[PATCH resend 2/9] efi: move MS-DOS stub out of generic PE header definition

Ard Biesheuvel
2022-08-18T08:55:55Z

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 <ardb@kernel.org>
---
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 e8a976a22f15..8bef81663853 100644
--- a/grub-core/kern/efi/efi.c
+++ b/grub-core/kern/efi/efi.c
@@ -302,7 +302,7 @@ 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;
@@ -313,7 +313,8 @@ 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];

--
2.35.1