linux-kernel - [PATCH 1/2] arm64: vdso: Work around invalid absolute relocations from GCC

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Message-Id: <20250429-vdso-absolute-reloc-v1-1-987a0afd10b5@linutronix.de>
Date: Tue, 29 Apr 2025 14:55:36 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Catalin Marinas <catalin.marinas@....com>, 
 Will Deacon <will@...nel.org>, Nam Cao <namcao@...utronix.de>, 
 Anna-Maria Behnsen <anna-maria@...utronix.de>, 
 Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...nel.org>, 
 Vincenzo Frascino <vincenzo.frascino@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Jan Stancek <jstancek@...hat.com>, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH 1/2] arm64: vdso: Work around invalid absolute relocations
 from GCC

All vDSO code needs to be completely position independent.
Symbol references are marked as hidden so the compiler is forced to emit
PC-relative relocations.
However GCC emits absolute relocations for symbol-relative references with
an offset >= 64KiB. This is the case in __arch_get_vdso_u_timens_data()
with a page size of 64KiB.

Work around the issue by preventing the optimizer from seeing the offsets.

Reported-by: Jan Stancek <jstancek@...hat.com>
Closes: https://lore.kernel.org/lkml/aApGPAoctq_eoE2g@t14ultra/
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120002
Fixes: 83a2a6b8cfc5 ("vdso/gettimeofday: Prepare do_hres_timens() for introduction of struct vdso_clock")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 arch/arm64/include/asm/vdso/gettimeofday.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
index 92a2b59a9f3df4d20feb483e6d8ebd1d813b7932..3322c7047d84fecae316a2904f1adec0cb458f6f 100644
--- a/arch/arm64/include/asm/vdso/gettimeofday.h
+++ b/arch/arm64/include/asm/vdso/gettimeofday.h
@@ -99,6 +99,19 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
 	return res;
 }
 
+#if IS_ENABLED(CONFIG_CC_IS_GCC) && IS_ENABLED(CONFIG_PAGE_SIZE_64KB)
+static __always_inline const struct vdso_time_data *__arch_get_vdso_u_time_data(void)
+{
+	const struct vdso_time_data *ret = &vdso_u_time_data;
+
+	/* Work around invalid absolute relocations */
+	OPTIMIZER_HIDE_VAR(ret);
+
+	return ret;
+}
+#define __arch_get_vdso_u_time_data __arch_get_vdso_u_time_data
+#endif /* IS_ENABLED(CONFIG_CC_IS_GCC) && IS_ENABLED(CONFIG_PAGE_SIZE_64KB) */
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* __ASM_VDSO_GETTIMEOFDAY_H */

-- 
2.49.0