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

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);

Answer 1

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.