path: root/lib/strncpy_from_user.c (plain)
blob: 5a07f19059c3637c33413e66586201bb629af93a

1	#include <linux/compiler.h>
2	#include <linux/export.h>
3	#include <linux/kasan-checks.h>
4	#include <linux/thread_info.h>
5	#include <linux/uaccess.h>
6	#include <linux/kernel.h>
7	#include <linux/errno.h>
8
9	#include <asm/byteorder.h>
10	#include <asm/word-at-a-time.h>
11
12	#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
13	#define IS_UNALIGNED(src, dst) 0
14	#else
15	#define IS_UNALIGNED(src, dst) \
16	(((long) dst | (long) src) & (sizeof(long) - 1))
17	#endif
18
19	/*
20	* Do a strncpy, return length of string without final '\0'.
21	* 'count' is the user-supplied count (return 'count' if we
22	* hit it), 'max' is the address space maximum (and we return
23	* -EFAULT if we hit it).
24	*/
25	static inline long do_strncpy_from_user(char *dst, const char __user *src,
26	unsigned long count, unsigned long max)
27	{
28	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
29	unsigned long res = 0;
30
31	/*
32	* Truncate 'max' to the user-specified limit, so that
33	* we only have one limit we need to check in the loop
34	*/
35	if (max > count)
36	max = count;
37
38	if (IS_UNALIGNED(src, dst))
39	goto byte_at_a_time;
40
41	while (max >= sizeof(unsigned long)) {
42	unsigned long c, data;
43
44	/* Fall back to byte-at-a-time if we get a page fault */
45	unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
46
47	*(unsigned long *)(dst+res) = c;
48	if (has_zero(c, &data, &constants)) {
49	data = prep_zero_mask(c, data, &constants);
50	data = create_zero_mask(data);
51	return res + find_zero(data);
52	}
53	res += sizeof(unsigned long);
54	max -= sizeof(unsigned long);
55	}
56
57	byte_at_a_time:
58	while (max) {
59	char c;
60
61	unsafe_get_user(c,src+res, efault);
62	dst[res] = c;
63	if (!c)
64	return res;
65	res++;
66	max--;
67	}
68
69	/*
70	* Uhhuh. We hit 'max'. But was that the user-specified maximum
71	* too? If so, that's ok - we got as much as the user asked for.
72	*/
73	if (res >= count)
74	return res;
75
76	/*
77	* Nope: we hit the address space limit, and we still had more
78	* characters the caller would have wanted. That's an EFAULT.
79	*/
80	efault:
81	return -EFAULT;
82	}
83
84	/**
85	* strncpy_from_user: - Copy a NUL terminated string from userspace.
86	* @dst: Destination address, in kernel space. This buffer must be at
87	* least @count bytes long.
88	* @src: Source address, in user space.
89	* @count: Maximum number of bytes to copy, including the trailing NUL.
90	*
91	* Copies a NUL-terminated string from userspace to kernel space.
92	*
93	* On success, returns the length of the string (not including the trailing
94	* NUL).
95	*
96	* If access to userspace fails, returns -EFAULT (some data may have been
97	* copied).
98	*
99	* If @count is smaller than the length of the string, copies @count bytes
100	* and returns @count.
101	*/
102	long strncpy_from_user(char *dst, const char __user *src, long count)
103	{
104	unsigned long max_addr, src_addr;
105
106	if (unlikely(count <= 0))
107	return 0;
108
109	max_addr = user_addr_max();
110	src_addr = (unsigned long)src;
111	if (likely(src_addr < max_addr)) {
112	unsigned long max = max_addr - src_addr;
113	long retval;
114
115	kasan_check_write(dst, count);
116	check_object_size(dst, count, false);
117	user_access_begin();
118	retval = do_strncpy_from_user(dst, src, count, max);
119	user_access_end();
120	return retval;
121	}
122	return -EFAULT;
123	}
124	EXPORT_SYMBOL(strncpy_from_user);
125