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) /* * For some reason some GCC-8 architectures (nios2, alpha) have trouble * determining that the ss_done state is impossible in __scoped_seqlock_next()
