Extract target CPU architecture from unikernel binary

[gntouts]

Currently, we rely on a non-standard ARCH instruction provided by the user to determine the CPU architecture for the container image.

This could be extracted by reading the ELF header of the unikernel binary, using GetBinaryArchitecture().

To do that, we should iterate over every COPY instruction and find the one that matches the file defined from com.urunc.unikernel.binary LABEL.

[ananos]

Good idea, and implementation.

We will have to handle a few corner cases, such as a Unikraft Nginx image (not sure why it shows as 32bit):

$ file app-nginx_kvm-x86_64
app-nginx_kvm-x86_64: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), statically linked, stripped

$ file app-redis_kvm-x86_64
app-redis_kvm-x86_64: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), statically linked, stripped

but:

# file /store/tmp_uni_binaries/app-redis_kvm-x86_64 
/store/tmp_uni_binaries/app-redis_kvm-x86_64: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped

must have something to do with the target chosen in the unikraft config. In any case, let's assume that all x86 architectures are 64bit (amd64) and all ARM architectures are aarch64. I assume it's as simple as:

	case elf.EM_386:
		return "amd64"
	case elf.EM_X86_64:
		return "amd64"
	case elf.EM_ARM:
		return "arm64"
	case elf.EM_AARCH64:
		return "arm64"

[ananos]

it seems we cannot detect unikraft aarch64 binaries correctly (see #32). It seems the format used for some cases is PECOFF, so we need to account for this in the code.