linux-kernel - Re: [PATCH] x86: Don't discard .exit.text and .exit.data at link-time

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Message-ID: <CAMe9rOp1SJvsjSMtDFi4HWKPpu2eePCDiedTPAndUEL5-HSU1w@mail.gmail.com>
Date:   Thu, 30 Jan 2020 12:06:06 -0800
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH] x86: Don't discard .exit.text and .exit.data at link-time

On Thu, Jan 30, 2020 at 11:58 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Jan 30, 2020 at 11:45:15AM -0800, H.J. Lu wrote:
> > On Thu, Jan 30, 2020 at 11:40 AM Kees Cook <keescook@...omium.org> wrote:
> > >
> > > On Thu, Jan 30, 2020 at 10:00:48AM -0800, H.J. Lu wrote:
> > > > Since .exit.text and .exit.data sections are discarded at runtime, we
> > > > should undefine EXIT_TEXT and EXIT_DATA to exclude .exit.text and
> > > > .exit.data sections from default discarded sections.
> > >
> > > This is just a correctness fix, yes? The EXIT_TEXT and EXIT_DATA were
> > > already included before the /DISCARD/ section here, so there's no
> > > behavioral change with this patch, correct?
> >
> > That is correct.  I was confused by EXIT_TEXT and EXIT_DATA in generic
> > DISCARDS.   My patch just makes it more explicit.
>
> Okay, so to that end and because this isn't arch-specific, I'd like to
> see this be a behavioral flag, and then the generic DISCARDS macro can
> be adjusted. This lets all architectures implement this without having
> to scatter undef/define lines in each arch.
>
> Something like this:
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index e00f41aa8ec4..f242d3b4814d 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -894,11 +894,17 @@
>   * section definitions so that such archs put those in earlier section
>   * definitions.
>   */
> -#define DISCARDS                                                       \
> -       /DISCARD/ : {                                                   \
> +#ifdef RUNTIME_DISCARD_EXIT
> +#define EXIT_DISCARDS
> +#else
> +#define EXIT_DISCARDS                                                  \
>         EXIT_TEXT                                                       \
>         EXIT_DATA                                                       \
> -       EXIT_CALL                                                       \
> +       EXIT_CALL
> +#endif
> +#define DISCARDS                                                       \
> +       /DISCARD/ : {                                                   \
> +       EXIT_DISCARDS                                                   \
>         *(.discard)                                                     \
>         *(.discard.*)                                                   \
>         *(.modinfo)                                                     \
>
> Then x86 and all other architectures that do this can just use
> #define RUNTIME_DISCARD_EXIT
> at the top (like EMITS_PT_NOTE, etc).
>

It should work.

Thanks.

-- 
H.J.