On Tue, Mar 05, 2019 at 10:58:01PM +0900, Masami Hiramatsu wrote:

> Could you tell me why WARN_ON_ONCE(!in_task()) is needed in access_ok()?

That came from here:

  lkml.kernel.org/r/20190225145240.GB32534@hirez.programming.kicks-ass.net

Because in-irq usage is dodgy, since we don't actually know what mm or
ds it loaded.

> > I dislike that whole KERNEL_DS thing, but obviously that's not something
> > that's going away.
> > 
> > Would something like:
> > 
> > 	WARN_ON_ONCE(!(in_task || segment_eq(get_fs(), USER_DS)))
> > 
> > Work? Then we allow KERNEL_DS in task context, but for interrupt and
> > others require USER_DS.
> 
> But what would this mean? I can't understand why we limit using
> access_ok() so strictly and narrow the cases.

Because it's been a source of bugs. Any sanity checking we can put in
seems like a good thing at this point.