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

[Jann Horn]

On Fri, Sep 24, 2021 at 1:59 AM Vito Caputo <vcaputo@xxxxxxxxxxx> wrote:
> On Thu, Sep 23, 2021 at 04:31:05PM -0700, Kees Cook wrote:
> > The /proc/$pid/wchan file has been broken by default on x86_64 for 4
> > years now[1]. As this remains a potential leak of either kernel
> > addresses (when symbolization fails) or limited observation of kernel
> > function progress, just remove the contents for good.
> >
> > Unconditionally set the contents to "0" and also mark the wchan
> > field in /proc/$pid/stat with 0.
> >
> > This leaves kernel/sched/fair.c as the only user of get_wchan(). But
> > again, since this was broken for 4 years, was this profiling logic
> > actually doing anything useful?
> >
> > [1] https://lore.kernel.org/lkml/20210922001537.4ktg3r2ky3b3r6yp@treble/
> >
> > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Cc: Vito Caputo <vcaputo@xxxxxxxxxxx>
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> <snip>
>
>
> Please don't deliberately break WCHANs wholesale.  This is a very
> useful tool for sysadmins to get a vague sense of where processes are
> spending time in the kernel on production systems without affecting
> performance or having to restart things under instrumentation.

Wouldn't /proc/$pid/stack be more useful for that anyway? As long as
you have root privileges, you can read that to get the entire stack,
not just a single method name.

(By the way, I guess that might be an alternative to ripping wchan out
completely - require CAP_SYS_ADMIN like for /proc/$pid/stack?)