Re: [PATCH v6 13/27] x86/boot/64: Build head64.c as mcmodel large when PIE is enabled

[Kirill A. Shutemov]

On Thu, Jan 31, 2019 at 11:24:20AM -0800, Thomas Garnier wrote:
> The __startup_64 function assumes all symbols have relocated addresses
> instead of the current boot virtual address. PIE generated code favor
> relative addresses making all virtual and physical address math incorrect.
> If PIE is enabled, build head64.c as mcmodel large instead to ensure absolute
> references on all memory access. Add a global __force_order variable required
> when using a large model with read_cr* functions.
> 
> To build head64.c as mcmodel=large, disable the retpoline gcc flags.
> This code is used at early boot and removed later, it doesn't need
> retpoline mitigation.
> 
> Position Independent Executable (PIE) support will allow to extend the
> KASLR randomization range below 0xffffffff80000000.
> 
> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxxxx>
> ---
>  arch/x86/kernel/Makefile | 6 ++++++
>  arch/x86/kernel/head64.c | 3 +++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 00b7e27bc2b7..1f98f52eab9f 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -22,6 +22,12 @@ CFLAGS_REMOVE_early_printk.o = -pg
>  CFLAGS_REMOVE_head64.o = -pg
>  endif
>  
> +ifdef CONFIG_X86_PIE
> +# Remove PIE and retpoline flags that are incompatible with mcmodel=large
> +CFLAGS_REMOVE_head64.o += -fPIE -mindirect-branch=thunk-extern -mindirect-branch-register
> +CFLAGS_head64.o = -mcmodel=large
> +endif
> +
>  KASAN_SANITIZE_head$(BITS).o				:= n
>  KASAN_SANITIZE_dumpstack.o				:= n
>  KASAN_SANITIZE_dumpstack_$(BITS).o			:= n
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 16b1cbd3a61e..22e81275495b 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -63,6 +63,9 @@ EXPORT_SYMBOL(vmemmap_base);
>  
>  #define __head	__section(.head.text)
>  
> +/* Required for read_cr3 when building as PIE */
> +unsigned long __force_order;
> +

I believe it only needed for GCC < 5. Newer GCC can eliminate the
reference. See my comment in arch/x86/boot/compressed/pgtable_64.c.

Maybe we should expand the comment here too?

-- 
 Kirill A. Shutemov