1 // SPDX-License-Identifier: GPL-2.0 2 #include <linux/kernel.h> 3 #include <linux/export.h> 4 #include <linux/uaccess.h> 5 #include <linux/mm.h> 6 7 #include <asm/word-at-a-time.h> 8 9 /* Set bits in the first 'n' bytes when loaded from memory */ 10 #ifdef __LITTLE_ENDIAN 11 # define aligned_byte_mask(n) ((1ul << 8*(n))-1) 12 #else 13 # define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n))) 14 #endif 15 16 /* 17 * Do a strnlen, return length of string *with* final '\0'. 18 * 'count' is the user-supplied count, while 'max' is the 19 * address space maximum. 20 * 21 * Return 0 for exceptions (which includes hitting the address 22 * space maximum), or 'count+1' if hitting the user-supplied 23 * maximum count. 24 * 25 * NOTE! We can sometimes overshoot the user-supplied maximum 26 * if it fits in a aligned 'long'. The caller needs to check 27 * the return value against "> max". 28 */ 29 static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max) 30 { 31 const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS; 32 unsigned long align, res = 0; 33 unsigned long c; 34 35 /* 36 * Truncate 'max' to the user-specified limit, so that 37 * we only have one limit we need to check in the loop 38 */ 39 if (max > count) 40 max = count; 41 42 /* 43 * Do everything aligned. But that means that we 44 * need to also expand the maximum.. 45 */ 46 align = (sizeof(unsigned long) - 1) & (unsigned long)src; 47 src -= align; 48 max += align; 49 50 unsafe_get_user(c, (unsigned long __user *)src, efault); 51 c |= aligned_byte_mask(align); 52 53 for (;;) { 54 unsigned long data; 55 if (has_zero(c, &data, &constants)) { 56 data = prep_zero_mask(c, data, &constants); 57 data = create_zero_mask(data); 58 return res + find_zero(data) + 1 - align; 59 } 60 res += sizeof(unsigned long); 61 /* We already handled 'unsigned long' bytes. Did we do it all ? */ 62 if (unlikely(max <= sizeof(unsigned long))) 63 break; 64 max -= sizeof(unsigned long); 65 unsafe_get_user(c, (unsigned long __user *)(src+res), efault); 66 } 67 res -= align; 68 69 /* 70 * Uhhuh. We hit 'max'. But was that the user-specified maximum 71 * too? If so, return the marker for "too long". 72 */ 73 if (res >= count) 74 return count+1; 75 76 /* 77 * Nope: we hit the address space limit, and we still had more 78 * characters the caller would have wanted. That's 0. 79 */ 80 efault: 81 return 0; 82 } 83 84 /** 85 * strnlen_user: - Get the size of a user string INCLUDING final NUL. 86 * @str: The string to measure. 87 * @count: Maximum count (including NUL character) 88 * 89 * Context: User context only. This function may sleep if pagefaults are 90 * enabled. 91 * 92 * Get the size of a NUL-terminated string in user space. 93 * 94 * Returns the size of the string INCLUDING the terminating NUL. 95 * If the string is too long, returns a number larger than @count. User 96 * has to check the return value against "> count". 97 * On exception (or invalid count), returns 0. 98 * 99 * NOTE! You should basically never use this function. There is 100 * almost never any valid case for using the length of a user space 101 * string, since the string can be changed at any time by other 102 * threads. Use "strncpy_from_user()" instead to get a stable copy 103 * of the string. 104 */ 105 long strnlen_user(const char __user *str, long count) 106 { 107 unsigned long max_addr, src_addr; 108 109 if (unlikely(count <= 0)) 110 return 0; 111 112 max_addr = user_addr_max(); 113 src_addr = (unsigned long)untagged_addr(str); 114 if (likely(src_addr < max_addr)) { 115 unsigned long max = max_addr - src_addr; 116 long retval; 117 118 if (user_access_begin(str, max)) { 119 retval = do_strnlen_user(str, count, max); 120 user_access_end(); 121 return retval; 122 } 123 } 124 return 0; 125 } 126 EXPORT_SYMBOL(strnlen_user); 127