Re: [PATCH 2/2] efi/arm64: increase the PE/COFF alignment so the kernel can run in place

[Ard Biesheuvel <ardb@kernel.org>]

Hi Arvind,

Thanks for taking a look.

On Sat, 28 Mar 2020 at 15:06, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Thu, Mar 26, 2020 at 05:59:05PM +0100, Ard Biesheuvel wrote:
> > Update the PE/COFF metadata so that the UEFI image loader will load the
> > kernel image at an offset that allows it to execute in place.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> >  arch/arm64/kernel/efi-header.S | 2 +-
> >  arch/arm64/kernel/image-vars.h | 7 +++++++
> >  2 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/efi-header.S b/arch/arm64/kernel/efi-header.S
> > index 914999ccaf8a..f9ee1c2a5fd6 100644
> > --- a/arch/arm64/kernel/efi-header.S
> > +++ b/arch/arm64/kernel/efi-header.S
> > @@ -32,7 +32,7 @@ optional_header:
> >
> >  extra_header_fields:
> >       .quad   0                                       // ImageBase
> > -     .long   SZ_4K                                   // SectionAlignment
> > +     .long   PECOFF_SECTION_ALIGNMENT                // SectionAlignment
> >       .long   PECOFF_FILE_ALIGNMENT                   // FileAlignment
> >       .short  0                                       // MajorOperatingSystemVersion
> >       .short  0                                       // MinorOperatingSystemVersion
> > diff --git a/arch/arm64/kernel/image-vars.h b/arch/arm64/kernel/image-vars.h
> > index be0a63ffed23..7a7fa3ba7b2f 100644
> > --- a/arch/arm64/kernel/image-vars.h
> > +++ b/arch/arm64/kernel/image-vars.h
> > @@ -15,6 +15,13 @@
> >  __efistub_kernel_size                = _edata - _text;
> >  __efistub_primary_entry_offset       = primary_entry - _text;
> >
> > +#ifndef CONFIG_RELOCATABLE
> > +PECOFF_SECTION_ALIGNMENT = SZ_4K;
> > +#elif THREAD_ALIGN > SEGMENT_ALIGN
> > +PECOFF_SECTION_ALIGNMENT = THREAD_ALIGN;
> > +#else
> > +PECOFF_SECTION_ALIGNMENT = SEGMENT_ALIGN;
> > +#endif
> >
> >  /*
> >   * The EFI stub has its own symbol namespace prefixed by __efistub_, to
> > --
> > 2.17.1
> >
>
> The section virtual addresses and (possibly) size of image need to be
> updated to be a multiple of PECOFF_SECTION_ALIGNMENT, no?

Indeed. I spotted that after sending this patch - both _end and
_initdata_begin need to be aligned to this value. I also noticed that
it is unclear whether values over 64 KB are permitted: the PE/COFF
spec mentions that 64 KB is the max for FileAlignment, and that
SectionAlignment should be larger or equal to that.

So I think it would be better to set this value to 64 KB
unconditionally, and round up the sections to 64 KB. This means 64k
pagesize kernels with vmap'ed stack have a 50% chance of ending up at
an offset that requires moving the image, but this is still an
improvement over doing it all the time.