lib/strncpy_from_user.c

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