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Date:   Fri, 22 Jan 2021 11:33:41 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Alexander Lobakin <alobakin@pm.me>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        kernel-hardening@lists.openwall.com,
        linux-hardening@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on
 recursive rebuilds
Message-ID: <202101221133.389539337D@keescook>
References: <20210121184544.659998-1-alobakin@pm.me>
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On Thu, Jan 21, 2021 at 06:45:55PM +0000, Alexander Lobakin wrote:
> CC_FLAGS_LTO gets initialized only via +=, never with := or =.
> When building with CONFIG_TRIM_UNUSED_KSYMS, Kbuild may perform
> several kernel rebuilds to satisfy symbol dependencies. In this
> case, value of CC_FLAGS_LTO is concatenated each time, which
> triggers a full rebuild.
> Initialize it with := to fix this.
> 
> Fixes: dc5723b02e52 ("kbuild: add support for Clang LTO")
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>

Ah-ha, good catch; thanks!

I'll get this into the tree.

-Kees

> ---
>  Makefile | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 668909e7a460..2233951666f7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -895,10 +895,10 @@ endif
>  
>  ifdef CONFIG_LTO_CLANG
>  ifdef CONFIG_LTO_CLANG_THIN
> -CC_FLAGS_LTO	+= -flto=thin -fsplit-lto-unit
> +CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
>  KBUILD_LDFLAGS	+= --thinlto-cache-dir=$(extmod-prefix).thinlto-cache
>  else
> -CC_FLAGS_LTO	+= -flto
> +CC_FLAGS_LTO	:= -flto
>  endif
>  CC_FLAGS_LTO	+= -fvisibility=hidden
>  
> -- 
> 2.30.0
> 
> 

-- 
Kees Cook