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

[H.J. Lu]

On Mon, Jul 15, 2019 at 3:35 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.

-- 
H.J.