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?)
