linux-kernel - Re: [REGRESSION][BISECTED] kbuild: CFLAGS=-w no longer works

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Message-ID: <20251009174120.GA442207@ax162>
Date: Thu, 9 Oct 2025 10:41:20 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Askar Safin <safinaskar@...il.com>
Cc: linux-kbuild@...r.kernel.org, Masahiro Yamada <masahiroy@...nel.org>,
	regressions@...ts.linux.dev, linux-kernel@...r.kernel.org,
	miguel.ojeda.sandonis@...il.com, nsc@...nel.org, ojeda@...nel.org,
	sam@...too.org, thomas.weissschuh@...utronix.de,
	Daniel Xu <dxu@...uu.xyz>
Subject: Re: [REGRESSION][BISECTED] kbuild: CFLAGS=-w no longer works

Hi Askar,

On Thu, Oct 09, 2025 at 10:51:49AM +0300, Askar Safin wrote:
> #regzbot introduced: d1d0963121769d8d16150b913fe886e48efefa51
> 
> As well as I understand, if you want to disable warnings, you
> should pass "CFLAGS=-w" to "make". Starting with d1d096312176,
> this no longer works.
> 
> Steps to reproduce:
> 
> $ cd linux
> $ git clean -f -q -d -x  # To clean everything not controlled by git
> $ echo 'CONFIG_64BIT=y' > /tmp/minimini
> $ make allnoconfig KCONFIG_ALLCONFIG=/tmp/minimini
> $ make -j32 CFLAGS=-w

As Nicolas pointed out, CFLAGS has not be hooked into Kbuild for a long
time, see commit 69ee0b352242 ("kbuild: do not pick up CFLAGS from the
environment"). You would need to use KCFLAGS=-w to accomplish the same
thing. KCFLAGS is only for the kernel build though, you would need
HOSTCFLAGS=-w as well to ignore warnings when building the host
programs such as objtool.

It does seem like the objtool Makefile should be hardened against CFLAGS
being passed in from the command line so that the build does not break
like this though.

Cheers,
Nathan