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 <[email protected]> > > > > 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.
