Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Jun 21, 2021 at 01:54:56PM +0000, Al Viro wrote:
> On Tue, Jun 15, 2021 at 02:58:12PM -0700, Linus Torvalds wrote:
> 
> > And I think our horrible "kernel threads return to user space when
> > done" is absolutely horrifically nasty. Maybe of the clever sort, but
> > mostly of the historical horror sort.
> 
> How would you prefer to handle that, then?  Separate magical path from
> kernel_execve() to switch to userland?  We used to have something of
> that sort, and that had been a real horror...
> 
> As it is, it's "kernel thread is spawned at the point similar to
> ret_from_fork(), runs the payload (which almost never returns) and
> then proceeds out to userland, same way fork(2) would've done."
> That way kernel_execve() doesn't have to do anything magical.
> 
> Al, digging through the old notes and current call graph...

	FWIW, the major assumption back then had been that get_signal(),
signal_delivered() and all associated machinery (including coredumps)
runs *only* from SIGPENDING/NOTIFY_SIGNAL handling.

	And "has complete registers on stack" is only a part of that;
there was other fun stuff in the area ;-/  Do we want coredumps for
those, and if we do, will the de_thread stuff work there?