1 // SPDX-License-Identifier: GPL-2.0 2 #include <linux/kernel.h> 3 #include <linux/export.h> 4 #include <linux/uaccess.h> 5 6 #include <asm/word-at-a-time.h> 7 8 /* Set bits in the first 'n' bytes when loaded from memory */ 9 #ifdef __LITTLE_ENDIAN 10 # define aligned_byte_mask(n) ((1ul << 8*(n))-1) 11 #else 12 # define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n))) 13 #endif 14 15 /* 16 * Do a strnlen, return length of string *with* final '\0'. 17 * 'count' is the user-supplied count, while 'max' is the 18 * address space maximum. 19 * 20 * Return 0 for exceptions (which includes hitting the address 21 * space maximum), or 'count+1' if hitting the user-supplied 22 * maximum count. 23 * 24 * NOTE! We can sometimes overshoot the user-supplied maximum 25 * if it fits in a aligned 'long'. The caller needs to check 26 * the return value against "> max". 27 */ 28 static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max) 29 { 30 const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS; 31 unsigned long align, res = 0; 32 unsigned long c; 33 34 /* 35 * Truncate 'max' to the user-specified limit, so that 36 * we only have one limit we need to check in the loop 37 */ 38 if (max > count) 39 max = count; 40 41 /* 42 * Do everything aligned. But that means that we 43 * need to also expand the maximum.. 44 */ 45 align = (sizeof(unsigned long) - 1) & (unsigned long)src; 46 src -= align; 47 max += align; 48 49 unsafe_get_user(c, (unsigned long __user *)src, efault); 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 /* We already handled 'unsigned long' bytes. Did we do it all ? */ 61 if (unlikely(max <= sizeof(unsigned long))) 62 break; 63 max -= sizeof(unsigned long); 64 unsafe_get_user(c, (unsigned long __user *)(src+res), efault); 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 efault: 80 return 0; 81 } 82 83 /** 84 * strnlen_user: - Get the size of a user string INCLUDING final NUL. 85 * @str: The string to measure. 86 * @count: Maximum count (including NUL character) 87 * 88 * Context: User context only. This function may sleep if pagefaults are 89 * enabled. 90 * 91 * Get the size of a NUL-terminated string in user space. 92 * 93 * Returns the size of the string INCLUDING the terminating NUL. 94 * If the string is too long, returns a number larger than @count. User 95 * has to check the return value against "> count". 96 * On exception (or invalid count), returns 0. 97 * 98 * NOTE! You should basically never use this function. There is 99 * almost never any valid case for using the length of a user space 100 * string, since the string can be changed at any time by other 101 * threads. Use "strncpy_from_user()" instead to get a stable copy 102 * of the string. 103 */ 104 long strnlen_user(const char __user *str, long count) 105 { 106 unsigned long max_addr, src_addr; 107 108 if (unlikely(count <= 0)) 109 return 0; 110 111 max_addr = user_addr_max(); 112 src_addr = (unsigned long)str; 113 if (likely(src_addr < max_addr)) { 114 unsigned long max = max_addr - src_addr; 115 long retval; 116 117 if (user_access_begin(str, max)) { 118 retval = do_strnlen_user(str, count, max); 119 user_access_end(); 120 return retval; 121 } 122 } 123 return 0; 124 } 125 EXPORT_SYMBOL(strnlen_user); 126