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Date:   Fri, 18 May 2018 09:27:59 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Peter Anvin <h.peter.anvin@intel.com>,
        kernel test robot <xiaolong.ye@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Anvin <hpa@zytor.com>, tipbuild@zytor.com,
        LKP <lkp@01.org>
Subject: Re: [PATCH] objtool: Detect assembly code falling through to INT3
 padding
Message-ID: <20180518072759.GA4326@gmail.com>
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> > With the following commit:
> > 
> >   51bad67ffbce ("x86/asm: Pad assembly functions with INT3 instructions")
> > 
> > ... asm function alignments are padded with INT3, so it's no longer safe
> > to fall through to an aligned function.  Make sure we catch any such
> > cases with objtool.
> > 
> > Note this only adds checking for 64-bit, since objtool doesn't support
> > x86-32.
> > 
> > Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > ---
> >  arch/x86/kernel/head_64.S       |  2 --
> >  tools/objtool/arch.h            |  3 ++-
> >  tools/objtool/arch/x86/decode.c |  2 +-
> >  tools/objtool/check.c           | 11 ++++++++++-
> >  4 files changed, 13 insertions(+), 5 deletions(-)
> 
> Ok, this is cool, it addresses the robustness problem that INT3 padding introduced 
> very nicely.
> 
> The concept of built-in kernel tooling working at the machine code level is just 
> so powerful - we should have added our own KCC compiler 20 years ago.

Hm, so a problem is that if we change the padding on 32-bit as well we won't have 
this detection there, because objtool doesn't work on 32-bit.

Thanks,

	Ingo