* [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds
@ 2021-01-21 18:45 Alexander Lobakin
2021-01-22 19:33 ` Kees Cook
0 siblings, 1 reply; 2+ messages in thread
From: Alexander Lobakin @ 2021-01-21 18:45 UTC (permalink / raw)
To: Kees Cook
Cc: Sami Tolvanen, Masahiro Yamada, Michal Marek, Alexander Lobakin,
kernel-hardening, linux-hardening, linux-kbuild, linux-kernel
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>
---
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=$(extmod-prefix).thinlto-cache
else
-CC_FLAGS_LTO += -flto
+CC_FLAGS_LTO := -flto
endif
CC_FLAGS_LTO += -fvisibility=hidden
--
2.30.0
^ permalink raw reply related [flat|nested] 2+ messages in thread* Re: [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds
2021-01-21 18:45 [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds Alexander Lobakin
@ 2021-01-22 19:33 ` Kees Cook
0 siblings, 0 replies; 2+ messages in thread
From: Kees Cook @ 2021-01-22 19:33 UTC (permalink / raw)
To: Alexander Lobakin
Cc: Sami Tolvanen, Masahiro Yamada, Michal Marek, kernel-hardening,
linux-hardening, linux-kbuild, linux-kernel
On Thu, Jan 21, 2021 at 06:45:55PM +0000, Alexander Lobakin wrote:
> CC_FLAGS_LTO gets initialized only via +=, never with := or =.
> When building with CONFIG_TRIM_UNUSED_KSYMS, Kbuild may perform
> several kernel rebuilds to satisfy symbol dependencies. In this
> case, value of CC_FLAGS_LTO is concatenated each time, which
> triggers a full rebuild.
> Initialize it with := to fix this.
>
> Fixes: dc5723b02e52 ("kbuild: add support for Clang LTO")
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Ah-ha, good catch; thanks!
I'll get this into the tree.
-Kees
> ---
> Makefile | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 668909e7a460..2233951666f7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -895,10 +895,10 @@ endif
>
> ifdef CONFIG_LTO_CLANG
> ifdef CONFIG_LTO_CLANG_THIN
> -CC_FLAGS_LTO += -flto=thin -fsplit-lto-unit
> +CC_FLAGS_LTO := -flto=thin -fsplit-lto-unit
> KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod-prefix).thinlto-cache
> else
> -CC_FLAGS_LTO += -flto
> +CC_FLAGS_LTO := -flto
> endif
> CC_FLAGS_LTO += -fvisibility=hidden
>
> --
> 2.30.0
>
>
--
Kees Cook
^ permalink raw reply [flat|nested] 2+ messages in thread
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2021-01-21 18:45 [PATCH kspp-next] kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds Alexander Lobakin
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