
69 | } |
70 | |
71 | /** |
72 | * strnlen_user: - Get the size of a user string INCLUDING final NUL. |
73 | * @str: The string to measure. |
74 | * @count: Maximum count (including NUL character) |
75 | * |
76 | * Context: User context only. This function may sleep if pagefaults are |
77 | * enabled. |
78 | * |
79 | * Get the size of a NUL-terminated string in user space. |
80 | * |
81 | * Returns the size of the string INCLUDING the terminating NUL. |
82 | * If the string is too long, returns a number larger than @count. User |
83 | * has to check the return value against "> count". |
84 | * On exception (or invalid count), returns 0. |
85 | * |
86 | * NOTE! You should basically never use this function. There is |
87 | * almost never any valid case for using the length of a user space |
88 | * string, since the string can be changed at any time by other |
89 | * threads. Use "strncpy_from_user()" instead to get a stable copy |
90 | * of the string. |
91 | */ |
92 | long strnlen_user(const char __user *str, long count) |
93 | { |
94 | unsigned long max_addr, src_addr; |
95 | |
96 | if (unlikely(count <= 0)) |
97 | return 0; |
98 | |
99 | max_addr = TASK_SIZE_MAX; |
100 | src_addr = (unsigned long)untagged_addr(str); |
101 | if (likely(src_addr < max_addr)) { |
102 | unsigned long max = max_addr - src_addr; |
103 | long retval; |
104 | |
105 | /* |
106 | * Truncate 'max' to the user-specified limit, so that |
107 | * we only have one limit we need to check in the loop |
108 | */ |
109 | if (max > count) |
110 | max = count; |
111 | |
112 | if (user_read_access_begin(str, max)) { |
113 | retval = do_strnlen_user(src: str, count, max); |
114 | user_read_access_end(); |
115 | return retval; |
116 | } |
117 | } |
118 | return 0; |
119 | } |
120 | EXPORT_SYMBOL(strnlen_user); |
121 | |