How UEFI selects the image consistent with its architecture from the option rom, loads and executes it?

[xuchao1234]

I didn't find any relevant processing in the "ProcessOpRomImage" function. Please tell me the location of the relevant code, Thank very much!

[Kostr]

Hi!

I'm not an expert, but I've tried to investigate this subject myself.

According to the UEFI specification target Machine Type is encoded in the header of a ROM image:

You can find this field in the relevant EFI_PCI_EXPANSION_ROM_HEADER structure https://github.com/tianocore/edk2/blob/master/MdePkg/Include/IndustryStandard/Pci22.h

///
/// EFI PCI Expansion ROM Header
/// Section 13.4.2, Unified Extensible Firmware Interface Specification, Version 2.1
///
typedef struct {
  UINT16    Signature;  ///< 0xaa55
  UINT16    InitializationSize;
  UINT32    EfiSignature; ///< 0x0EF1
  UINT16    EfiSubsystem;
  UINT16    EfiMachineType;                      // <-------------
  UINT16    CompressionType;
  UINT8     Reserved[8];
  UINT16    EfiImageHeaderOffset;
  UINT16    PcirOffset;
} EFI_PCI_EXPANSION_ROM_HEADER;

But it is not processed neither in the PCI ROM parsing code, nor in the loadpcirom UEFI shell command code:

edk2/MdeModulePkg/Bus/Pci/PciBusDxe/PciOptionRomSupport.c

Line 76 in 7c0ad2c

if ((Pcir->CodeType == PCI_CODE_TYPE_EFI_IMAGE) &&

edk2/ShellPkg/Library/UefiShellDebug1CommandsLib/LoadPciRom.c

Line 268 in 7c0ad2c

if ((Pcir->CodeType == PCI_CODE_TYPE_EFI_IMAGE) &&

So it looks like EDKII just loads every image without checking its machine type. And it is responsibility of the LoadImage() function to fail if the architecture is wrong.

Here is the relevant info from the UEFI specification that supports this conclusion: