linux-kernel - [PATCH 1/3] arm64: uapi: Provide correct __BITS_PER

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Message-Id: <20250821-vdso-arm64-compat-bitsperlong-v1-1-700bcabe7732@linutronix.de>
Date: Thu, 21 Aug 2025 09:56:44 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Catalin Marinas <catalin.marinas@....com>, 
 Will Deacon <will@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
 Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...nel.org>, 
 Vincenzo Frascino <vincenzo.frascino@....com>
Cc: John Stultz <jstultz@...gle.com>, linux-arm-kernel@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH 1/3] arm64: uapi: Provide correct __BITS_PER_LONG for the
 compat vDSO

The generic vDSO library uses the UAPI headers. On arm64 __BITS_PER_LONG is
always '64' even when used from the compat vDSO. In that case __GENMASK()
does an illegal bitshift, invoking undefined behaviour.

Change __BITS_PER_LONG to also work when used from the comapt vDSO.
To not confuse real userspace, only do this when building the kernel.

Reported-by: John Stultz <jstultz@...gle.com>
Closes: https://lore.kernel.org/lkml/CANDhNCqvKOc9JgphQwr0eDyJiyG4oLFS9R8rSFvU0fpurrJFDg@mail.gmail.com/
Fixes: cd3557a7618b ("vdso/gettimeofday: Add support for auxiliary clocks")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 arch/arm64/include/uapi/asm/bitsperlong.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arm64/include/uapi/asm/bitsperlong.h b/arch/arm64/include/uapi/asm/bitsperlong.h
index 485d60bee26ca313ad15797f230efe10072befc9..d59730975f30c0764577ec20dfafa33373311e21 100644
--- a/arch/arm64/include/uapi/asm/bitsperlong.h
+++ b/arch/arm64/include/uapi/asm/bitsperlong.h
@@ -17,7 +17,12 @@
 #ifndef __ASM_BITSPERLONG_H
 #define __ASM_BITSPERLONG_H
 
+#if defined(__KERNEL__) && !defined(__aarch64__)
+/* Used by the compat vDSO */
+#define __BITS_PER_LONG 32
+#else
 #define __BITS_PER_LONG 64
+#endif
 
 #include <asm-generic/bitsperlong.h>
 

-- 
2.50.1