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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 27 Oct 2022 09:41:54 -0700
Message-ID: <CAG_fn=WWiOvPRnw1ao-UPavY-U7rLFLa_a6b-k2noHKLunN+Yw@mail.gmail.com>
Subject: Re: [PATCH 3/5] Kconfig.debug: disable CONFIG_FRAME_WARN for KMSAN by default
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kbuild@vger.kernel.org
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On Thu, Oct 27, 2022 at 7:33 AM Masahiro Yamada <masahiroy@kernel.org> wrot=
e:
>
> On Tue, Oct 25, 2022 at 6:22 AM Alexander Potapenko <glider@google.com> w=
rote:
> >
> > KMSAN adds a lot of instrumentation to the code, which results in
> > increased stack usage (up to 2048 bytes and more in some cases).
> > It's hard to predict how big the stack frames can be, so we disable
> > the warnings for KMSAN instead.
> >
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Masahiro Yamada <masahiroy@kernel.org>
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: linux-kbuild@vger.kernel.org
> > Link: https://github.com/google/kmsan/issues/89
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > ---
> >  lib/Kconfig.debug | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 3fc7abffc7aa2..29280072dc0e4 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -400,8 +400,9 @@ config FRAME_WARN
> >         default 1536 if (!64BIT && XTENSA)
> >         default 1024 if !64BIT
> >         default 2048 if 64BIT
> > +       default 0 if KMSAN
>
>
>
> This is wrong.
>
> Kconfig picks up the first default entry which has
> true 'if' condition.
>
>
> Since (!64BIT || 64BIT) covers all the possible cases,
> this patch is just adding dead code.
>
Thanks for clarifying!
What we actually need is to forcefully disable CONFIG_FRAME_WARN under
KMSAN, but adding constructs like "select FRAME_WARN 0" or "select
FRAME_WARN=3D0" doesn't work, right?

I'll move "default 0 if KMSAN" to the beginning of FRAME_WARN declaration t=
hen.

>
>
>
>
>
> >         help
> > -         Tell gcc to warn at build time for stack frames larger than t=
his.
> > +         Tell the compiler to warn at build time for stack frames larg=
er than this.
> >           Setting this too low will cause a lot of warnings.
> >           Setting it to 0 disables the warning.
> >
> > --
> > 2.38.0.135.g90850a2211-goog
> >
>
>
> --
> Best Regards
> Masahiro Yamada


--=20
Alexander Potapenko
Software Engineer

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