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

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Message-ID: <20260119111037-4decf57f-2094-4fac-bcf4-03506791b197@linutronix.de>
Date: Mon, 19 Jan 2026 11:13:08 +0100
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: David Laight <david.laight.linux@...il.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, "H. Peter Anvin" <hpa@...or.com>, 
	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 10:06:19AM +0000, David Laight wrote:
> On Fri, 16 Jan 2026 08:40:27 +0100
> Thomas Weißschuh <thomas.weissschuh@...utronix.de> 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");
> 
> nak...
> 
> You can't assume the compiler has _Static_assert().
> All the ones that do probably define __SIZEOF_LONG__.
> You could use something 'old-school' like:
> typedef char __inconsistent_long_size[1 - 2 * (sizeof(long) * 8 != __BITS_PER_LONG))];

This is only used when building the kernel, it never actually reaches
userspace. And all supported compilers for the kernel do have _Static_assert().
As indicated by other users of _Static_assert() we have elsewhere in the tree.

> 
> 	David
> 
> > +#endif
> > +#endif /* __KERNEL__ */
> > +
> >  #endif /* _UAPI__ASM_GENERIC_BITS_PER_LONG */