linux-kernel - Re: [PATCH] lib/Kconfig.debug: disable FRAME

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Message-ID: <20231019155600.GB60597@dev-arch.thelio-3990X>
Date:   Thu, 19 Oct 2023 08:56:00 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Hamza Mahfooz <hamza.mahfooz@....com>,
        linux-kernel@...r.kernel.org,
        Rodrigo Siqueira <rodrigo.siqueira@....com>,
        Harry Wentland <harry.wentland@....com>,
        Alex Deucher <alexander.deucher@....com>,
        stable@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Nick Terrell <terrelln@...com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        Li Hua <hucool.lihua@...wei.com>, Rae Moar <rmoar@...gle.com>,
        rust-for-linux@...r.kernel.org, bpf@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] lib/Kconfig.debug: disable FRAME_WARN for kasan and kcsan

On Thu, Oct 19, 2023 at 02:53:01PM +0200, Arnd Bergmann wrote:
> On Thu, Oct 19, 2023, at 12:04, Alexander Potapenko wrote:
> > So the remaining option would be to just increase the frame size every
> > time a new function surpasses the limit.
> 
> That is clearly not an option, though we could try to
> add Kconfig dependencies that avoid the known bad combinations,
> such as annotating the AMD GPU driver as
> 
>       depends on (CC_IS_GCC || CLANG_VERSION >=180000) || !(KASAN || KCSAN)

This would effectively disable the AMDGPU driver for allmodconfig, which
is somewhat unfortunate as it is an easy testing target.

Taking a step back, this is all being done because of a couple of
warnings in the AMDGPU code. If fixing those in the source is too much
effort (I did note [1] that GCC is at the current limit for that file
even with Rodrigo's series applied [2]), couldn't we just take the
existing workaround that this Makefile has for this file and its high
stack usage and just extend it slightly for clang?

diff --git a/drivers/gpu/drm/amd/display/dc/dml2/Makefile b/drivers/gpu/drm/amd/display/dc/dml2/Makefile
index 66431525f2a0..fd49e3526c0d 100644
--- a/drivers/gpu/drm/amd/display/dc/dml2/Makefile
+++ b/drivers/gpu/drm/amd/display/dc/dml2/Makefile
@@ -58,7 +58,7 @@ endif
 endif
 
 ifneq ($(CONFIG_FRAME_WARN),0)
-frame_warn_flag := -Wframe-larger-than=2048
+frame_warn_flag := -Wframe-larger-than=$(if $(CONFIG_CC_IS_CLANG),3072,2048)
 endif
 
 CFLAGS_$(AMDDALPATH)/dc/dml2/display_mode_core.o := $(dml2_ccflags) $(frame_warn_flag)

That would address the immediate concern of the warning breaking builds
with CONFIG_WERROR=y while not raising the limit for other files in the
kernel (just this one file in AMDGPU) and avoiding disabling the whole
driver. The number could be lower, I think ~2500 bytes is the most usage
I see with Rodrigo's series applied, so maybe 2800 would be a decent
limit? Once there is a fix in the compiler, this expression could be
changed to use clang-min-version or something of that sort.

[1]: https://lore.kernel.org/20231017172231.GA2348194@dev-arch.thelio-3990X/
[2]: https://lore.kernel.org/20231016142031.241912-1-Rodrigo.Siqueira@amd.com/

Cheers,
Nathan