How to Resolve \'Invalid Argument\' Errors When Implementing `ioctl()` in Go?

IOCTL Implementation in Go

Question:

In translating C code involving ioctl() to Go, an issue arises when converting 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){
   int ret_val = ioctl(file_desc, IOCTL_MBOX_PROPERTY, buf);
   return ret_val;
}

The corresponding Go code:

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

        if err != nil {
                log.Fatalln("mBoxProperty() : ", err)
        }

}

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)
}

causes an "invalid argument" error. What is the potential issue and how can it be resolved?

Answer:

To rectify the issue, consider the following:

• Utilize wrappers for ioctl() in "golang.org/x/sys/unix". Particularly, unix.IoctlSetInt might be suitable for your use case.
• Exercise caution when handing over small memory buffers to the kernel. Go's garbage collector can free objects that it deems unused, potentially moving them as well. This can create discrepancies between the kernel's operations and the Go code's expectations.
• Envisage writing a modest extension using cgo to malloc() an appropriate buffer and passing it to ioctl. Memory allocated via malloc() is not subject to garbage collection, hence mitigating the aforementioned issues.

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