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

[Kees Cook]

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?

Can we actually use __builtin_return_address(0) in __schedule?

-- 
Kees Cook