Accept ELFs with an OS/ABI of ELFOSABI_GNU

[255]

RP2 ELF files typically have their EI_OSABI field set to ELFOSABI_NONE. However, when Clang uses GNU extensions, it sets EI_OSABI to ELFOSABI_GNU. The binary is still fully compatible with RP2; ELFOSABI_GNU just indicates that GNU extensions are used in the ELF.

For example, Clang uses the SHF_GNU_RETAIN section flag when compiling a C++17 inline variable with a non-default constructor. This results in an ELF with an EI_OSABI of ELFOSABI_GNU. Picotool currently rejects these ELFs, even though the binary is compatible with the device.

[will-v-pi]

Could you provide a sample ELF file which has ELFOSABI_GNU, so that we can check that the rest of the ELF file handling still works with it?

Also, could you target this PR at the develop branch rather than master

[255]

I created a minimal example to demonstrate the issue. I built it in Pigweed using Bazel and Clang (see http://pwrev.dev/229671).

#include "pico/stdlib.h"

struct HasNonDefaultConstructor {
  HasNonDefaultConstructor() {}
};

// An inline variable with a non-default destruction causes Clang to use the
// SHF_GNU_RETAIN section flag, which changes the OS/ABI field to ELFOSABI_GNU.
// If you remove `inline` on the next line, the ELF will use ELFOSABI_NONE.
inline HasNonDefaultConstructor variable;

int main() {
  gpio_init(PICO_DEFAULT_LED_PIN);
  gpio_set_dir(PICO_DEFAULT_LED_PIN, GPIO_OUT);

  while (true) {
    gpio_put(PICO_DEFAULT_LED_PIN, true);
    sleep_ms(100);
    gpio_put(PICO_DEFAULT_LED_PIN, false);
    sleep_ms(100);
  }
}

I uploaded two versions of the ELF, one as shown above and one with inline removed: minimal_example.zip.

Inspecting the ELFs, you can see that when the variable is inline, Clang sets the GNU_RETAIN flag for .bss:

$ readelf -t minimal_example.elf minimal_example_without_inline.elf | grep -A2 "File:\|\.bss"
File: minimal_example.elf
There are 23 section headers, starting at offset 0x25500:

--
  [12] .bss
       NOBITS          200003b8 0203b4 0002d8 00   0   0  8
       [00200003]: WRITE, ALLOC, GNU_RETAIN
--
File: minimal_example_without_inline.elf
There are 23 section headers, starting at offset 0x25480:

--
  [12] .bss
       NOBITS          200003b8 0203b4 0002d4 00   0   0  8
       [00000003]: WRITE, ALLOC

Because of that, it sets the OS/ABI to GNU (3) instead of NONE (0) (which readelf reports as System V).

$ readelf -h minimal_example.elf minimal_example_without_inline.elf | grep "File:\|ABI:"
File: minimal_example.elf
  OS/ABI:                            UNIX - GNU
File: minimal_example_without_inline.elf
  OS/ABI:                            UNIX - System V

I flashed these binaries to a Pico using openocd and a debug probe, and they both work as expected. Picotool 2.0.0 refuses to flash minimal_example.elf even though it is compatible.

[will-v-pi]

Thanks, those elfs look like they'll be fine with the rest of picotool - I'll merge this