How to access efi executable present in another partition using gnu-efi?

Question

I have to create 2 fat16 partitions in a disk and place one of the efi binary in another fat16 partition.

To begin with, wrote a basic HelloWorld.efi application (second stage) and for the first stage, used below-mentioned 2 apis to execute the HelloWorld.efi (both binaries are present in the same partition)

Currently I am trying to figure out a way to access the HelloWorld.efi present in another partition. Using these apis, I can access any file present in the same partition but don't know what api(s) to use to access another partition? Any suggestions? Thanks.

efi_status = BS->HandleProtocol(image_handle, &EFI_LOADED_IMAGE_GUID,
                (void **)&shim_li);

efi_status = BS->HandleProtocol(device, &EFI_SIMPLE_FILE_SYSTEM_GUID,
                (void **) &drive);

score 1 · Accepted Answer · answered Sep 15 '22 at 06:25

The simplest way would be to check if the file exists inside any filesystem.

The code below uses EDK2 but should work with gnu-efi with minimal modifications.

EFI_STATUS Status;
EFI_HANDLE* Handles;
UINTN HandleCount;
UINTN HandleIndex;
EFI_DEVICE_PATH_PROTOCOL* FilePath;
EFI_HANDLE ImageHandle;
CHAR16* ExitData;
UINTN ExitDataLength;

Status = gBS->LocateHandleBuffer(ByProtocol, &gEfiSimpleFileSystemProtocolGuid, NULL, &HandleCount, &Handles);
// error handling

for (HandleIndex = 0; HandleIndex < HandleCount; HandleIndex++) {
    FilePath = FileDevicePath(Handles[HandleIndex], L"MyFolder\\HelloWorld.efi");
    if (FilePath == NULL) {
        continue;
    }
    
    Status = gBS->LoadImage(TRUE, gImageHandle, FilePath, NULL, 0, &ImageHandle);
    gBS->FreePool(FilePath);
    if (Status != EFI_SUCCESS) {
        continue;
    }
    // Set LoadOptions and Watchdog if needed
    Status = gBS->StartImage(ImageHandle, &ExitDataLength, &ExitData);
    // error handling
}
gBS->FreePool(Handles);

If you know the partition id you can parse the device path of the filesystem and search for the correct partition.

How to access efi executable present in another partition using gnu-efi?

1 Answers1