Re: [PATCH 03/11] fs: add new read_uptr and write_uptr file operations

[Luis Chamberlain]

On Sat, Jun 27, 2020 at 09:33:03AM -0700, Linus Torvalds wrote:
> The real problem with
> "set_fs()" has been that we've occasionally had bugs where we ended up
> running odd paths that we really didn't _intend_ to run with kernel
> pointers. The classic example is the SCSI "write as ioctl" example,
> where a write to a SCSI generic device would do various odd things and
> follow pointers and what-not. Then you get into real trouble when
> "splice()" ends up truiong to write a kernel buffer, and because of
> "set_fs()" suddenly the sg code started accessing kernel memory
> willy-nilly.

So the semantics of this interface can create chaos fast if not used
carefully and conservatively.

Christoph, it would be great if you're future series can include some
version of a verbiage for the motivation for the culling of set_fs().
Maybe it was just me, but the original motivation wasn't clear at first
and took some thread digging to get it.

  Luis