Re: [PATCH] kasan: turn off asan-stack for clang-8 and earlier

[Nick Desaulniers]

+ Evgenii

On Wed, Feb 20, 2019 at 9:36 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Feb 20, 2019 at 6:00 PM Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
> > On 2/20/19 5:51 PM, Arnd Bergmann wrote:
> > > On Wed, Feb 20, 2019 at 3:45 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> > > I would have to some more research, but I expect several hundred
> > > patches before we get to a clean randconfig build with a broken
> > > compiler.
> >
> > Manually maintaining asan-stack parameter for the sake of one broken compiler isn't a great idea either.
> >
> > Couple alternative suggestions:
> >
> > 1) If we can't fix the problem or the cost of fixing is too high, maybe just hide it? Disable -Wframe-larger-then on pre clang-9 compilers.
> >
> > 2) Fallback cflags. The idea is to try to compile every the file with "-mllvm -asan-stack=1 -Wframe-larger-than=2048 -Werror" at first,
> >  and fallback to "-mllvm -asan-stack=0" if failed. So it would be something similar to $(call cc-option, -mllvm -asan-stack=1 -Wframe-larger-than=2048 -Werror, -mllvm -asan-stack=0)
> >  except that "cc-option" tries options only once on some code example while  we need to try options on every file that we actually compile.
> >  Honestly, I'm not sure that it's worthy to hack Kbuild engine for that particular use-case.
>
> My original plan was to put this under CONFIG_KASAN_EXTRA to allow you
> to still enable it in older compilers, but you just removed that option ;-)
>
> Maybe bringing it back would be a compromise? That way it's hidden from
> all the build testing bots (because of the !CONFIG_COMPILE_TEST dependency),
> but anyone who really wants it can still have the option, and set
> CONFIG_FRAME_WARN
> to whichever value they like.
>
>      Arnd

I like Evgenii's idea:
https://bugs.llvm.org/show_bug.cgi?id=38809#c10

Even though something like that wouldn't make the clang-8 train, I
think it's ok.

While I myself share Arnd's goal of driving compiler warnings to zero,
in general I'd prefer not to disable warning-producing-features or
disable warnings outright for cases where we have some ideas of
changes we can make to the compiler.  There's probably a list now of
false warnings produced by old versions of Clang from bugs in Clang
that we fixed.  I'm not interested in additionally trying to work
around those somehow in kernel sources.

Qian previously pointed out that most drivers don't produce this
warning under KASAN+Clang.  While 114 is a lot, what are the chances
that someone NEEDS a KASAN+Clang build to compile warning free and
happen to include one of these problematic drivers?  And if there is a
chance they do observe the warning, are we doing a disservice by
disabling the feature (-asan-stack=1) outright for the whole kernel,
or disabling the warning (`-Wstack-frame-larger-than=`) which can flag
issues unrelated to KASAN?

To Evgenii's idea, I vote that the compiler is incorrect here, and we
shouldn't start turning things off.  Evgenii, do you have some sense
of how to tune the inliner as you described?
-- 
Thanks,
~Nick Desaulniers