linux-kernel - [PATCH v3 2/3] riscv: vdso: Disable LTO for the vDSO

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Message-Id: <20250611-vdso-absolute-reloc-v3-2-47897d73784b@linutronix.de>
Date: Wed, 11 Jun 2025 11:22:13 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Alexandre Ghiti <alex@...ti.fr>, Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
 Vincenzo Frascino <vincenzo.frascino@....com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH v3 2/3] riscv: vdso: Disable LTO for the vDSO

All the other architectures supporting LTO (x86, arm64, loongarch) do not
use it for the vDSO.

Its is problematic for some upcoming compile-time validation of the
generated object code.
The LTO object files do not contain the necessary relocation information
and -flto-fat-objects is not compatible with clang < 16.

For consistency and to enable the mentioned compile-time checks,
disable LTO for the vDSO.
The vDSO heavily uses __always_inline anyways.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 arch/riscv/kernel/vdso/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index c19c3c76f7c9f6b2f7523a59269de3b683656323..9f1bf5bae9bd473e43d9fd3022e9e1a185128b73 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -49,7 +49,7 @@ CPPFLAGS_vdso.lds += -DHAS_VGETTIMEOFDAY
 endif
 
 # Disable -pg to prevent insert call site
-CFLAGS_REMOVE_VDSO = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
+CFLAGS_REMOVE_VDSO = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
 CFLAGS_REMOVE_vgettimeofday.o = $(CFLAGS_REMOVE_VDSO)
 CFLAGS_REMOVE_getrandom.o = $(CFLAGS_REMOVE_VDSO)
 CFLAGS_REMOVE_hwprobe.o = $(CFLAGS_REMOVE_VDSO)

-- 
2.49.0