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// SPDX-License-Identifier: GPL-2.0
#include <linux/compiler.h>
#include <linux/export.h>
#include <linux/fault-inject-usercopy.h>
#include <linux/kasan-checks.h>
#include <linux/thread_info.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/mm.h>
#include <asm/byteorder.h>
#include <asm/word-at-a-time.h>
#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
#define IS_UNALIGNED(src, dst) 0
#else
#define IS_UNALIGNED(src, dst) \
(((long) dst | (long) src) & (sizeof(long) - 1))
#endif
/*
* Do a strncpy, return length of string without final '\0'.
* 'count' is the user-supplied count (return 'count' if we
* hit it), 'max' is the address space maximum (and we return
* -EFAULT if we hit it).
*/
static __always_inline long do_strncpy_from_user(char *dst, const char __user *src,
unsigned long count, unsigned long max)
{
const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
unsigned long res = 0;
if (IS_UNALIGNED(src, dst))
goto byte_at_a_time;
while (max >= sizeof(unsigned long)) {
unsigned long c, data, mask;
/* Fall back to byte-at-a-time if we get a page fault */
unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
/*
* Note that we mask out the bytes following the NUL. This is
* important to do because string oblivious code may read past
* the NUL. For those routines, we don't want to give them
* potentially random bytes after the NUL in `src`.
*
* One example of such code is BPF map keys. BPF treats map keys
* as an opaque set of bytes. Without the post-NUL mask, any BPF
* maps keyed by strings returned from strncpy_from_user() may
* have multiple entries for semantically identical strings.
*/
if (has_zero(c, &data, &constants)) {
data = prep_zero_mask(c, data, &constants);
data = create_zero_mask(data);
mask = zero_bytemask(data);
*(unsigned long *)(dst+res) = c & mask;
return res + find_zero(data);
}
*(unsigned long *)(dst+res) = c;
res += sizeof(unsigned long);
max -= sizeof(unsigned long);
}
byte_at_a_time:
while (max) {
char c;
unsafe_get_user(c,src+res, efault);
dst[res] = c;
if (!c)
return res;
res++;
max--;
}
/*
* Uhhuh. We hit 'max'. But was that the user-specified maximum
* too? If so, that's ok - we got as much as the user asked for.
*/
if (res >= count)
return res;
/*
* Nope: we hit the address space limit, and we still had more
* characters the caller would have wanted. That's an EFAULT.
*/
efault:
return -EFAULT;
}
/**
* strncpy_from_user: - Copy a NUL terminated string from userspace.
* @dst: Destination address, in kernel space. This buffer must be at
* least @count bytes long.
* @src: Source address, in user space.
* @count: Maximum number of bytes to copy, including the trailing NUL.
*
* Copies a NUL-terminated string from userspace to kernel space.
*
* On success, returns the length of the string (not including the trailing
* NUL).
*
* If access to userspace fails, returns -EFAULT (some data may have been
* copied).
*
* If @count is smaller than the length of the string, copies @count bytes
* and returns @count.
*/
long strncpy_from_user(char *dst, const char __user *src, long count)
{
unsigned long max_addr, src_addr;
might_fault();
if (should_fail_usercopy())
return -EFAULT;
if (unlikely(count <= 0))
return 0;
kasan_check_write(dst, count);
check_object_size(dst, count, false);
if (can_do_masked_user_access()) {
long retval;
src = masked_user_access_begin(src);
retval = do_strncpy_from_user(dst, src, count, count);
user_read_access_end();
return retval;
}
max_addr = TASK_SIZE_MAX;
src_addr = (unsigned long)untagged_addr(src);
if (likely(src_addr < max_addr)) {
unsigned long max = max_addr - src_addr;
long retval;
/*
* 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;
if (user_read_access_begin(src, max)) {
retval = do_strncpy_from_user(dst, src, count, max);
user_read_access_end();
return retval;
}
}
return -EFAULT;
}
EXPORT_SYMBOL(strncpy_from_user);
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