Re: [PATCH 3/3] x86/EFI: don't accept 64-bit base relocations on page tables

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 19/08/16 08:52, Jan Beulich wrote:
> Page tables get pre-populated with physical addresses which, due to
> living inside the Xen image, will never exceed 32 bits in width. That
> in turn results in the tool generating the relocations to produce
> 32-bit relocations for them instead of the 64-bit ones needed for
> relocating virtual addresses. Hence instead of special casing page
> tables in the processing of 64-bit relocations, let's be more rigid
> and refuse them (as being indicative of something else having gone
> wrong in the build process).
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Is it an ABI requirement to use the minimal available relocation?  It is
certainly suboptimal to use a 64bit relocation when a 32bit one would
do, but I wouldn't bet that it is unconditional avoided by all toolchains.

It is currently the case that Xen needs to live below 4GB physical, so
from that point of view a 64bit relocation will not be required in the
pagetables.

~Andrew

>
> --- a/xen/arch/x86/efi/efi-boot.h
> +++ b/xen/arch/x86/efi/efi-boot.h
> @@ -81,12 +81,9 @@ static void __init efi_arch_relocate_ima
>                  }
>                  break;
>              case PE_BASE_RELOC_DIR64:
> -                if ( delta )
> -                {
> -                    *(u64 *)addr += delta;
> -                    if ( in_page_tables(addr) )
> -                        *(intpte_t *)addr += xen_phys_start;
> -                }
> +                if ( in_page_tables(addr) )
> +                    blexit(L"Unexpected relocation type");
> +                *(u64 *)addr += delta;
>                  break;
>              default:
>                  blexit(L"Unsupported relocation type");
>
>
>
>
>
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> https://lists.xen.org/xen-devel


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