linux-kernel - Re: linux-next: Tree for Apr 22 (objtool warnings)

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Date:   Thu, 23 Apr 2020 11:06:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: linux-next: Tree for Apr 22 (objtool warnings)

On Wed, Apr 22, 2020 at 11:44:06AM -0500, Josh Poimboeuf wrote:
> On Wed, Apr 22, 2020 at 08:35:29AM -0700, Randy Dunlap wrote:
> > On 4/22/20 12:10 AM, Stephen Rothwell wrote:
> > > Hi all,
> > > 
> > > Changes since 20200421:
> > > 
> > 
> > on x86_64:
> 
> In both cases the unreachable instruction happens immediately after a
> call to a function which is truncated with a UD2 (because of
> UBSAN_TRAP).
> 
> When I remove UBSAN_TRAP, the UD2s are replaced with calls to
> __ubsan_handle_type_mismatch_v1().

Hrm, these are coming out of CONFIG_UBSAN_MISC, yes? It seems that the
UBSAN checks that are non-recoverable all inject unreachable checks
afterwards, from what I can see.

> Kees, any idea?

Isn't this another version of the earlier unreachable-ud2 issue?

Regardless, the type_mismatch it triggered for misalignment and
object-size checks, and the alignment check is likely going to always
misfire on x86. The randconfig includes that config:

CONFIG_UBSAN_ALIGNMENT=y

So perhaps the config should be strengthened to disallow it under
COMPILE_TEST?

config UBSAN_ALIGNMENT
        def_bool !UBSAN_NO_ALIGNMENT
	depends on !COMPILE_TEST

-- 
Kees Cook