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

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Message-ID: <20260119103758.3afb5927@pumpkin>
Date: Mon, 19 Jan 2026 10:37:58 +0000
From: David Laight <david.laight.linux@...il.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
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, 19 Jan 2026 11:13:08 +0100
Thomas Weißschuh <thomas.weissschuh@...utronix.de> wrote:

> 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.

Don't you need a check that it isn't wrong on a user system?
Which is what I thought it was doing.

The earlier check can also just be:

#if defined(__SIZEOF_LONG__) && __BITS_PER_LONG != 8 * __SIZEOF_LONG__
#error Inconsistent word size. Check uapi/asm/bitsperlong.h
#endif

	David

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