linux/lib/strnlen_user.c

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

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

/*
 * 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 __always_inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
{ … }

/**
 * 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)
{ … }
EXPORT_SYMBOL(…);