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Message-ID: <202101221133.389539337D@keescook>
Date: Fri, 22 Jan 2021 11:33:41 -0800
From: Kees Cook <keescook@...omium.org>
To: Alexander Lobakin <alobakin@...me>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
Masahiro Yamada <masahiroy@...nel.org>,
Michal Marek <michal.lkml@...kovi.net>,
kernel-hardening@...ts.openwall.com,
linux-hardening@...r.kernel.org, linux-kbuild@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on
recursive rebuilds
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@...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
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