[Xen-devel] [PATCH v2 05/31] OvmfPkg/XenOvmf: Creating an ELF header

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

This header replace the embedded variable store.

The ELF header explain to a loader to load the binary at the address
1MB, then jump to the PVH entry point which will be created in a later
patch.

That patch include generate_elf_header.c which can be use to regenerate
the ELF header, but this will be a manual step.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---

Notes:
    This "ELF header" might not be the best way, but with it the Xen
    toolstack can load OVMF like any other PVH-compatible kernel without
    modification. Is there a better way?
    
    The generate_elf_header.c file was used to generate the series of hex
    number. It isn't at a right place but I'm not sure where to put this C
    file. Maybe in the commit message or maybe it could be forgotten since
    I've added all the comments in the .fdf file.

 OvmfPkg/XenOvmf.fdf   | 82 +++++++++++++++++++-
 generate_elf_header.c | 78 +++++++++++++++++++
 2 files changed, 157 insertions(+), 3 deletions(-)

diff --git a/OvmfPkg/XenOvmf.fdf b/OvmfPkg/XenOvmf.fdf
index 295e30ca3f..20ebacd673 100644
--- a/OvmfPkg/XenOvmf.fdf
+++ b/OvmfPkg/XenOvmf.fdf
@@ -21,8 +21,8 @@ [Defines]
 !include OvmfPkg.fdf.inc
 
 #
-# Build the variable store and the firmware code as one unified flash device
-# image.
+# This will allow the flash device image to be recognize as an ELF, with first
+# an ELF headers, then the firmware code.
 #
 [FD.OVMF]
 BaseAddress   = $(FW_BASE_ADDRESS)
@@ -31,7 +31,83 @@ [FD.OVMF]
 BlockSize     = $(BLOCK_SIZE)
 NumBlocks     = $(FW_BLOCKS)
 
-!include VarStore.fdf.inc
+!if ($(FD_SIZE_IN_KB) == 1024) || ($(FD_SIZE_IN_KB) == 2048)
+0x00000000|0x0000e000
+!endif
+!if $(FD_SIZE_IN_KB) == 4096
+0x00000000|0x00040000
+!endif
+#was NV_VARIABLE_STORE
+DATA = {
+  # ELF file header
+  0x7f, 0x45, 0x4c, 0x46, # e_ident[0..3]: Magic number
+  0x01, # File class: 32-bit objects
+  0x01, # Data encoding: 2's complement, little endian
+  0x01, # File version
+  0x03, # OS ABI identification: Object uses GNU ELF extensions
+  0x00, # ABI version
+  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  # e_ident[EI_PAD...]
+
+  0x02, 0x00, # e_type = Executable file
+  0x03, 0x00, # e_machine = Intel 80386
+  0x01, 0x00, 0x00, 0x00, # e_version
+  0xd0, 0xff, 0x2f, 0x00, # e_entry: Entry point virtual address
+  0x34, 0x00, 0x00, 0x00, # e_phoff: Program header table file offset
+  0x00, 0x00, 0x00, 0x00, # e_shoff: Section header table file offset
+  0x00, 0x00, 0x00, 0x00, # e_flags: Processor-specific flags
+  0x34, 0x00, #    e_ehsize: ELF header size
+  0x20, 0x00, # e_phentsize: Program header table entry size
+  0x01, 0x00, #     e_phnum: Program header table entry count
+  0x00, 0x00, # e_shentsize: Section header table entry size
+  0x00, 0x00, #     e_shnum: Section header table entry count
+  0xff, 0xff, # e_shstrndx
+
+  # ELF Program segment header
+  0x01, 0x00, 0x00, 0x00, # p_type = Loadable program segment
+  0x00, 0x00, 0x00, 0x00, # p_offset
+  0x00, 0x00, 0x10, 0x00, # p_vaddr: Segment virtual address
+  0x00, 0x00, 0x10, 0x00, # p_paddr: Segment physical address
+  0x00, 0x00, 0x20, 0x00, # p_filesz: Segment size in file
+  0x00, 0x00, 0x20, 0x00, # p_memsz: Segment size in memory
+  0x07, 0x00, 0x00, 0x00, # p_flags = Segment is executable | writable | 
readable
+  0x00, 0x00, 0x00, 0x00  # p_align
+
+}
+
+!if ($(FD_SIZE_IN_KB) == 1024) || ($(FD_SIZE_IN_KB) == 2048)
+0x0000e000|0x00001000
+!endif
+!if $(FD_SIZE_IN_KB) == 4096
+0x00040000|0x00001000
+!endif
+#NV_EVENT_LOG
+
+!if ($(FD_SIZE_IN_KB) == 1024) || ($(FD_SIZE_IN_KB) == 2048)
+0x0000f000|0x00001000
+!endif
+!if $(FD_SIZE_IN_KB) == 4096
+0x00041000|0x00001000
+!endif
+#NV_FTW_WORKING
+DATA = {
+  # EFI_FAULT_TOLERANT_WORKING_BLOCK_HEADER->Signature = 
gEdkiiWorkingBlockSignatureGuid         =
+  #  { 0x9e58292b, 0x7c68, 0x497d, { 0xa0, 0xce, 0x65,  0x0, 0xfd, 0x9f, 0x1b, 
0x95 }}
+  0x2b, 0x29, 0x58, 0x9e, 0x68, 0x7c, 0x7d, 0x49,
+  0xa0, 0xce, 0x65,  0x0, 0xfd, 0x9f, 0x1b, 0x95,
+  # Crc:UINT32            #WorkingBlockValid:1, WorkingBlockInvalid:1, Reserved
+  0x2c, 0xaf, 0x2c, 0x64, 0xFE, 0xFF, 0xFF, 0xFF,
+  # WriteQueueSize: UINT64
+  0xE0, 0x0F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
+}
+
+!if ($(FD_SIZE_IN_KB) == 1024) || ($(FD_SIZE_IN_KB) == 2048)
+0x00010000|0x00010000
+!endif
+!if $(FD_SIZE_IN_KB) == 4096
+0x00042000|0x00042000
+!endif
+#NV_FTW_SPARE
+
 
