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Linux/lib/strncpy_from_user.c

  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, long count, unsigned long max)
 26 {
 27         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 28         long res = 0;
 29 
 30         /*
 31          * Truncate 'max' to the user-specified limit, so that
 32          * we only have one limit we need to check in the loop
 33          */
 34         if (max > count)
 35                 max = count;
 36 
 37         if (IS_UNALIGNED(src, dst))
 38                 goto byte_at_a_time;
 39 
 40         while (max >= sizeof(unsigned long)) {
 41                 unsigned long c, data;
 42 
 43                 /* Fall back to byte-at-a-time if we get a page fault */
 44                 unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
 45 
 46                 *(unsigned long *)(dst+res) = c;
 47                 if (has_zero(c, &data, &constants)) {
 48                         data = prep_zero_mask(c, data, &constants);
 49                         data = create_zero_mask(data);
 50                         return res + find_zero(data);
 51                 }
 52                 res += sizeof(unsigned long);
 53                 max -= sizeof(unsigned long);
 54         }
 55 
 56 byte_at_a_time:
 57         while (max) {
 58                 char c;
 59 
 60                 unsafe_get_user(c,src+res, efault);
 61                 dst[res] = c;
 62                 if (!c)
 63                         return res;
 64                 res++;
 65                 max--;
 66         }
 67 
 68         /*
 69          * Uhhuh. We hit 'max'. But was that the user-specified maximum
 70          * too? If so, that's ok - we got as much as the user asked for.
 71          */
 72         if (res >= count)
 73                 return res;
 74 
 75         /*
 76          * Nope: we hit the address space limit, and we still had more
 77          * characters the caller would have wanted. That's an EFAULT.
 78          */
 79 efault:
 80         return -EFAULT;
 81 }
 82 
 83 /**
 84  * strncpy_from_user: - Copy a NUL terminated string from userspace.
 85  * @dst:   Destination address, in kernel space.  This buffer must be at
 86  *         least @count bytes long.
 87  * @src:   Source address, in user space.
 88  * @count: Maximum number of bytes to copy, including the trailing NUL.
 89  *
 90  * Copies a NUL-terminated string from userspace to kernel space.
 91  *
 92  * On success, returns the length of the string (not including the trailing
 93  * NUL).
 94  *
 95  * If access to userspace fails, returns -EFAULT (some data may have been
 96  * copied).
 97  *
 98  * If @count is smaller than the length of the string, copies @count bytes
 99  * and returns @count.
100  */
101 long strncpy_from_user(char *dst, const char __user *src, long count)
102 {
103         unsigned long max_addr, src_addr;
104 
105         if (unlikely(count <= 0))
106                 return 0;
107 
108         max_addr = user_addr_max();
109         src_addr = (unsigned long)src;
110         if (likely(src_addr < max_addr)) {
111                 unsigned long max = max_addr - src_addr;
112                 long retval;
113 
114                 kasan_check_write(dst, count);
115                 check_object_size(dst, count, false);
116                 user_access_begin();
117                 retval = do_strncpy_from_user(dst, src, count, max);
118                 user_access_end();
119                 return retval;
120         }
121         return -EFAULT;
122 }
123 EXPORT_SYMBOL(strncpy_from_user);
124 

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