Re: [PATCH] Makefile: LTO: have linker check -Wframe-larger-than

From: Nick Desaulniers <ndesaulniers@google.com>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Sami Tolvanen <samitolvanen@google.com>,
	Candle Sun <candlesea@gmail.com>,
	Fangrui Song <maskray@google.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Nathan Chancellor <nathan@kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] Makefile: LTO: have linker check -Wframe-larger-than
Date: Fri, 12 Mar 2021 13:38:51 -0800	[thread overview]
Message-ID: <CAKwvOdkNoo+7pDiiNdnciRhHB8LpVTOr7ROpDxfEnLS-Qn9dJA@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdnPhpKRs6SePCUCPs_2MUFbWgJiaf9F9J+aQZGESSQ9yA@mail.gmail.com>

On Fri, Mar 12, 2021 at 9:55 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Mar 11, 2021 at 5:09 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > -Wframe-larger-than= requires stack frame information, which the
> > frontend cannot provide. This diagnostic is emitted late during
> > compilation once stack frame size is available.
> >
> > When building with LTO, the frontend simply lowers C to LLVM IR and does
> > not have stack frame information, so it cannot emit this diagnostic.
> > When the linker drives LTO, it restarts optimizations and lowers LLVM IR
> > to object code. At that point, it has stack frame information but
> > doesn't know to check for a specific max stack frame size.
> >
> > I consider this a bug in LLVM that we need to fix. There are some
> > details we're working out related to LTO such as which value to use when
> > there are multiple different values specified per TU, or how to
> > propagate these to compiler synthesized routines properly, if at all.
> >
> > Until it's fixed, ensure we don't miss these. At that point we can wrap
> > this in a compiler version guard or revert this based on the minimum
> > support version of Clang.
> >
> > The error message is not generated during link:
> >   LTO     vmlinux.o
> > ld.lld: warning: stack size limit exceeded (8224) in foobarbaz
> >
> > Cc: Sami Tolvanen <samitolvanen@google.com>
> > Reported-by: Candle Sun <candlesea@gmail.com>
> > Suggested-by: Fangrui Song <maskray@google.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > ---
> > LTO users might want to `make clean` or `rm -rf .thinlto-cache` to test
> > this.
> >
> >  Makefile | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/Makefile b/Makefile
> > index f9b54da2fca0..74566b1417b8 100644
> > --- a/Makefile
> > +++ b/Makefile
>
> Candle sent me a private message that we probably also want coverage
> for kernel modules. Let me revise this and test/send a v2.

False alarm, seems specific to Android's LTO support pre-5.11. I will
fix that in Android trees.  This patch is still relevant going
forward.

>
> > @@ -910,6 +910,11 @@ CC_FLAGS_LTO       += -fvisibility=hidden
> >
> >  # Limit inlining across translation units to reduce binary size
> >  KBUILD_LDFLAGS += -mllvm -import-instr-limit=5
> > +
> > +# Check for frame size exceeding threshold during prolog/epilog insertion.
> > +ifneq ($(CONFIG_FRAME_WARN),0)
> > +KBUILD_LDFLAGS += -plugin-opt=-warn-stack-size=$(CONFIG_FRAME_WARN)
> > +endif
> >  endif
> >
> >  ifdef CONFIG_LTO
> > --
> > 2.31.0.rc2.261.g7f71774620-goog
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers

-- 
Thanks,
~Nick Desaulniers