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lib/strnlen_user.c
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// SPDX-License-Identifier: GPL-2.0 |
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#include <linux/kernel.h> #include <linux/export.h> #include <linux/uaccess.h> |
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#include <linux/mm.h> |
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#include <linux/bitops.h> |
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#include <asm/word-at-a-time.h> |
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/* * Do a strnlen, return length of string *with* final '\0'. * 'count' is the user-supplied count, while 'max' is the * address space maximum. * * Return 0 for exceptions (which includes hitting the address * space maximum), or 'count+1' if hitting the user-supplied * maximum count. * * NOTE! We can sometimes overshoot the user-supplied maximum * if it fits in a aligned 'long'. The caller needs to check * the return value against "> max". */ static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max) { const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS; |
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unsigned long align, res = 0; |
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unsigned long c; /* * Truncate 'max' to the user-specified limit, so that * we only have one limit we need to check in the loop */ if (max > count) max = count; /* * Do everything aligned. But that means that we * need to also expand the maximum.. */ |
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align = (sizeof(unsigned long) - 1) & (unsigned long)src; |
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src -= align; max += align; |
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unsafe_get_user(c, (unsigned long __user *)src, efault); |
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c |= aligned_byte_mask(align); for (;;) { unsigned long data; if (has_zero(c, &data, &constants)) { data = prep_zero_mask(c, data, &constants); data = create_zero_mask(data); return res + find_zero(data) + 1 - align; } res += sizeof(unsigned long); |
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/* We already handled 'unsigned long' bytes. Did we do it all ? */ if (unlikely(max <= sizeof(unsigned long))) |
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break; max -= sizeof(unsigned long); |
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unsafe_get_user(c, (unsigned long __user *)(src+res), efault); |
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} res -= align; /* * Uhhuh. We hit 'max'. But was that the user-specified maximum * too? If so, return the marker for "too long". */ if (res >= count) return count+1; /* * Nope: we hit the address space limit, and we still had more * characters the caller would have wanted. That's 0. */ |
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efault: |
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return 0; } /** * strnlen_user: - Get the size of a user string INCLUDING final NUL. * @str: The string to measure. * @count: Maximum count (including NUL character) * |
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* Context: User context only. This function may sleep if pagefaults are * enabled. |
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* * Get the size of a NUL-terminated string in user space. * * Returns the size of the string INCLUDING the terminating NUL. |
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* If the string is too long, returns a number larger than @count. User * has to check the return value against "> count". |
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* On exception (or invalid count), returns 0. |
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* * NOTE! You should basically never use this function. There is * almost never any valid case for using the length of a user space * string, since the string can be changed at any time by other * threads. Use "strncpy_from_user()" instead to get a stable copy * of the string. |
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*/ long strnlen_user(const char __user *str, long count) { unsigned long max_addr, src_addr; if (unlikely(count <= 0)) return 0; max_addr = user_addr_max(); |
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src_addr = (unsigned long)untagged_addr(str); |
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if (likely(src_addr < max_addr)) { unsigned long max = max_addr - src_addr; |
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long retval; |
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if (user_access_begin(str, max)) { retval = do_strnlen_user(str, count, max); user_access_end(); return retval; } |
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} return 0; } EXPORT_SYMBOL(strnlen_user); |