+
++This is a neat trick I learnt recently while experimenting with building a +small EFI bootloader shim. +Most guides will recommend testing using qemu and EDK2, but there +actually is an even simpler way to test (some) EFI applications entirely +without qemu or rebooting. + +
+U-Boot can be compiled as a simple user-space Linux executable using the +"sandbox" pseudo-architecture. This is mostly used in the U-Boot test suite but +we can also use it as a simple user-space EFI test environment. +Obviously only the UEFI subset supported by U-boot will work. + +
+Building the sandbox binary is as easy as fetching the source code from + + https://github.com/u-boot/u-boot +, selecting the Sandbox configuration and building the binary via: + +
+$ make sandbox_defconfig all ++ +
+The resulting u-boot file is a regular ELF Linux binary. +Running it with -h gives us a list of supported command line options. + +
+$ ./u-boot -h +U-Boot 2024.07-rc2 (May 19 2024 - 17:27:46 +0200) + +u-boot, a command line test interface to U-Boot + +Usage: u-boot [options] +Options: + --autoboot_keyed Allow keyed autoboot + -b, --boot Run distro boot commands + -c, --command <arg> Execute U-Boot command + -d, --fdt <arg> Specify U-Boot's control FDT + -D, --default_fdt Use the default u-boot.dtb control FDT in U-Boot directory + -h, --help Display help + -i, --interactive Enter interactive mode + -j, --jump <arg> Jumped from previous U-Boot + -k, --select_unittests <arg> Select unit tests to run + -K, --double_lcd Double the LCD display size in each direction + -l, --show_lcd Show the sandbox LCD display + -L, --log_level <arg> Set log level (0=panic, 7=debug) + -m, --memory <arg> Read/write ram_buf memory contents from file + -n, --ignore_missing Ignore missing state on read + -p, --program <arg> U-Boot program name + -r, --read Read the state FDT on startup + --rm_memory Remove memory file after reading + -s, --state <arg> Specify the sandbox state FDT + -S, --signals Handle signals (such as SIGSEGV) in sandbox + -t, --terminal <arg> Set terminal to raw/cooked mode + -T, --test_fdt Use the test.dtb control FDT in U-Boot directory + -u, --unittests Run unit tests + -v, --verbose Show test output + -w, --write Write state FDT on exit ++ +
+More info on sandbox mode and other interesting configuration options can be +found in the official U-Boot documentation at [1]. + +
+Running the binary without arguments allows us to access a regular U-Boot shell, +load files and execute them via UEFI. +The last missing puzzle piece is getting the binary from the host file system +into U-Boot. For this we can use the host interface documented at +[2]. + +
+host load allows as to load a file from the host file system directly +into U-Boot's address space. +The easiest way to load and execute a binary is loading it to the pre-defined +kernel_addr_r address and then executing it via bootefi. + +
+For simplicity we use the helloworld.efi application that ships +with U-Boot: + +
+=> host load hostfs - $kernel_addr_r lib/efi_loader/helloworld.efi +5632 bytes read in 0 ms +=> bootefi $kernel_addr_r +No EFI system partition +No EFI system partition +Failed to persist EFI variables +Missing TPMv2 device for EFI_TCG_PROTOCOL +Missing RNG device for EFI_RNG_PROTOCOL +Booting /lib\efi_loader\helloworld.efi +Hello, world! +Running on UEFI 2.10 +Have ACPI 2.0 table +Load options:+ ++File path: /lib\efi_loader\helloworld.efi +Missing device handle +=> +
+And there we have our "Hello, world!". Of course we can also run more +complex applications than that as long as they work in U-Boot. +Even graphical EFI apps should not be a problem thanks to built-in SDL support. + +
+Finally, to make things even easier we can use the -c flag to pass +those commands directly to u-boot and skip the interactive session entirely: + +
+$ ./u-boot -c "host load hostfs - \$kernel_addr_r lib/efi_loader/helloworld.efi; bootefi \$kernel_addr_r" ++ +