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authorLinus Torvalds <torvalds@linux-foundation.org>2012-05-26 14:06:38 -0400
committerLinus Torvalds <torvalds@linux-foundation.org>2012-05-26 14:33:53 -0400
commita08c5356a3aaf638c41897ae4169de18db89595e (patch)
treefe0d1cb48a26cc000c199d484a139799d559a178 /lib/strnlen_user.c
parent36126f8f2ed8168eb13aa0662b9b9585cba100a9 (diff)
lib: add generic strnlen_user() function
This adds a new generic optimized strnlen_user() function that uses the <asm/word-at-a-time.h> infrastructure to portably do efficient string handling. In many ways, strnlen is much simpler than strncpy, and in particular we can always pre-align the words we load from memory. That means that all the worries about alignment etc are a non-issue, so this one can easily be used on any architecture. You obviously do have to do the appropriate word-at-a-time.h macros. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib/strnlen_user.c')
-rw-r--r--lib/strnlen_user.c138
1 files changed, 138 insertions, 0 deletions
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
new file mode 100644
index 000000000000..90900ecfeb54
--- /dev/null
+++ b/lib/strnlen_user.c
@@ -0,0 +1,138 @@
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 << 8*(7-(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);