Re: [PATCH] [RFC] ubsan: turn off UBSAN_ALIGNMENT on s390

[Peter Zijlstra]

On Fri, May 15, 2026 at 06:53:57PM +0200, Heiko Carstens wrote:
> On Fri, May 15, 2026 at 11:19:54AM +0200, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@xxxxxxxx>
> > 
> > Testing randconfig builds on s390 with gcc-15, I came across a number of
> > seemingly unrelated build failures that ended up all being caused
> > by the -fsanitize=alignment option:
> > 
> > s390-linux-ld: kernel/sched/build_policy.o: in function `thread_group_cputime':
> > include/linux/seqlock.h:1286:(.text+0x1f738): undefined reference to `__scoped_seqlock_bug'
> 
> Does this only happen with __scoped_seqlock_bug()?
> I just enabled UBSAN_ALIGNMENT, and with gcc-16 I can see this too.
> 
> > What I observe here is a huge increase in generated calls to
> > __ubsan_handle_type_mismatch_v1() that ends up thowing off a number of
> > compiler optimizations that the kernel relies on.
> > 
> > I have not been able to figure out why this happens on s390 but not arm64,
> > arm or x86, if other toolchain versions are affected by the same thing,
> > and if this is a problem in gcc or in the kernel itself, e.g. some
> > variable being identified as unaligned when it should be aligned.
> > 
> > This clearly needs more investigation to figure out properly what is
> > going on, but turning it off is currently required for randconfig testing.
> ...
> > ---
> >  lib/Kconfig.ubsan | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> > index 1ecaae7064d2..3fc03a6b5af4 100644
> > --- a/lib/Kconfig.ubsan
> > +++ b/lib/Kconfig.ubsan
> > @@ -152,6 +152,7 @@ config UBSAN_ENUM
> >  
> >  config UBSAN_ALIGNMENT
> >  	bool "Perform checking for misaligned pointer usage"
> > +	depends on !S390 || BROKEN
> 
> Wouldn't it be more appropriate to extend the ifdef at __scoped_seqlock_bug()
> which emits an empty function for exactly this reason for some gcc versions
> and kernel configs?
> 
> That is: add CONFIG_UBSAN_ALIGNMENT to the list (copy-pasted - white space
> damage below)?
> 
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index 5a40252b8334..18affa4d21a6 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -1259,7 +1259,7 @@ static __always_inline void __scoped_seqlock_cleanup(struct ss_tmp *sst)
>  
>  extern void __scoped_seqlock_invalid_target(void);
>  
> -#if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 90000) || defined(CONFIG_KASAN)
> +#if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 90000) || defined(CONFIG_KASAN) || defined(CONFIG_UBSAN_ALIGNMENT)

Right, so I have a GCC bug and some mail with Kees about all of this.

I wanted to add: CONFIG_UBSAN

But for 'raisins' the whole bounds checking thing is mangled into UBSAN,
so everybody + dog has UBSAN on in their .config :/

IIRC Kees was going to look at untangling bounds checking from UBSAN and
make UBSAN pure debugging stuff (again). But I don't think he's managed
to find time for this.

Anyway, yes, I suppose we can do as proposed.