Re: [PATCH] x86/process: Silence KASAN warnings in get_wchan()

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Tue, Oct 6, 2015 at 12:26 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> >
> >> On 10/05/2015 07:39 PM, Andi Kleen wrote:
> >> >> But, I think I have the solution.
> >> >> We could have some blacklist - list of function names which we should be ignored.
> >> >> In kasan_report() we could resolve return address to function name and compare it with name in list.
> >> >> If name in list -> ignore report.
> >> >
> >> > I think annotating statements is cleaner than functions, even if it
> >> > is more code. Much better documentation
> >> >
> >>
> >> I agree with that, that's why I suggested to add READ_ONCE_NOCHECK():
> >>       READ_ONCE_NOCHECK()
> >>       {
> >>               kasan_disable_current();
> >>               READ_ONCE();
> >>               kasan_enable_current();
> >>       }
> >>
> >> Anywone objects?
> >
> > Sounds good to me! As long as it's hidden from plain .c files I'm a happy camper.
> >
> > This should probably also be faster for KASAN than triggering a warning and having
> > to parse a blacklist, right?
> >
> >> > If disabling with an attribute doesn't work, you could put it into a special
> >> > section with __attribute__((section ...)) and check the start/end symbol
> >> > before reporting. That's how kprobes solves similar issues. It also has the
> >> > advantage that it stops inlining.
> >>
> >> Yes, it might be better. Although, because of broken -fconserve-stack, this may
> >> not work in some cases - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63533
> >> Function splitter may split original function into two parts and it always puts
> >> one split part in default .text section.
> >
> > We do a _ton_ of such section tricks in the kernel (all of exception handling is
> > based on that) - if that's broken by -fconserve-stack then the kernel is broken
> > much more widely.
> >
> > So unless KASAN wants to do something special here you can rely on sections just
> > fine.
> 
> Kprobes is moving away from a section approach for some reason (not
> sure why), but the kprobe approach should work, too.

Do you mean NOKPROBE_SYMBOL() vs __kprobes?

So one concern is with functions being in multiple blacklists, so yeah, the 
NOKPROBE_SYMBOL() approach might be more robust than __kprobes.

But note that NOKPROBE_SYMBOL() itself is still section based:

#define __NOKPROBE_SYMBOL(fname)                        \
static unsigned long __used                             \
        __attribute__((section("_kprobe_blacklist")))   \
        _kbl_addr_##fname = (unsigned long)fname;

Thanks,

	Ingo