Re: [PATCH] seqlock: Allow UBSAN to fail optimizing

[Salvatore Bonaccorso <carnil@debian.org>]

Hi Peter,

[Adding Kees as well]

On Thu, Jan 29, 2026 at 12:07:33PM +0100, Peter Zijlstra wrote:
> 
> Very similar to commit:
> 
>   b94d45b6bbb4 ("seqlock: Allow KASAN to fail optimizing")
> 
> mark UBSAN as failing to optimize and not suitable for release builds.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202601290910.uwScjRjc-lkp@intel.com/
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123873
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> 
> Debian folks, can you please remove CONFIG_UBSAN=y from your builds,
> this is, per GCC devs, not a feature suitable for production!
> 
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index c00063dffba3..436f05bf5dd7 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -1259,14 +1259,14 @@ 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)
>  /*
>   * For some reason some GCC-8 architectures (nios2, alpha) have trouble
>   * determining that the ss_done state is impossible in __scoped_seqlock_next()
>   * below.
>   *
> - * Similarly KASAN is known to confuse compilers enough to break this. But we
> - * don't care about code quality for KASAN builds anyway.
> + * Similarly *SAN is known to confuse compilers enough to break this. But we
> + * don't care about code quality for *SAN builds anyway.
>   */
>  static inline void __scoped_seqlock_bug(void) { }
>  #else

Kees, Peter approached the Debian kernel list above to drop
CONFIG_UBSAN again, which, so I think we need to revert your
6cfadabfe015 ("Enable UBSAN_BOUNDS and UBSAN_SHIFT"):
https://salsa.debian.org/kernel-team/linux/-/commit/6cfadabfe015fa0d659fc8e3efd495cbcae3e44e

I have make a MR for our packaging for the change in
https://salsa.debian.org/kernel-team/linux/-/merge_requests/1804

Regards,
Salvatore