[PATCH v5 1/6] 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,
introduce a minimal struct grub_msdos_image_header type based on the
PE/COFF spec's description of the image header, and use the offset
recorded at file position 0x3c to discover the actual location of the PE
signature and the COFF image header.

The remaining fields are moved into a struct grub_pe_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 |  8 ++++++--
 include/grub/efi/pe32.h  | 16 ++++++++++++----
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/grub-core/kern/efi/efi.c b/grub-core/kern/efi/efi.c
index e8a976a22f15..cf49d6357e00 100644
--- a/grub-core/kern/efi/efi.c
+++ b/grub-core/kern/efi/efi.c
@@ -302,7 +302,8 @@ grub_addr_t
 grub_efi_modules_addr (void)
 {
   grub_efi_loaded_image_t *image;
-  struct grub_pe32_header *header;
+  struct grub_msdos_image_header *header;
+  struct grub_pe_image_header *pe_image_header;
   struct grub_pe32_coff_header *coff_header;
   struct grub_pe32_section_table *sections;
   struct grub_pe32_section_table *section;
@@ -314,7 +315,10 @@ grub_efi_modules_addr (void)
     return 0;
 
   header = image->image_base;
-  coff_header = &(header->coff_header);
+  pe_image_header
+    = (struct grub_pe_image_header *) ((char *) header
+                                       + header->pe_image_header_offset);
+  coff_header = &(pe_image_header->coff_header);
   sections
     = (struct grub_pe32_section_table *) ((char *) coff_header
                                          + sizeof (*coff_header)
diff --git a/include/grub/efi/pe32.h b/include/grub/efi/pe32.h
index 0ed8781f0376..98c4ff177e19 100644
--- a/include/grub/efi/pe32.h
+++ b/include/grub/efi/pe32.h
@@ -48,6 +48,17 @@
 
 #define GRUB_PE32_MAGIC                        0x5a4d
 
+struct grub_msdos_image_header
+{
+  /* This is always 'MZ'. (GRUB_PE32_MAGIC)  */
+  grub_uint16_t msdos_magic;
+
+  grub_uint16_t reserved[29];
+
+  /* The file offset of the PE image header. */
+  grub_uint32_t pe_image_header_offset;
+};
+
 /* According to the spec, the minimal alignment is 512 bytes...
    But some examples (such as EFI drivers in the Intel
    Sample Implementation) use 32 bytes (0x20) instead, and it seems
@@ -254,11 +265,8 @@ struct grub_pe32_section_table
 
 #define GRUB_PE32_SIGNATURE_SIZE 4
 
-struct grub_pe32_header
+struct grub_pe_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