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

Thomas Weißschuh
2026-01-19T10:56:35Z

On Mon, Jan 19, 2026 at 10:37:58AM +0000, David Laight wrote:
> On Mon, 19 Jan 2026 11:13:08 +0100
> Thomas Weißschuh <thomas.weissschuh@linutronix.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@linutronix.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@linutronix.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.

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.

> 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

The if defined(__SIZEOF_LONG__) is also unnecessary as that is always present
with the supported kernel compilers. So we can drop one level of ifdeffery.

Testing __CHAR_BIT__ == 8 would again test the compiler implementation and not
the UAPI headers, so I'd rather not do it. Using __CHAR_BIT__ in the test is
done for consistency with the generic implementation.

Thomas