Re: [PATCH] proc: Disable /proc/$pid/wchan

[Mark Rutland]

On Fri, Sep 24, 2021 at 07:26:22AM -0700, Kees Cook wrote:
> On Fri, Sep 24, 2021 at 02:54:24PM +0100, Mark Rutland wrote:
> > On Thu, Sep 23, 2021 at 06:16:16PM -0700, Kees Cook wrote:
> > > On Thu, Sep 23, 2021 at 05:22:30PM -0700, Vito Caputo wrote:
> > > > Instead of unwinding stacks maybe the kernel should be sticking an
> > > > entrypoint address in the current task struct for get_wchan() to
> > > > access, whenever userspace enters the kernel?
> > > 
> > > wchan is supposed to show where the kernel is at the instant the
> > > get_wchan() happens. (i.e. recording it at syscall entry would just
> > > always show syscall entry.)
> > 
> > It's supposed to show where a blocked task is blocked; the "wait
> > channel".
> > 
> > I'd wanted to remove get_wchan since it requires cross-task stack
> > walking, which is generally painful.
> 
> Right -- this is the "fragile" part I'm worried about.
> 
> > We could instead have the scheduler entrypoints snapshot their caller
> > into a field in task_struct. If there are sufficiently few callers, that
> > could be an inline wrapper that passes a __func__ string. Otherwise, we
> > still need to symbolize.
> 
> Hmm. Does PREEMPT break this?

Within the core scheduler functions interrupts should be disabled, and
as long as we only update task_struct there we shouldn't have a race.

> Can we actually use __builtin_return_address(0) in __schedule?

We'd need to do this in a few entry points above __schedule, since the
currently get_wchan walks until !in_sched_functions(). It should be
possible, though we might need to make sure those the nexus points
aren't inlined.

Thanks,
Mark.