linux-kernel - [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds

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Date:   Thu, 21 Jan 2021 18:45:55 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Kees Cook <keescook@...omium.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Alexander Lobakin <alobakin@...me>,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds

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>
---
 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