Re: [PATCH v3] efi: Request desired alignment via the PE/COFF headers

[Linn Crosetto <linn-VXdhtT5mjnY@public.gmane.org>]

Michael Brown <mbrown@...> writes:

> 
> The kernel will align itself to the nearest boundary specified by the
> kernel_alignment field in the bzImage header.  If the kernel is loaded
> to an address which is not sufficiently aligned, it will therefore use
> memory beyond that indicated solely by the init_size field.
> 
> The PE/COFF headers now include a .bss section to describe the
> requirements of the init_size field, but do not currently expose the
> alignment requirement.  Consequently, a kernel loaded via the PE entry
> point may still end up overwriting unexpected areas of memory.
> 
> Fix by exposing the desired alignment via the SectionAlignment field
> in the PE/COFF headers.  Despite its name, this field provides an
> overall alignment requirement for the loaded file.  (Naturally, the
> FileAlignment field describes the alignment for individual sections.)
> 
> There is no way in the PE/COFF headers to express the concept of
> min_alignment; we therefore do not expose the minimum (as opposed to
> preferred) alignment.
> 
> Signed-off-by: Michael Brown <mbrown@...>
> ---
>  arch/x86/boot/header.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 7a6d43a..16ef025 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
>  <at>  <at>  -154,7 +154,7  <at>  <at>  extra_header_fields:
>  #else
>  	.quad	0				# ImageBase
>  #endif
> -	.long	0x20				# SectionAlignment
> +	.long	CONFIG_PHYSICAL_ALIGN		# SectionAlignment
>  	.long	0x20				# FileAlignment
>  	.word	0				# 
MajorOperatingSystemVersion
>  	.word	0				# 
MinorOperatingSystemVersion

There may be a problem with this change. The specification says that the 
SizeOfImage field must be a multiple of SectionAlignment. That was the case 
when SectionAlignment was 0x20, but now that it is so large it would 
require quite an increase in the last section to comply. A kernel I am 
looking at now has:

Alignment of sections:	0x1000000
Size of image:	0x105a000