linux-kernel - Re: [PATCH] framewarn: expand KASAN

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Message-ID: <20180921095516.GA17326@flashbox>
Date:   Fri, 21 Sep 2018 02:55:16 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Alexander Potapenko <glider@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] framewarn: expand KASAN_EXTRA exception to KASAN

On Fri, Sep 21, 2018 at 11:45:07AM +0200, Dmitry Vyukov wrote:
> On Fri, Sep 21, 2018 at 11:25 AM, Andrey Ryabinin
> <aryabinin@...tuozzo.com> wrote:
> > On 09/21/2018 04:50 AM, Andy Lutomirski wrote:
> >> This patch seems reasonable, but you emailed the wrong people :)
> >>
> >> On Thu, Sep 20, 2018 at 5:15 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
> >>>
> >>> It turns out that KASAN in general will bloat stack frames in unexpected
> >>> ways, not just KASAN_EXTRA. So, this patch trivially changes that
> >>> default to be associated with KASAN instead of KASAN_EXTRA.
> >>>
> >
> > KASAN_EXTRA bloats stack more than just KASAN, that's why the limit is higher than just for KASAN.
> > If want more details, tead the changelog from commit e7c52b84fb18f08ce49b6067ae6285aca79084a8
> >
> > If anything causes "stack frame > 2048" warning for KASAN we should at least try to fix it,
> > I mean reduce stack usage.
> 
> 
> +Nick who is also hitting these warnings on clang/arm64 build. As far
> as I understand the situation there is much worse.
> 
> I would be good to understand/fix the worst offenders. But the stack
> size increase with KASAN is a real, inherent thing. So if we live very
> close the edge, we can get people using different compilers and/or
> versions of compilers constantly breaking each other. And clang hits
> this warnings in lots of places today just because the current code
> was tailored to gcc over long period, i.e. allowing more locals where
> gcc happened to handle that better and having fewer locals where gcc
> happened to handle it worse. But for another compiler all these
> assumptions are significantly perturbed.
> 
> Nick, do you know what frame size limit eliminates the bulk of
> warnings on clang? Is 3072 a reasonable limit allowing to fix the
> remaining outliners?
> 

Hi Dmitry,

I know I'm not Nick and I hope I am not butting in but I've been
following this thread due to these warnings cropping up in Clang.
We've been tracking them on GitHub and judging from the values
there, I would argue that 3072 is a good starting value.

Link: https://github.com/ClangBuiltLinux/linux/issues/39

Cheers,
Nathan

> 
> >>> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> >>> ---
> >>>  lib/Kconfig.debug | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> >>> index 4966c4fbe7f7..39078a080e29 100644
> >>> --- a/lib/Kconfig.debug
> >>> +++ b/lib/Kconfig.debug
> >>> @@ -222,7 +222,7 @@ config ENABLE_MUST_CHECK
> >>>  config FRAME_WARN
> >>>         int "Warn for stack frames larger than (needs gcc 4.4)"
> >>>         range 0 8192
> >>> -       default 3072 if KASAN_EXTRA
> >>> +       default 3072 if KASAN
> >>>         default 2048 if GCC_PLUGIN_LATENT_ENTROPY
> >>>         default 1280 if (!64BIT && PARISC)
> >>>         default 1024 if (!64BIT && !PARISC)
> >>> --
> >>> 2.19.0