Re: UBSAN: run-time undefined behavior sanity checker

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 01/22/2016 08:15 AM, Dave Jones wrote:
> On Thu, Jan 21, 2016 at 08:57:17PM +0000, Linux Kernel wrote:
>  > Web:        https://git.kernel.org/torvalds/c/c6d308534aef6c99904bf5862066360ae067abc4
>  > Commit:     c6d308534aef6c99904bf5862066360ae067abc4
>  > Parent:     68920c973254c5b71a684645c5f6f82d6732c5d6
>  > Refname:    refs/heads/master
>  > Author:     Andrey Ryabinin <aryabinin@virtuozzo.com>
>  > AuthorDate: Wed Jan 20 15:00:55 2016 -0800
>  > Committer:  Linus Torvalds <torvalds@linux-foundation.org>
>  > CommitDate: Wed Jan 20 17:09:18 2016 -0800
>  > 
>  >     UBSAN: run-time undefined behavior sanity checker
>  >     
>  >     UBSAN uses compile-time instrumentation to catch undefined behavior
>  >     (UB).  Compiler inserts code that perform certain kinds of checks before
>  >     operations that could cause UB.  If check fails (i.e.  UB detected)
>  >     __ubsan_handle_* function called to print error message.
>  >     
>  >     So the most of the work is done by compiler.  This patch just implements
>  >     ubsan handlers printing errors.
>  >     
>  >     GCC has this capability since 4.9.x [1] (see -fsanitize=undefined
>  >     option and its suboptions).
>  >     However GCC 5.x has more checkers implemented [2].
>  >     Article [3] has a bit more details about UBSAN in the GCC.
> 
> If I enable this and CONFIG_UBSAN_ALIGNMENT, the kernel doesn't boot,
> and hangs really early (pretty much as soon as I hit return in grub)
> far too early for serial console or even tty output.
> 
> Compiler is debian unstable's 5.3.1 20160114
> 
> I don't know if this is worth chasing down, I chose to just disable it,
> but figured I'd post in case other people stumble across the same issue.
> 

Likely caused by unaligned access in very early code, which ends up in too early printk() call.
You could try to disable instrumentation (UBSAN_SANITIZE := n) in early code.

Be aware that CONFIG_UBSAN_ALIGNMENT causes a *lot* of spam in dmesg. Since x86 supports unaligned
accesses, the significant amount of that spam just a false-positive reports.