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