[PATCH v2 3/8] efi: move MS-DOS stub out of generic PE header definition

[Ard Biesheuvel]

The PE/COFF spec permits the COFF signature and file header to appear
anywhere in the file, and the actual offset is recorded in 4 byte
little endian field at offset 0x3c of the image.

When GRUB is emitted as a PE/COFF binary, we reuse the 128 byte MS-DOS
stub (even for non-x86 architectures), putting the COFF signature and
file header at offset 0x80. However, other PE/COFF images may use
different values, and non-x86 Linux kernels use an offset of 0x40
instead.

So let's get rid of the grub_pe32_header struct from pe32.h, given that
it does not represent anything defined by the PE/COFF spec. Instead,
use the GRUB_PE32_MSDOS_STUB_SIZE macro explicitly to reference the
COFF header in the only place in the code where we rely on this.

The remaining fields are moved into a struct grub_coff_image_header,
which we will use later to access COFF header fields of arbitrary
images (and which may therefore appear at different offsets)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
---
 grub-core/kern/efi/efi.c | 5 +++--
 include/grub/efi/pe32.h  | 5 +----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/grub-core/kern/efi/efi.c b/grub-core/kern/efi/efi.c
index e0165e74c587..9e5a72be538d 100644
--- a/grub-core/kern/efi/efi.c
+++ b/grub-core/kern/efi/efi.c
@@ -282,7 +282,7 @@ grub_addr_t
 grub_efi_modules_addr (void)
 {
   grub_efi_loaded_image_t *image;
-  struct grub_pe32_header *header;
+  struct grub_coff_image_header *header;
   struct grub_pe32_coff_header *coff_header;
   struct grub_pe32_section_table *sections;
   struct grub_pe32_section_table *section;
@@ -293,7 +293,8 @@ grub_efi_modules_addr (void)
   if (! image)
     return 0;
 
-  header = image->image_base;
+  header = (struct grub_coff_image_header *) ((char *) image->image_base
+                                             + GRUB_PE32_MSDOS_STUB_SIZE);
   coff_header = &(header->coff_header);
   sections
     = (struct grub_pe32_section_table *) ((char *) coff_header
diff --git a/include/grub/efi/pe32.h b/include/grub/efi/pe32.h
index 0ed8781f0376..a2da4b318c85 100644
--- a/include/grub/efi/pe32.h
+++ b/include/grub/efi/pe32.h
@@ -254,11 +254,8 @@ struct grub_pe32_section_table
 
 #define GRUB_PE32_SIGNATURE_SIZE 4
 
-struct grub_pe32_header
+struct grub_coff_image_header
 {
-  /* This should be filled in with GRUB_PE32_MSDOS_STUB.  */
-  grub_uint8_t msdos_stub[GRUB_PE32_MSDOS_STUB_SIZE];
-
   /* This is always PE\0\0.  */
   char signature[GRUB_PE32_SIGNATURE_SIZE];
 
-- 
2.17.1