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

Commit Message

Nathan Chancellor March 11, 2025, 7:43 p.m. UTC

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: [e-mail]
Link: https://github.com/llvm/llvm-project/commit/381599f1fe973afad3094e55ec99b1620dba7d8c
Signed-off-by: Nathan Chancellor <[e-mail]>
---
 arch/arm/Kconfig                   | 2 +-
 arch/arm/include/asm/vmlinux.lds.h | 6 ++++++
 init/Kconfig                       | 5 +++++
 3 files changed, 12 insertions(+), 1 deletion(-)

Patch

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