 $(VARS_SIZE)|$(FVMAIN_SIZE)
 FV = FVMAIN_COMPACT
diff --git a/generate_elf_header.c b/generate_elf_header.c
new file mode 100644
index 0000000000..5bb87df0d6
--- /dev/null
+++ b/generate_elf_header.c
@@ -0,0 +1,78 @@
+#include "elf.h"
+#include "stdio.h"
+#include "stddef.h"
+
+/*
+ * TODO:
+ *   this header will need a XEN_ELFNOTE_PHYS32_ENTRY
+ *   right now, it works without, but that might change.
+ */
+
+void print_hdr(void *s, size_t size) {
+  char *c = s;
+
+  while (size--) {
+    printf("0x%02hhx, ", *(c++));
+  }
+}
+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;
+
+  /* ELF file header */
+  Elf32_Ehdr hdr = {
+    .e_ident = ELFMAG,
+    .e_type = ET_EXEC,
+    .e_machine = EM_386,
+    .e_version = EV_CURRENT,
+    // PVH entry point
+    .e_entry = ovmf_base_address + ovmf_blob_size - 0x30,
+  };
+
+  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;
+  hdr.e_flags = R_386_NONE;
+  hdr.e_ehsize = sizeof (hdr);
+  /* about program header */
+  hdr.e_phoff = sizeof (hdr);
+  /* about section header */
+  hdr.e_shentsize = 0;
+  hdr.e_shnum = 0;
+  hdr.e_shstrndx = -1;
+
+  /* program header */
+  Elf32_Phdr phdr = {
+    .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, // is this needed?
+  };
+  phdr.p_vaddr = phdr.p_paddr;
+
+  hdr.e_phnum += 1;
+  hdr.e_phentsize += sizeof (phdr);
+
+  /* section header */
+  Elf32_Shdr shdr = {
+  };
+
+  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# hdr.e_entry\n");
+  print_hdr(&hdr.e_entry, sizeof(hdr.e_entry));
+  printf("\n# the rest\n");
+  print_hdr(&hdr.e_entry + 1, hdr_size - entry_off - sizeof(hdr.e_entry));
+  printf("\n# Program segment header\n");
+  print_hdr(&phdr, sizeof (phdr));
+
+  return 0;
+}
-- 
Anthony PERARD


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