How to Implement IOCTLs in Golang and Avoid \'Invalid Argument\' Errors?

IOCTL Implementation in Golang

When porting userspace code from C to Golang, handling IOCTL can be a challenge. This article addresses a specific issue encountered while attempting to convert the following C code:

#define MAJOR_NUM 100
#define IOCTL_MBOX_PROPERTY _IOWR(MAJOR_NUM, 0, char *)
static int mbox_property(int file_desc, void *buf) {
    return ioctl(file_desc, IOCTL_MBOX_PROPERTY, buf);
}

The Golang equivalent:

func mBoxProperty(f *os.File, buf [256]int64) {
    err := Ioctl(f.Fd(), IOWR(100, 0, 8), uintptr(unsafe.Pointer(&buf[0])))
}

func Ioctl(fd, op, arg uintptr) error {
    _, _, ep := syscall.Syscall(syscall.SYS_IOCTL, fd, op, arg)
    if ep != 0 {
        return syscall.Errno(ep)
    }
    return nil
}

func IOWR(t, nr, size uintptr) uintptr {
    return IOC(IocRead|IocWrite, t, nr, size)
}

func IOC(dir, t, nr, size uintptr) uintptr {
    return (dir << IocDirshift) | (t << IocTypeshift) | (nr << IocNrshift) | (size << IocSizeshift)
}

Running this code results in an "invalid argument" error. The issue lies in how IOCTL() is being called.

A Wrapper Solution

The "golang.org/x/sys/unix" package provides ioctl(2) wrappers. For this specific case, the unix.IoctlSetInt wrapper can be employed.

Memory Considerations

It's important to note that IoctlSetInt and other IOCTL wrappers hand over a small memory buffer to the kernel. Go's garbage collector doesn't recognize this, potentially moving or deallocating the memory while it's still being used by the kernel.

To address this, consider using a C extension via cgo to create a buffer with malloc. This ensures the buffer won't be moved or freed, avoiding potential memory issues.

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