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 <aryabi...@virtuozzo.com> > > AuthorDate: Wed Jan 20 15:00:55 2016 -0800 > > Committer: Linus Torvalds <torva...@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.
