linux-kernel - [PATCH v3] LoongArch: Replace -ffreestanding with finer-grained -fno-builtin's

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Date:   Sun,  6 Aug 2023 18:36:20 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Huacai Chen <chenhuacai@...nel.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Xi Ruoyao <xry111@...111.site>, loongarch@...ts.linux.dev,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        WANG Xuerui <git@...0n.name>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH v3] LoongArch: Replace -ffreestanding with finer-grained -fno-builtin's

From: WANG Xuerui <git@...0n.name>

As explained by Nick in the original issue: the kernel usually does a
good job of providing library helpers that have similar semantics as
their ordinary userspace libc equivalents, but -ffreestanding disables
such libcall optimization and other related features in the compiler,
which can lead to unexpected things such as CONFIG_FORTIFY_SOURCE not
working (!).

However, due to the desire for better control over unaligned accesses
with respect to CONFIG_ARCH_STRICT_ALIGN, and also for avoiding the
GCC bug https://gcc.gnu.org/PR109465, we do want to still disable
optimizations for the memory libcalls (memcpy, memmove and memset for
now). Use finer-grained -fno-builtin-* toggles to achieve this without
losing source fortification and other libcall optimizations.

Closes: https://github.com/ClangBuiltLinux/linux/issues/1897
Reported-by: Nathan Chancellor <nathan@...nel.org>
Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: WANG Xuerui <git@...0n.name>
---

Changes in v3 (no functional changes):

- Move explanatory words mentioning -ffreestanding from the comments into
  commit message.

Changes in v2:

- Keep the memory operation builtins disabled, add comments, and tweak the
  commit message along the way.

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

diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index b1e5db51b61c..3a0a3b209fc1 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -83,7 +83,7 @@ KBUILD_CFLAGS_KERNEL		+= -fPIE
 LDFLAGS_vmlinux			+= -static -pie --no-dynamic-linker -z notext
 endif

-cflags-y += -ffreestanding
+cflags-y += -fno-builtin-memcpy -fno-builtin-memmove -fno-builtin-memset
 cflags-y += $(call cc-option, -mno-check-zero-division)

 load-y		= 0x9000000000200000
-- 
2.40.0