linux-kernel - Re: UBSAN: run-time undefined behavior sanity checker

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Message-ID: <20160122051539.GA1326@codemonkey.org.uk>
Date:	Fri, 22 Jan 2016 00:15:39 -0500
From:	Dave Jones <davej@...emonkey.org.uk>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: UBSAN: run-time undefined behavior sanity checker

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@...tuozzo.com>
 > AuthorDate: Wed Jan 20 15:00:55 2016 -0800
 > Committer:  Linus Torvalds <torvalds@...ux-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.

That aside though, neat feature. I look forward to breaking kernels with it :)

	Dave