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Date:   Thu, 21 Jan 2021 18:45:55 +0000
To:     Kees Cook <keescook@chromium.org>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Alexander Lobakin <alobakin@pm.me>,
        kernel-hardening@lists.openwall.com,
        linux-hardening@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds
Message-ID: <20210121184544.659998-1-alobakin@pm.me>
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Precedence: bulk

CC_FLAGS_LTO gets initialized only via +=3D, never with :=3D or =3D.
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 :=3D to fix this.

Fixes: dc5723b02e52 ("kbuild: add support for Clang LTO")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 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
=20
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_THIN
-CC_FLAGS_LTO=09+=3D -flto=3Dthin -fsplit-lto-unit
+CC_FLAGS_LTO=09:=3D -flto=3Dthin -fsplit-lto-unit
 KBUILD_LDFLAGS=09+=3D --thinlto-cache-dir=3D$(extmod-prefix).thinlto-cache
 else
-CC_FLAGS_LTO=09+=3D -flto
+CC_FLAGS_LTO=09:=3D -flto
 endif
 CC_FLAGS_LTO=09+=3D -fvisibility=3Dhidden
=20
--=20
2.30.0