lib/strnlen_user.c

Source file repositories/reference/linux-study-clean/lib/strnlen_user.c

File Facts

System
Linux kernel
Corpus path
lib/strnlen_user.c
Extension
.c
Size
3626 bytes
Lines
130
Domain
Kernel Services
Bucket
lib
Inferred role
Kernel Services: exported/initcall integration point
Status
integration implementation candidate

Why This File Exists

Shared kernel service surface used by multiple subsystems, including helpers, cryptography, virtualization support, and async I/O infrastructure.

Dependency Surface

Detected Declarations

Annotated Snippet

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;
}

/**
 * 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;

	if (can_do_masked_user_access()) {
		long retval;

		str = masked_user_read_access_begin(str);
		retval = do_strnlen_user(str, count, count);
		user_read_access_end();
		return retval;
	}

	max_addr = TASK_SIZE_MAX;
	src_addr = (unsigned long)untagged_addr(str);
	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(str, max)) {
			retval = do_strnlen_user(str, count, max);
			user_read_access_end();
			return retval;
		}
	}
	return 0;
}
EXPORT_SYMBOL(strnlen_user);

Annotation

Implementation Notes