Re: [PATCH v2 1/2] x86: Add support for building a multiboot2 PE binary

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.03.2024 16:11, Ross Lagerwall wrote:
> In addition to building xen.efi and xen.gz, build xen-mbi.exe. The
> latter is a PE binary that can be used with a multiboot2 loader that
> supports loading PE binaries.

I have to admit I find .exe a strange extension outside of the Windows
world. Would it be an option to have no extension at all (xen-mbi), or
use xen.mbi?

> Using this option allows the binary to be signed and verified by Shim.
> This means the same xen-mbi.exe binary can then be used for BIOS boot,
> UEFI Boot and UEFI boot with Secure Boot verification (all with the
> convenience of GRUB2 as a bootloader).

With which "UEFI boot" really means "chainloader" from grub? That isn't
required though, is it? I.e. "UEFI boot" ought to work also without
involving grub?

> The new binary is created by modifying xen.efi:
> * Relocations are stripped since they are not needed.

Because of the changed entry point, aiui. What hasn't become clear to
me is what difference in functionality there's going to be between
booting this new image in "UEFI boot" mode vs using xen.efi. Clearly
xen.efi's own (EFI-level) command line options won't be available. But
it feels like there might be more.

> * The image base address is set to 0 since it must necessarily be below
>   4 GiB and the loader will relocate it anyway.

While technically okay, what is the reason for this adjustment? 

> * The PE entry point is set to the multiboot2 entry point rather than
>   the normal EFI entry point. This is only relevant for BIOS boot since
>   for EFI boot the entry point is specified via a multiboot2 tag.

Hmm, I may then be confused about the terminology further up: What is
"BIOS boot" then? So far I've taken that as the equivalent of xen.gz's
way of being booted (i.e. grub without EFI underneath).

> @@ -123,6 +124,19 @@ syms-warn-dup-$(CONFIG_ENFORCE_UNIQUE_SYMBOLS) := 
> --error-dup
>  
>  orphan-handling-$(call ld-option,--orphan-handling=warn) += 
> --orphan-handling=warn
>  
> +ifeq ($(XEN_BUILD_PE),y)
> +$(TARGET)-mbi.exe: $(TARGET).efi $(obj)/efi/modify-mbi-exe
> +     $(OBJCOPY) --remove-section=.reloc $< $@.tmp
> +     $(obj)/efi/modify-mbi-exe $@.tmp
> +     $(OBJCOPY) --set-start=0x$$($(NM) -pa $@.tmp | awk '/T start$$/{print 
> $$1}') $@.tmp $@.tmp2

I understand section removal is better done by objcopy. Changing the entry
point could be done by the new tool, though (by passing it the designated
address)?

As to stripping .reloc - generally this needs accompanying by setting the
"relocations stripped" flag in the COFF(?) header. Here, however, I take
it this is not only not needed, but actually not wanted. This imo wants
saying somewhere; the individual steps done here could do with brief
comments anyway, imo.

> --- /dev/null
> +++ b/xen/arch/x86/efi/modify-mbi-exe.c
> @@ -0,0 +1,77 @@
> +#include <stdio.h>
> +#include <stdint.h>
> +#include <unistd.h>
> +#include <fcntl.h>
> +
> +struct mz_hdr {
> +    uint16_t signature;
> +#define MZ_SIGNATURE 0x5a4d
> +    uint16_t last_page_size;
> +    uint16_t page_count;
> +    uint16_t relocation_count;
> +    uint16_t header_paras;
> +    uint16_t min_paras;
> +    uint16_t max_paras;
> +    uint16_t entry_ss;
> +    uint16_t entry_sp;
> +    uint16_t checksum;
> +    uint16_t entry_ip;
> +    uint16_t entry_cs;
> +    uint16_t relocations;
> +    uint16_t overlay;
> +    uint8_t reserved[32];
> +    uint32_t extended_header_base;
> +};
> +
> +struct coff_hdr {
> +    uint32_t signature;
> +    uint16_t cpu;
> +    uint16_t section_count;
> +    int32_t timestamp;
> +    uint32_t symbols_file_offset;
> +    uint32_t symbol_count;
> +    uint16_t opt_hdr_size;
> +    uint16_t flags;
> +};

I can't spot any use of this struct.

Jan

> +#define IMAGE_BASE_OFFSET 48
> +#define NEW_IMAGE_BASE 0x0
> +
> +int main(int argc, char **argv)
> +{
> +    int fd;
> +    struct mz_hdr mz_hdr;
> +    const uint64_t base_addr = NEW_IMAGE_BASE;
> +
> +    if ( argc != 2 )
> +    {
> +        fprintf(stderr, "usage: %s <image>\n", argv[0]);
> +        return 1;
> +    }
> +
> +    fd = open(argv[1], O_RDWR);
> +    if ( fd < 0 ||
> +         read(fd, &mz_hdr, sizeof(mz_hdr)) != sizeof(mz_hdr) )
> +    {
> +        perror(argv[1]);
> +        return 2;
> +    }
> +
> +    if ( mz_hdr.signature != MZ_SIGNATURE ||
> +         !mz_hdr.extended_header_base )
> +    {
> +        fprintf(stderr, "%s: Wrong DOS file format\n", argv[1]);
> +        return 2;
> +    }
> +
> +    if ( lseek(fd, mz_hdr.extended_header_base + IMAGE_BASE_OFFSET, 
> SEEK_SET) < 0 ||
> +         write(fd, &base_addr, sizeof(base_addr)) != sizeof(base_addr) )
> +    {
> +        perror(argv[1]);
> +        return 3;
> +    }
> +
> +    close(fd);
> +
> +    return 0;
> +}