xref: /openbmc/linux/lib/strncpy_from_user.c (revision ca55b2fe)
1 #include <linux/compiler.h>
2 #include <linux/export.h>
3 #include <linux/uaccess.h>
4 #include <linux/kernel.h>
5 #include <linux/errno.h>
6 
7 #include <asm/byteorder.h>
8 #include <asm/word-at-a-time.h>
9 
10 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
11 #define IS_UNALIGNED(src, dst)	0
12 #else
13 #define IS_UNALIGNED(src, dst)	\
14 	(((long) dst | (long) src) & (sizeof(long) - 1))
15 #endif
16 
17 /*
18  * Do a strncpy, return length of string without final '\0'.
19  * 'count' is the user-supplied count (return 'count' if we
20  * hit it), 'max' is the address space maximum (and we return
21  * -EFAULT if we hit it).
22  */
23 static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
24 {
25 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
26 	long res = 0;
27 
28 	/*
29 	 * Truncate 'max' to the user-specified limit, so that
30 	 * we only have one limit we need to check in the loop
31 	 */
32 	if (max > count)
33 		max = count;
34 
35 	if (IS_UNALIGNED(src, dst))
36 		goto byte_at_a_time;
37 
38 	while (max >= sizeof(unsigned long)) {
39 		unsigned long c, data;
40 
41 		/* Fall back to byte-at-a-time if we get a page fault */
42 		if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
43 			break;
44 		*(unsigned long *)(dst+res) = c;
45 		if (has_zero(c, &data, &constants)) {
46 			data = prep_zero_mask(c, data, &constants);
47 			data = create_zero_mask(data);
48 			return res + find_zero(data);
49 		}
50 		res += sizeof(unsigned long);
51 		max -= sizeof(unsigned long);
52 	}
53 
54 byte_at_a_time:
55 	while (max) {
56 		char c;
57 
58 		if (unlikely(__get_user(c,src+res)))
59 			return -EFAULT;
60 		dst[res] = c;
61 		if (!c)
62 			return res;
63 		res++;
64 		max--;
65 	}
66 
67 	/*
68 	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
69 	 * too? If so, that's ok - we got as much as the user asked for.
70 	 */
71 	if (res >= count)
72 		return res;
73 
74 	/*
75 	 * Nope: we hit the address space limit, and we still had more
76 	 * characters the caller would have wanted. That's an EFAULT.
77 	 */
78 	return -EFAULT;
79 }
80 
81 /**
82  * strncpy_from_user: - Copy a NUL terminated string from userspace.
83  * @dst:   Destination address, in kernel space.  This buffer must be at
84  *         least @count bytes long.
85  * @src:   Source address, in user space.
86  * @count: Maximum number of bytes to copy, including the trailing NUL.
87  *
88  * Copies a NUL-terminated string from userspace to kernel space.
89  *
90  * On success, returns the length of the string (not including the trailing
91  * NUL).
92  *
93  * If access to userspace fails, returns -EFAULT (some data may have been
94  * copied).
95  *
96  * If @count is smaller than the length of the string, copies @count bytes
97  * and returns @count.
98  */
99 long strncpy_from_user(char *dst, const char __user *src, long count)
100 {
101 	unsigned long max_addr, src_addr;
102 
103 	if (unlikely(count <= 0))
104 		return 0;
105 
106 	max_addr = user_addr_max();
107 	src_addr = (unsigned long)src;
108 	if (likely(src_addr < max_addr)) {
109 		unsigned long max = max_addr - src_addr;
110 		return do_strncpy_from_user(dst, src, count, max);
111 	}
112 	return -EFAULT;
113 }
114 EXPORT_SYMBOL(strncpy_from_user);
115