[luau] kernel module, /proc filesystem, and mmap

[Jimen Ching]

Hi all,

I am trying to write a kernel module that shares a global variable between
a userland application and the module.  In the kernel module, I use this:

	mode = ...; /* read/write for all... */
	entry = create_proc_entry("jimen", mode, NULL);
	entry->proc_fops = my_fops;

Where my_fops contains an mmap entry like so...

	int var __attribute__ ((aligned (PAGE_SIZE)));

	int
	my_mmap(struct file *filp, struct vm_area_struct *vma)
	{
		vma->vm_flags |= VM_RESERVED;
		if (remap_page_range(vma->vm_start, (unsigned long)&var,
			vma->vm_end - vma->vm_start, vma->vm_page_prot))
			return -EAGAIN;
		return 0;
	}

	struct file_operations my_fops =
	{
	mmap: my_mmap,
	};

Then from an application, I do this:

	int fd = open("/proc/jimen", O_RDWR);
	int *ptr = mmap(NULL, 4, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

The mmap system call seems to work without error.  'ptr' seems to have a
valid pointer.  But '*ptr' is -1.  I can't modify it or anything.  If I
add in MAP_ANONYMOUS to the flags, I am getting valid values and able to
modify '*ptr'.  But if the module modifies 'var', '*ptr' does not reflect
the change.  This kind of makes sense since MAP_ANONYMOUS means there is
no underlying file.  But without it, I'm not even getting a valid pointer.

I don't think the application's mmap call is at fault.  I think the
problem is with the kernel module.  Anyone know if I can mmap a kernel
module variable using a /proc filesystem entry?  All of the documentation
I found on the net implies this should work with a device in /dev.  But I
don't want to deal with mknod, so that is why I'm using /proc.  I didn't
read anything that suggests I can't use /proc instead of /dev.  Has anyone
done this before?

Any help is greatly appreciated.

--jc
-- 
Jimen Ching (WH6BRR)      jching at flex.com     wh6brr at uhm.ampr.org