OvmfXenElfHeaderGenerator.c

/** @file
  This program generates a hex array to be manually coppied into
  OvmfXen.fdf.

  The purpose is for the flash device image to be recognize as an ELF.

  Copyright (c) 2019, Citrix Systems, Inc.

  SPDX-License-Identifier: BSD-2-Clause-Patent
**/

#include "elf.h"
#include "stdio.h"
#include "stddef.h"

void print_hdr(void *s, size_t size)
{
  char *c = s;

  while (size--) {
    printf("0x%02hhx, ", *(c++));
  }
}

/* Format for the XEN_ELFNOTE_PHYS32_ENTRY program segment */
#define XEN_ELFNOTE_PHYS32_ENTRY 18
typedef struct {
  uint32_t name_size;
  uint32_t desc_size;
  uint32_t type;
  char name[4];
  uint32_t desc;
} xen_elfnote_phys32_entry;

int main(void)
{
  /* FW_SIZE */
  size_t ovmf_blob_size = 0x00200000;
  /* Load OVMF at 1MB when running as PVH guest */
  uint32_t ovmf_base_address = 0x00100000;
  /* Xen PVH entry point */
  uint32_t ovmfxen_pvh_entry_point = ovmf_base_address + ovmf_blob_size - 0x30;
  size_t offset_into_file = 0;

  /* ELF file header */
  Elf32_Ehdr hdr = {
    .e_ident = ELFMAG,
    .e_type = ET_EXEC,
    .e_machine = EM_386,
    .e_version = EV_CURRENT,
    .e_entry = ovmfxen_pvh_entry_point,
    .e_flags = R_386_NONE,
    .e_ehsize = sizeof (hdr),
    .e_phentsize = sizeof (Elf32_Phdr),
  };
  offset_into_file += sizeof (hdr);

  hdr.e_ident[EI_CLASS] = ELFCLASS32;
  hdr.e_ident[EI_DATA] = ELFDATA2LSB;
  hdr.e_ident[EI_VERSION] = EV_CURRENT;
  hdr.e_ident[EI_OSABI] = ELFOSABI_LINUX;
  /* Placing program headers just after hdr */
  hdr.e_phoff = sizeof (hdr);

  /* program header */
  Elf32_Phdr phdr_load = {
    .p_type = PT_LOAD,
    .p_offset = 0, /* load everything */
    .p_paddr = ovmf_base_address,
    .p_filesz = ovmf_blob_size,
    .p_memsz = ovmf_blob_size,
    .p_flags = PF_X | PF_W | PF_R,
    .p_align = 0,
  };
  phdr_load.p_vaddr = phdr_load.p_paddr;
  hdr.e_phnum += 1;
  offset_into_file += sizeof (phdr_load);

  /* Xen ELF Note. */

  xen_elfnote_phys32_entry xen_elf_note = {
    .type = XEN_ELFNOTE_PHYS32_ENTRY,
    .name = "Xen",
    .desc = ovmfxen_pvh_entry_point,
    .name_size =
      offsetof (xen_elfnote_phys32_entry, desc) -
      offsetof (xen_elfnote_phys32_entry, name),
    .desc_size =
      sizeof (xen_elfnote_phys32_entry) -
      offsetof (xen_elfnote_phys32_entry, desc),
  };
  Elf32_Phdr phdr_note = {
    .p_type = PT_NOTE,
    .p_filesz = sizeof (xen_elf_note),
    .p_memsz = sizeof (xen_elf_note),
    .p_flags = PF_R,
    .p_align = 0,
  };
  hdr.e_phnum += 1;
  offset_into_file += sizeof (phdr_note);
  phdr_note.p_offset = offset_into_file;
  phdr_note.p_paddr = ovmf_base_address + phdr_note.p_offset;
  phdr_note.p_vaddr = phdr_note.p_paddr;


  /*
   * print elf header
   */

  size_t i;
  size_t hdr_size = sizeof (hdr);
  size_t entry_off = offsetof(typeof(hdr), e_entry);

  printf("# ELF file header\n");
  print_hdr(&hdr, entry_off);
  printf("\n");
  print_hdr(&hdr.e_entry, sizeof (hdr.e_entry));
  printf(" # hdr.e_entry\n");
  print_hdr(&hdr.e_entry + 1, hdr_size - entry_off - sizeof (hdr.e_entry));

  printf("\n\n# ELF Program segment headers\n");
  printf("# - Load segment\n");
  for (i = 0; i < sizeof (phdr_load); i += 4) {
    print_hdr(((char*)&phdr_load) + i, 4);
    printf("\n");
  }
  printf("# - ELFNOTE segment\n");
  for (i = 0; i < sizeof (phdr_note); i += 4) {
    print_hdr(((char*)&phdr_note) + i, 4);
    printf("\n");
  }

  printf("\n# XEN_ELFNOTE_PHYS32_ENTRY\n");
  for (i = 0; i < sizeof (xen_elf_note); i += 4) {
    print_hdr(((char*)&xen_elf_note) + i, 4);
    printf("\n");
  }

  return 0;
}