Re: Should linuxkpi linux_file_ioctl support flexible array copy?

On Fri, 13 Feb 2026, yi zishun wrote:

Hi,

> Hey Hackers, I want to port udmabuf from Linux to FreeBSD, but I
> encountered a problem:
> udmabuf.h declares the interface which I don't want to touch or change:
>
>        ...
>        struct udmabuf_create_list {
>                __u32 flags;
>                __u32 count;
>                struct udmabuf_create_item list[];
>        };
>        #define UDMABUF_CREATE       _IOW('u', 0x42, struct udmabuf_create)
>        #define UDMABUF_CREATE_LIST  _IOW('u', 0x43, struct
> udmabuf_create_list)
>        ...
>
> It has a flexible array and uses `_IOW`, which means the FreeBSD native
> KPI (`cdevsw` ioctl) cannot get the userspace pointer to get the items
> list.
>
> So I turned to linuxkpi, but unfortunately I found that if I define
> `_IOW`,  Linuxkpi's `linux_file_ioctl` is only allowed to get the kernel
> space pointer, just like the FreeBSD KPI. So I can't copy the flexible
> array from userspace.
>
> So I have two ways to go:
> 1.Change the command to use `_IO` instead of `_IOW` to prevent copying
> by `sys_ioctl`. This means I will change the ABI.
> 2.Try to figure out a way to empower linuxkpi to skip `sys_ioctl`
> helping to get the userspace pointer. This will be a big piece of work.
>
> Any recommendations on how to handle this gracefully? Thanks~
>
> (BTW, I'm a kernel rookie hoping to tackle the GSoC 2026 'Port or
> reimplement udmabuf' project. For context, my current WIP code is here:
> https://github.com/yizishun/drm-kmod/blob/dev-udmabuf/drivers/dma-buf/udmabuf.c
> )

Let me ask two totally different questions:

(a) is this udmabuf or u-dma-buf?
(b) what are the expected consumers for your code?

/bz

-- 
Bjoern A. Zeeb                                                     r15:7