Hello world cross compiled for ARM is runing on both x86_64 and ARM

Issue

I was preparing a demonstration of user mode Qemu's (qemu-user package) qemu-arm. To do so, I used a simple hello world C program hello.c:

#include <stdio.h>

int main()
{
        printf("Oi, Qemu!\nPrograma C aqui!\n");
}

To cross compile it (statically linked), I used the cross toolchain from gcc-arm-linux-gnueabihf:

$ arm-linux-gnueabihf-gcc --version
arm-linux-gnueabihf-gcc (Ubuntu 9.3.0-10ubuntu1) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ arm-linux-gnueabihf-gcc hello.c -o hello_c_static -static

The output runs in both qemu-arm, Beaglebone Black AND ON THE PC.

How is it possible?!

EDIT

About the compiled executable:

file hello_c_static 
hello_c_static: ELF 32-bit LSB executable, ARM, EABI5 version 1
(GNU/Linux), statically linked,
BuildID[sha1]=6a33aaa5abb9a14fbc0ca4f2e7b432d6fa5d7067, for GNU/Linux 3.2.0,
not stripped

Solution

Check ls -l /proc/sys/fs/binfmt_misc/ to see if your x86 system is set up to transparently run qemu on ARM binaries for you, the same way it can be to run WIINE on Windows executables or whatever using Linux binfmt_misc. https://en.wikipedia.org/wiki/Binfmt_misc

Admin guide https://www.kernel.org/doc/Documentation/admin-guide/binfmt-misc.rst

---

The installer for the qemu-user package might have registered the executable formats it supports.

With no binfmts registers, you'll just have the register and status "files". On my system, WINE and Mono boot scripts have registered handlers so I also see CLR and DOSWin files. e.g.

$ cat /proc/sys/fs/binfmt_misc/DOSWin 
enabled
interpreter /usr/bin/wine
flags: 
offset 0
magic 4d5a

---

If there's somehow a different mechanism, try using strace ./some_arm_program to see what system calls happen when you execute it.

Maybe also suspend it while it's running (control-z) and look at /proc/$(pidof some_arm_program)/maps and other files.

(This last section was written after the OP commented that they only saw register and status files, not qemu-arm, but they've since change their comment. It looks like binfmt-support is the answer.)

Answered By - Peter Cordes