Re: [PATCH] powerpc: disable support for relative ksymtab references

[Nicholas Piggin <nicholas.piggin@gmail.com>]

On Wed, 29 Aug 2018 08:47:53 +0200
Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:

> The newly added code that emits ksymtab entries as pairs of 32-bit
> relative references interacts poorly with the way powerpc lays out
> its address space: when a module exports a per-CPU variable, the
> primary module region covering the ksymtab entry -and thus the 32-bit
> relative reference- is too far away from the actual per-CPU variable's
> base address (to which the per-CPU offsets are applied to obtain the
> respective address of each CPU's copy), resulting in corruption when
> the module loader attempts to resolve symbol references of modules
> that are loaded on top and link to the exported per-CPU symbol.
> 
> So let's disable this feature on powerpc. Even though it implements
> CONFIG_RELOCATABLE, it does not implement CONFIG_RANDOMIZE_BASE and
> so KASLR kernels (which are the main target of the feature) do not
> exist on powerpc anyway.

Let's go with this for now. We have a bit more work to do with module
loader, linker, percpu, or all of the above. Thanks everyone.

Linus can you merge this? I think Michael is on vacation this week and
things are pretty crashy without it here. Don't need to call him off
the beach for it.

Thanks,
Nick

> 
> Reported-by: Andreas Schwab <schwab@linux-m68k.org>
> Suggested-by: Nicholas Piggin <nicholas.piggin@gmail.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  arch/powerpc/Kconfig | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index db0b6eebbfa5..a80669209155 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -177,7 +177,6 @@ config PPC
>  	select HAVE_ARCH_KGDB
>  	select HAVE_ARCH_MMAP_RND_BITS
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
> -	select HAVE_ARCH_PREL32_RELOCATIONS
>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_CBPF_JIT			if !PPC64
> -- 
> 2.17.1
>