linux-kernel - Re: [PATCH v3 1/2] x86/mm: Identify the end of the kernel area to be reserved

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Date:   Tue, 23 Jul 2019 15:05:13 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Mike Lothian <mike@...eburn.co.uk>,
        Tom Lendacky <thomas.lendacky@....com>, bhe@...hat.com,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, lijiang@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v3 1/2] x86/mm: Identify the end of the kernel area to be
 reserved

On Mon, Jul 15, 2019 at 01:16:48PM -0700, H.J. Lu wrote:
> On Mon, Jul 15, 2019 at 3:35 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > On Mon, 15 Jul 2019, Thomas Gleixner wrote:
> > > On Mon, 15 Jul 2019, Mike Lothian wrote:
> > > > That build failure is from the current tip of Linus's tree
> > > > If the fix is in, then it hasn't fixed the issue
> > >
> > > The reverted commit caused a build fail with gold as well. Let me stare at
> > > your issue.
> >
> > So with gold the build fails in the reloc tool complaining about that
> > relocation:
> >
> >   Invalid absolute R_X86_64_32S relocation: __end_of_kernel_reserve
> >
> > The commit does:
> >
> > +extern char __end_of_kernel_reserve[];
> > +
> >
> >  void __init setup_arch(char **cmdline_p)
> >  {
> > +       /*
> > +        * Reserve the memory occupied by the kernel between _text and
> > +        * __end_of_kernel_reserve symbols. Any kernel sections after the
> > +        * __end_of_kernel_reserve symbol must be explicitly reserved with a
> > +        * separate memblock_reserve() or they will be discarded.
> > +        */
> >         memblock_reserve(__pa_symbol(_text),
> > -                        (unsigned long)__bss_stop - (unsigned long)_text);
> > +                        (unsigned long)__end_of_kernel_reserve - (unsigned long)_text);
> >
> > So it replaces __bss_stop with __end_of_kernel_reserve here.
> >
> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -368,6 +368,14 @@ SECTIONS
> >                 __bss_stop = .;
> >         }
> >
> > +       /*
> > +        * The memory occupied from _text to here, __end_of_kernel_reserve, is
> > +        * automatically reserved in setup_arch(). Anything after here must be
> > +        * explicitly reserved using memblock_reserve() or it will be discarded
> > +        * and treated as available memory.
> > +        */
> > +       __end_of_kernel_reserve = .;
> >
> > And from the linker script __bss_stop and __end_of_kernel_reserve are
> > exactly the same. From System.map (of a successful ld build):
> >
> > ffffffff82c00000 B __brk_base
> > ffffffff82c00000 B __bss_stop
> > ffffffff82c00000 B __end_bss_decrypted
> > ffffffff82c00000 B __end_of_kernel_reserve
> > ffffffff82c00000 B __start_bss_decrypted
> > ffffffff82c00000 B __start_bss_decrypted_unused
> >
> > So how on earth can gold fail with that __end_of_kernel_reserve change?
> >
> > For some unknown reason it turns that relocation into an absolute
> > one. That's clearly a gold bug^Wfeature and TBH, I'm more than concerned
> > about that kind of behaviour.
> >
> > If we just revert that commit, then what do we achieve? We paper over the
> > underlying problem, which is not really helping anything.
> >
> > Aside of that gold still fails to build the X32 VDSO and it does so for a
> > very long time....
> >
> > Until we really understand what the problem is, this stays as is.
> >
> > @H.J.: Any insight on that?
> >
> 
> Since building a workable kernel for different kernel configurations isn't a
> requirement for gold, I don't recommend gold for kernel.

Um, it worked before this commit, and now it doesn't.  "Some" companies
are using gold for linking the kernel today...

thanks,

greg k-h