Re: [PATCH v4 1/6] fs: add write-stream management ioctls

[Christoph Hellwig <hch@lst.de>]

On Thu, Jul 30, 2026 at 10:22:11AM -0700, Darrick J. Wong wrote:
> "The third argument is an untyped pointer to memory.  It's traditionally
> char  *argp (from the days before void * was valid C), and will be so
> named for this discussion."

That documentation is wrong or at least very incomplete.  A pointer
is the typical use, but there are plenty of ioctls passing scalar
values <= sizeof(long), going all the way back to early UNIX days.

> You can also see this reflected in the discussion of the R/W/WR variants
> of _IO:
> https://docs.kernel.org/driver-api/ioctl.html

_IO* are optional helpers, and plenty especially older ioctls are
defined without using them.

> Granted your kernel code can treat that argp as an integer and not a
> __user pointer if it wants to, but that goes against most ioctl
> implementations.  Further, subsystems that marshal ioctl information for
> passing through to another layer (e.g. fuse) assume that the third
> argument is a pointer and that it can copy sizeof(data_type) bytes at
> that address in and out of the kernel as part of marshalling.

That is not a good assumption and things will break.