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

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Message-Id: <4e4b1b5b-5f7d-4604-b5ef-0d0726263843@app.fastmail.com>
Date: Mon, 19 Jan 2026 15:57:49 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: "David Laight" <david.laight.linux@...il.com>,
 "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>, 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 Mon, Jan 19, 2026, at 14:41, Thomas Weißschuh wrote:
> On Mon, Jan 19, 2026 at 01:45:04PM +0100, Arnd Bergmann wrote:
>> On Mon, Jan 19, 2026, at 11:56, Thomas Weißschuh wrote:
>> > On Mon, Jan 19, 2026 at 10:37:58AM +0000, David Laight wrote:
>> >> 
>> >> Don't you need a check that it isn't wrong on a user system?
>> >> Which is what I thought it was doing.
>> >
>> > Not really. The overrides defined by arch/*/include/uapi/asm/bitsperlong.h are
>> > being tested here. If they work in the kernel build I assume they also work
>> > in userspace.
>> 
>> I think You could just move check into include/asm-generic/bitsperlong.h
>> to make this more obvious with the #ifdef __KERNEL__, and remove the
>> disabled check from my original version there.
>
> Ok. I'd like to keep your existing test though, as it tests something different
> and it would be nice to have that too at some point.

Sure, that works too. I wonder if one of the recent vdso cleanups
also happened to address the problem with the incorrect BITS_PER_LONG
being visible in the vdso code. Maybe we can already turn that on again.

    Arnd