Re: [PATCH v3 0/5] Support Secure Boot for multiboot2 Xen

[Bobby Eshleman <bobbyeshleman@xxxxxxxxx>]

On Fri, Jan 22, 2021 at 10:39:28AM +0100, Jan Beulich wrote:
> On 22.01.2021 01:51, Bobby Eshleman wrote:
> > I followed with v2 feedback and attempted to convert the PE/COFF header
> > into C instead of ASM.  Unfortunately, this was only possible for the
> > first part (Legacy) of the PE/COFF header.  The other parts required
> > addresses only available at link time (such as __2M_rwdata_end,
> > __pe_SizeOfImage, efi_mb_start address, etc...), which effectively ruled
> > out C.
> 
> I don't follow the conclusion drawn, so would you mind going into
> further detail?
> 

No problem at all, bad explanation on my part.  The core issue is
actually about the legality of casting 64-bit addresses to 32-bit values
in constant expressions, which then is sometimes complained about by GCC
in terms of load-time computability...

Taking __2M_rwdata_end as an example.  Attempting to use it in
the PE/COFF optional header in C looks something like:

extern const char __2M_rwdata_end[];
extern const char efi_pe_head_end[];

struct optional_header optional_header = {
...
    .code_size = (uint32_t)((unsigned long)&__2M_rwdata_end) -
                    (uint32_t)((unsigned long)&efi_pe_head_end,
...
}

GCC throws "error: initializer element is not constant" because casting
a 64-bit address to a 32-bit value is not a legal constant expression
for static storage class objects, even though we know that in practice
the address wouldn't ever be above 4GB.

efi_pe_head_end could potentially be calculated by header struct sizes,
but doing that predictably yields the same error.

If we drop the explicit casting, GCC throws 'error: initializer element
is not computable at load time'.

tl;dr:

I could not find a way to derive code size (data sections and all)
without using a symbol location, which is an illegal constant expression
operand for initializing static storage class objects... and I could not
find a way to define the header in C without using an object of static
storage class (global variable or static variable).

-Bob