Locking Memory Question

[John Baldwin]

On Wednesday, July 29, 2015 11:28:03 PM K. Macy wrote:
> >>
> >> Im not clear how I'd do that. the data being passed up from the kernel is a variable size. To use copyout I'd have to pass a
> >> pointer with a static buffer, right?
> >
> > Correct, you can pass along the size, and if it's not large enough
> > try again... Less than ideal...
> >
> >> Is there a way to malloc user space memory from within an ioctl call?
> >
> > Well, it is possible that you could do the equivalent of mmap, and pass
> > the address back along w/ a length, and leave it up to the user to
> > munmap it...  This is probably the nicest method if you the size is
> > really largely variable, and it's expensive if the userland process
> > allocated too much memory...  The down side is that this is more
> > complex to code...
> >
> 
> 
> Mach has the ability to send large "out of line messages". For smaller
> messages where it doesn't do VM tricks to avoid copying it does
> exactly this. In the receive path the kernel calls vm_allocate (which
> is essentially just a wrapper for mmap) then copies the buffer in to
> the newly allocated address space. The message itself contains the
> allocated address and size of the allocation. The receiver is expected
> to call vm_deallocate (munmap) when it's done with the data.
> 
> The implementation is mixed in with enough other code that it may not
> be a useful reference. Nonetheless, I wanted to point at that this
> isn't as strange as it might sound.

You can do this in FreeBSD by calling vm_mmap() with a NULL handle pointer
to simulate a MAP_ANON mapping.  Something like this:

	vm_mmap(&curproc->p_vmspace->vm_map, &addr, <size>, VM_PROT_READ |
	    VM_PROT_WRITE, VM_PROT_READ | VM_PROT_WRITE, MAP_SHARED, OBJT_DEFAULT,
		NULL, 0);

It's not great for a true shared memory buffer, but is fine for a one-time
copy.

-- 
John Baldwin