linux-kernel - Re: [PATCH 1/1] x86/purgatory: Change compiler flags to avoid relocation errors.

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Date:   Wed, 4 Sep 2019 22:34:13 -0700
From:   Andreas Smas <andreas@...elycoder.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Steve Wahl <steve.wahl@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Vaibhav Rustagi <vaibhavrustagi@...gle.com>,
        russ.anderson@....com, dimitri.sivanich@....com,
        mike.travis@....com, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 1/1] x86/purgatory: Change compiler flags to avoid
 relocation errors.

On Wed, Sep 4, 2019 at 3:19 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> + (folks recommended by ./scripts/get_maintainer.pl <patchfile>)
> (See also, step 7:
> https://nickdesaulniers.github.io/blog/2017/05/16/submitting-your-first-patch-to-the-linux-kernel-and-responding-to-feedback/)
>
> On Wed, Sep 4, 2019 at 2:45 PM Steve Wahl <steve.wahl@....com> wrote:
> >
> > The last change to this Makefile caused relocation errors when loading
>
> It's good to add a fixes tag like below when a patch fixes a
> regression, so that stable backports the fix as far back as the
> regression:
> Fixes: b059f801a937 ("x86/purgatory: Use CFLAGS_REMOVE rather than
> reset KBUILD_CFLAGS")
>
> > a kdump kernel.  This change restores the appropriate flags, without
> > reverting to the former practice of resetting KBUILD_CFLAGS.
> >
> > Signed-off-by: Steve Wahl <steve.wahl@....com>
> > ---
> >  arch/x86/purgatory/Makefile | 35 +++++++++++++++++++----------------
> >  1 file changed, 19 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> > index 8901a1f89cf5..9f0bfef1f5db 100644
> > --- a/arch/x86/purgatory/Makefile
> > +++ b/arch/x86/purgatory/Makefile
> > @@ -18,37 +18,40 @@ targets += purgatory.ro
> >  KASAN_SANITIZE := n
> >  KCOV_INSTRUMENT := n
> >
> > +# These are adjustments to the compiler flags used for objects that
> > +# make up the standalone porgatory.ro
> > +
> > +PURGATORY_CFLAGS_REMOVE := -mcmodel=kernel
> > +PURGATORY_CFLAGS := -mcmodel=large -ffreestanding -fno-zero-initialized-in-bss
>
> Thanks for confirming the fix.  While it sounds like -mcmodel=large is
> the only necessary change, I don't object to -ffreestanding of
> -fno-zero-initialized-in-bss being readded, especially since I think
> what you've done with PURGATORY_CFLAGS_REMOVE is more concise.

Without -ffreestanding this results in undefined symbols (as before this patch)

$ readelf -a arch/x86/purgatory/purgatory.ro|grep UND
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
    51: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND __stack_chk_fail

I just bumped into this issue as I discovered that kexec() no longer works after
the x86/purgatory: Use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS -commit
was merged.