cregit-Linux how code gets into the kernel

Release 4.16 lib/strnlen_user.c

Directory: lib
// SPDX-License-Identifier: GPL-2.0
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/uaccess.h>

#include <asm/word-at-a-time.h>

/* Set bits in the first 'n' bytes when loaded from memory */

#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)

#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))

 * 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; long align, res = 0; 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.. */ align = (sizeof(long) - 1) & (unsigned long)src; src -= align; max += align; unsafe_get_user(c, (unsigned long __user *)src, efault); 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); /* We already handled 'unsigned long' bytes. Did we do it all ? */ if (unlikely(max <= sizeof(unsigned long))) break; max -= sizeof(unsigned long); unsafe_get_user(c, (unsigned long __user *)(src+res), efault); } 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. */ efault: return 0; }


Linus Torvalds22599.12%375.00%
Jan Kara20.88%125.00%

/** * strnlen_user: - Get the size of a user string INCLUDING final NUL. * @str: The string to measure. * @count: Maximum count (including NUL character) * * Context: User context only. This function may sleep if pagefaults are * enabled. * * Get the size of a NUL-terminated string in user space. * * Returns the size of the string INCLUDING the terminating NUL. * If the string is too long, returns a number larger than @count. User * has to check the return value against "> count". * On exception (or invalid count), returns 0. * * 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. */
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(); src_addr = (unsigned long)str; if (likely(src_addr < max_addr)) { unsigned long max = max_addr - src_addr; long retval; user_access_begin(); retval = do_strnlen_user(str, count, max); user_access_end(); return retval; } return 0; }


Linus Torvalds90100.00%2100.00%


Overall Contributors

Linus Torvalds35598.89%457.14%
Jan Kara20.56%114.29%
Greg Kroah-Hartman10.28%114.29%
Paul Mackerras10.28%114.29%
Directory: lib
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