System Call Issues

The following sections discuss system call issues.

What does EOVERFLOW mean?

The EOVERFLOW return value is returned from a system call whenever one or more fields of the data structure used to pass information out of the kernel is too small to hold the value.

A number of 32-bit system calls now return EOVERFLOW when faced with large objects on the 64-bit kernel. While this was already true when dealing with large files, the fact that daddr_t, dev_t, time_t, and its derivative types struct timeval and timespec_t now contain 64-bit quantities might allow more EOVERFLOW return values to be observed by 32-bit applications.

Beware ioctl(2)

Some ioctl(2) calls have been rather poorly specified in the past. Unfortunately, ioctl(2) is completely insulated from compile-time type checking; therefore it can be a source of bugs that are difficult to track down.

Consider two ioctl(2) calls - one that manipulates a pointer to 32-bit quantity (IOP32), the other that manipulates a pointer to a long quantity (IOPLONG).

The following code sample works as part of a 32-bit application:

Example 6-1

	int a, d;
	long b;
	...
	if (ioctl(d, IOP32, ) == -1)
			return (errno);
	if (ioctl(d, IOPLONG, ) == -1)
			return (errno);

Both ioctl(2) calls work correctly when this code fragment is compiled and run as part of a 32-bit application.

Both ioctl(2) calls also return success when this code fragment is compiled and run as a 64-bit application. However, neither ioctl(2) works correctly. The first ioctl(2) passes a container that is too big, and on a big-endian implementation, the kernel will copy in or copy out from the wrong part of the 64-bit word. Even on a little-endian implementation, the container probably contains stack garbage in the upper 32-bits. The second ioctl(2) will copy in or copy out too much, either reading an incorrect value, or corrupting adjacent variables on the user stack.