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From:   Masahiro Yamada <masahiroy@kernel.org>
To:     linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org,
        Russell King <linux@armlinux.org.uk>
Cc:     linux-kernel@vger.kernel.org, Ard Biesheuvel <ardb@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>
Subject: [PATCH] ARM: change link order of $(mmy-y) to avoid veneers
Date:   Mon, 19 Jun 2023 23:37:25 +0900
Message-Id: <20230619143725.57967-1-masahiroy@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

The kernel compiled with multi_v7_defconfig + CONFIG_KASAN=y +
CONFIG_KASAN_INLINE=y does not boot.

I do not think KASAN is the direct reason of the boot failure.
CONFIG_KASAN_INLINE is just one example configuration that grows the
image size significantly and makes a big distance between function
callers and callees.

I see some veneers for __get_user_* in the bad kernel image. I am
not perfectly clear, but __get_user_* may not work with veneers for
some reasons.

If I move the link order of arch/arm/lib/getuser.S, the veneers are
gone, and the kernel gets working again.

I do not see a good reason that $(mmu-y) must be added to lib-y because
all the code in $(mmu-y) is mandatory. Add it to obj-y to move the code
to lower address.

[1] multi_v7_defconfig (works)

 $ arm-linux-gnueabihf-nm vmlinux | grep __get_user_1
 c072a450 T __get_user_1
 c17ea033 r __kstrtab___get_user_1
 c18119fe r __kstrtabns___get_user_1
 c17c4878 r __ksymtab___get_user_1

[2] multi_v7_defconfig + CONFIG_KASAN_INLINE (does not work)

 $ arm-linux-gnueabihf-nm vmlinux | grep __get_user_1
 c341ec2c T __get_user_1
 c06e3580 t ____get_user_1_veneer
 c0adc6c0 t ____get_user_1_veneer
 c12cf054 t ____get_user_1_veneer
 c43f42cc r __kstrtab___get_user_1
 c441c128 r __kstrtabns___get_user_1
 c43cead8 r __ksymtab___get_user_1

[3] multi_v7_defconfig + CONFIG_KASAN_INLINE + this patch (works)

 $ arm-linux-gnueabihf-nm vmlinux | grep __get_user_1
 c10975b0 T __get_user_1
 c43f42cc r __kstrtab___get_user_1
 c441c128 r __kstrtabns___get_user_1
 c43cead8 r __ksymtab___get_user_1

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 arch/arm/lib/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index 650404be6768..4d092ef87a1d 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -28,7 +28,7 @@ endif
 # using lib_ here won't override already available weak symbols
 obj-$(CONFIG_UACCESS_WITH_MEMCPY) += uaccess_with_memcpy.o
 
-lib-$(CONFIG_MMU) += $(mmu-y)
+obj-$(CONFIG_MMU) += $(mmu-y)
 
 ifeq ($(CONFIG_CPU_32v3),y)
   lib-y	+= io-readsw-armv3.o io-writesw-armv3.o
-- 
2.39.2