linux-kernel - Re: [PATCH 4/4] asm-generic/bitsperlong.h: Add sanity checks for __BITS_PER

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Message-Id: <5424652d-3607-4718-b32a-a9591437c133@app.fastmail.com>
Date: Fri, 16 Jan 2026 08:46:22 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
 "David S . Miller" <davem@...emloft.net>,
 "Andreas Larsson" <andreas@...sler.com>, "Andy Lutomirski" <luto@...nel.org>,
 "Thomas Gleixner" <tglx@...nel.org>, "Ingo Molnar" <mingo@...hat.com>,
 "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, "Heiko Carstens" <hca@...ux.ibm.com>,
 "Vasily Gorbik" <gor@...ux.ibm.com>,
 "Alexander Gordeev" <agordeev@...ux.ibm.com>,
 "Christian Borntraeger" <borntraeger@...ux.ibm.com>,
 "Sven Schnelle" <svens@...ux.ibm.com>
Cc: sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
 Linux-Arch <linux-arch@...r.kernel.org>, linux-s390@...r.kernel.org
Subject: Re: [PATCH 4/4] asm-generic/bitsperlong.h: Add sanity checks for
 __BITS_PER_LONG

On Fri, Jan 16, 2026, at 08:40, Thomas Weißschuh wrote:
> The value of __BITS_PER_LONG from architecture-specific logic should
> always match the generic one if that is available. It should also match
> the actual C type 'long'.
>
> Mismatches can happen for example when building the compat vDSO. Either
> during the compilation, see commit 9a6d3ff10f7f ("arm64: uapi: Provide
> correct __BITS_PER_LONG for the compat vDSO"), or when running sparse
> when mismatched CHECKFLAGS are inherited from the kernel build.
>
> Add some consistency checks which detect such issues early and clearly.
> The tests are added to the UAPI header to make sure it is also used when
> building the vDSO as that is not supposed to use regular kernel headers.
>
> The kernel-interal BITS_PER_LONG is not checked as it is derived from
> CONFIG_64BIT and therefore breaks for the compat vDSO. See the similar,
> deactivated check in include/asm-generic/bitsperlong.h.
>
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>

Acked-by: Arnd Bergmann <arnd@...db.de>