2021-01-21 19:50:17

by Alexander Lobakin

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 <[email protected]>
---
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

2021-01-22 23:18:30

by Kees Cook

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 <[email protected]>

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