# Linux kernel misc notes
###### tags: `Linux kernel`
{%hackmd theme-dark %}

## Copy from user

static __always_inline unsigned long __must_check
copy_from_user(void *to, const void __user *from, unsigned long n)
{
	if (check_copy_size(to, n, false))
		n = _copy_from_user(to, from, n);
	return n;
}

static __always_inline __must_check bool
check_copy_size(const void *addr, size_t bytes, bool is_source)
{
	int sz = __builtin_object_size(addr, 0);
	if (unlikely(sz >= 0 && sz < bytes)) {
		if (!__builtin_constant_p(bytes))
			copy_overflow(sz, bytes);
		else if (is_source)
			__bad_copy_from();
		else
			__bad_copy_to();
		return false;
	}
	if (WARN_ON_ONCE(bytes > INT_MAX))
		return false;
	check_object_size(addr, bytes, is_source);
	return true;
}

static inline __must_check unsigned long
_copy_from_user(void *to, const void __user *from, unsigned long n)
{
	unsigned long res = n;
	might_fault();
	if (!should_fail_usercopy() && likely(access_ok(from, n))) {
		instrument_copy_from_user_before(to, from, n);
		res = raw_copy_from_user(to, from, n);
		instrument_copy_from_user_after(to, from, n, res);
	}
	if (unlikely(res))
		memset(to + (n - res), 0, res);
	return res;
}