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 * Do everything aligned. But that means that we 31 * need to also expand the maximum.. 32 */ 33 align = (sizeof(unsigned long) - 1) & (unsigned long)src; 34 src -= align; 35 max += align; 36 37 unsafe_get_user(c, (unsigned long __user *)src, efault); 38 c |= aligned_byte_mask(align); 39 40 for (;;) { 41 unsigned long data; 42 if (has_zero(c, &data, &constants)) { 43 data = prep_zero_mask(c, data, &constants); 44 data = create_zero_mask(data); 45 return res + find_zero(data) + 1 - align; 46 } 47 res += sizeof(unsigned long); 48 /* We already handled 'unsigned long' bytes. Did we do it all ? */ 49 if (unlikely(max <= sizeof(unsigned long))) 50 break; 51 max -= sizeof(unsigned long); 52 unsafe_get_user(c, (unsigned long __user *)(src+res), efault); 53 } 54 res -= align; 55 56 /* 57 * Uhhuh. We hit 'max'. But was that the user-specified maximum 58 * too? If so, return the marker for "too long". 59 */ 60 if (res >= count) 61 return count+1; 62 63 /* 64 * Nope: we hit the address space limit, and we still had more 65 * characters the caller would have wanted. That's 0. 66 */ 67 efault: 68 return 0; 69 } 70 71 /** 72 * strnlen_user: - Get the size of a user string INCLUDING final NUL. 73 * @str: The string to measure. 74 * @count: Maximum count (including NUL character) 75 * 76 * Context: User context only. This function may sleep if pagefaults are 77 * enabled. 78 * 79 * Get the size of a NUL-terminated string in user space. 80 * 81 * Returns the size of the string INCLUDING the terminating NUL. 82 * If the string is too long, returns a number larger than @count. User 83 * has to check the return value against "> count". 84 * On exception (or invalid count), returns 0. 85 * 86 * NOTE! You should basically never use this function. There is 87 * almost never any valid case for using the length of a user space 88 * string, since the string can be changed at any time by other 89 * threads. Use "strncpy_from_user()" instead to get a stable copy 90 * of the string. 91 */ 92 long strnlen_user(const char __user *str, long count) 93 { 94 unsigned long max_addr, src_addr; 95 96 if (unlikely(count <= 0)) 97 return 0; 98 99 max_addr = TASK_SIZE_MAX; 100 src_addr = (unsigned long)untagged_addr(str); 101 if (likely(src_addr < max_addr)) { 102 unsigned long max = max_addr - src_addr; 103 long retval; 104 105 /* 106 * Truncate 'max' to the user-specified limit, so that 107 * we only have one limit we need to check in the loop 108 */ 109 if (max > count) 110 max = count; 111 112 if (user_read_access_begin(str, max)) { 113 retval = do_strnlen_user(str, count, max); 114 user_read_access_end(); 115 return retval; 116 } 117 } 118 return 0; 119 } 120 EXPORT_SYMBOL(strnlen_user); 121