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

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Message-ID: <20260119163559-b20b14d7-56ca-4f17-8800-83f618d778b8@linutronix.de>
Date: Mon, 19 Jan 2026 16:39:26 +0100
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: "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, Arnd Bergmann <arnd@...db.de>, 
	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@...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 07:33:13AM -0800, H. Peter Anvin wrote:
> On 2026-01-15 23:40, 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>
> > ---
> >  include/uapi/asm-generic/bitsperlong.h | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/include/uapi/asm-generic/bitsperlong.h b/include/uapi/asm-generic/bitsperlong.h
> > index fadb3f857f28..9d762097ae0c 100644
> > --- a/include/uapi/asm-generic/bitsperlong.h
> > +++ b/include/uapi/asm-generic/bitsperlong.h
> > @@ -28,4 +28,18 @@
> >  #define __BITS_PER_LONG_LONG 64
> >  #endif
> >  
> > +/* Consistency checks */
> > +#ifdef __KERNEL__
> > +#if defined(__CHAR_BIT__) && defined(__SIZEOF_LONG__)
> > +#if __BITS_PER_LONG != (__CHAR_BIT__ * __SIZEOF_LONG__)
> > +#error Inconsistent word size. Check uapi/asm/bitsperlong.h
> > +#endif
> > +#endif
> > +
> > +#ifndef __ASSEMBLER__
> > +_Static_assert(sizeof(long) * 8 == __BITS_PER_LONG,
> > +	       "Inconsistent word size. Check uapi/asm/bitsperlong.h");
> > +#endif
> > +#endif /* __KERNEL__ */
> > +
> >  #endif /* _UAPI__ASM_GENERIC_BITS_PER_LONG */
> > 
> 
> Do we actually support any compilers which *don't* define __SIZEOF_LONG__?

When building the kernel not. I used this pattern because it is used
further up in the file. There it makes sense as it is actually a userspace
header which needs to support all kinds of compilers.
But this new check is gated behind __KERNEL__ anyways...
For the next revision I will move it into the regular kernel-internal
bitsperlong.h. That will be less confusing and still handle the vDSO build,
due to the way our header hierarchy works.


Thomas