linux-kernel - [PATCH 1/2] ARM: Require linker to support KEEP within OVERLAY for DCE

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Message-Id: <20250311-arm-fix-vectors-with-linker-dce-v1-1-ec4c382e3bfd@kernel.org>
Date: Tue, 11 Mar 2025 20:43:42 +0100
From: Nathan Chancellor <nathan@...nel.org>
To: Russell King <linux@...linux.org.uk>
Cc: Christian Eggers <ceggers@...i.de>, Arnd Bergmann <arnd@...db.de>, 
 Linus Walleij <linus.walleij@...aro.org>, 
 Yuntao Liu <liuyuntao12@...wei.com>, linux-arm-kernel@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, stable@...r.kernel.org, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH 1/2] ARM: Require linker to support KEEP within OVERLAY for
 DCE

ld.lld prior to 21.0.0 does not support using the KEEP keyword within an
overlay description, which may be needed to avoid discarding necessary
sections within an overlay with '--gc-sections', which can be enabled
for the kernel via CONFIG_LD_DEAD_CODE_DATA_ELIMINATION.

Disallow CONFIG_LD_DEAD_CODE_DATA_ELIMINATION without support for KEEP
within OVERLAY and introduce a macro, OVERLAY_KEEP, that can be used to
conditionally add KEEP when it is properly supported to avoid breaking
old versions of ld.lld.

Cc: stable@...r.kernel.org
Link: https://github.com/llvm/llvm-project/commit/381599f1fe973afad3094e55ec99b1620dba7d8c
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 arch/arm/Kconfig                   | 2 +-
 arch/arm/include/asm/vmlinux.lds.h | 6 ++++++
 init/Kconfig                       | 5 +++++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 835b5f100e92..f3f6b7a33b79 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -121,7 +121,7 @@ config ARM
 	select HAVE_KERNEL_XZ
 	select HAVE_KPROBES if !XIP_KERNEL && !CPU_ENDIAN_BE32 && !CPU_V7M
 	select HAVE_KRETPROBES if HAVE_KPROBES
-	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if (LD_VERSION >= 23600 || LD_IS_LLD)
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if (LD_VERSION >= 23600 || LD_CAN_USE_KEEP_IN_OVERLAY)
 	select HAVE_MOD_ARCH_SPECIFIC
 	select HAVE_NMI
 	select HAVE_OPTPROBES if !THUMB2_KERNEL
diff --git a/arch/arm/include/asm/vmlinux.lds.h b/arch/arm/include/asm/vmlinux.lds.h
index d60f6e83a9f7..0f8ef1ed725e 100644
--- a/arch/arm/include/asm/vmlinux.lds.h
+++ b/arch/arm/include/asm/vmlinux.lds.h
@@ -34,6 +34,12 @@
 #define NOCROSSREFS
 #endif
 
+#ifdef CONFIG_LD_CAN_USE_KEEP_IN_OVERLAY
+#define OVERLAY_KEEP(x)		KEEP(x)
+#else
+#define OVERLAY_KEEP(x)		x
+#endif
+
 /* Set start/end symbol names to the LMA for the section */
 #define ARM_LMA(sym, section)						\
 	sym##_start = LOADADDR(section);				\
diff --git a/init/Kconfig b/init/Kconfig
index d0d021b3fa3b..fc994f5cd5db 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -129,6 +129,11 @@ config CC_HAS_COUNTED_BY
 	# https://github.com/llvm/llvm-project/pull/112636
 	depends on !(CC_IS_CLANG && CLANG_VERSION < 190103)
 
+config LD_CAN_USE_KEEP_IN_OVERLAY
+	# ld.lld prior to 21.0.0 did not support KEEP within an overlay description
+	# https://github.com/llvm/llvm-project/pull/130661
+	def_bool LD_IS_BFD || LLD_VERSION >= 210000
+
 config RUSTC_HAS_COERCE_POINTEE
 	def_bool RUSTC_VERSION >= 108400
 

-- 
2.48.1