linux-kernel - Re: [PATCH] objtool: Detect assembly code falling through to INT3 padding

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Message-ID: <20180518071814.GB26358@gmail.com>
Date:   Fri, 18 May 2018 09:18:14 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Peter Anvin <h.peter.anvin@...el.com>,
        kernel test robot <xiaolong.ye@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Anvin <hpa@...or.com>, tipbuild@...or.com,
        LKP <lkp@...org>
Subject: Re: [PATCH] objtool: Detect assembly code falling through to INT3
 padding


* Josh Poimboeuf <jpoimboe@...hat.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@...utronix.de>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.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.

Thanks,

	Ingo