Re: UBSAN: run-time undefined behavior sanity checker

From: Andrey Ryabinin
Date: Mon Jan 25 2016 - 09:02:56 EST


On 01/22/2016 08:00 PM, Dave Jones wrote:
> On Fri, Jan 22, 2016 at 07:43:55PM +0300, Andrey Ryabinin wrote:
> > 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@xxxxxxxxxxxxx>
> > > > AuthorDate: Wed Jan 20 15:00:55 2016 -0800
> > > > Committer: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > > 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.
>
> So disabling that option fixed booting on one machine, but every other I've
> tried it on hangs the same way, really early. Any thoughts on how to chase this down ?
>

Try to disable instrumentation for early code, like in the patch bellow.
Also send me you .config please. Perhaps I will be able to reproduce this.


diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index b1b78ff..d39a954 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -20,6 +20,8 @@ KASAN_SANITIZE_head$(BITS).o := n
KASAN_SANITIZE_dumpstack.o := n
KASAN_SANITIZE_dumpstack_$(BITS).o := n

+UBSAN_SANITIZE := n
+
CFLAGS_irq.o := -I$(src)/../include/asm/trace

obj-y := process_$(BITS).o signal.o