Running ARM executables on x86

Posted sometime before 2012

Preamble

This is an old note that I found lying around on my hard drive. It may not be accurate. I have added as much background around it as I can remember, but the original was just 4 lines of shell script.

The original purpose for this was to build an ARM cross compiler environment that would run on my fast x86 machine. The target platform had limited memory and disk capacity, and was not capabale of even having GCC installed. I ended up setting up an ARM chroot within which I could run ARM executables. I didn’t go for fully blown virtualisation because I wanted to substitute commonly used binaries for native x86 ones. I substituted GCC for a cross-compiling GCC, and I substituted tar. This meant that cross-compiling a project that executed intermediate results would still work. For example, when compiling Python, the initial interpreter is built using GCC, this interpreter is then executed to coordinate the rest of the build. Without the ability to run arbitrary ARM executables, the cross compile phase would have stopped here, unable to run the Python executable it had just built with our cross compiler.

I strongly recommend NOT using this approach for cross compiling code. It works really well, reall fast, about 90% of the time, but a lot of subtle issues can crop up. Instead do one of the following:

If your target has enough grunt to do basic tasks, and you are only compiling C/C++ projects, try using distcc.
Just emulate the whole system. Build times may go up, but you will save time in the long run not having to deal with bizarre issues.

The Instructions

The Linux kernel has a special feature called binfmt_misc which allows you to register user-space programs as interpreters for files that you attempt to execute, similar how #! is used, only more flexible.

These instructions are for getting the kernel to run ARM binaries using QEMU from within an ARM chroot. We use the static version of QEMU so that it is easy to get running in the chroot we will be using, which is going to contain mostly ARM libraries and executables.

The following command regesters qemu-arm-static as the interpreter for any files that contain the specified magic number. This sequence of bytes is present in ELF files which target ARM.

echo ":qemu-arm-static:M::\x7fELF\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x28\x00::/usr/bin/qemu-arm-static:" > /proc/sys/fs/binfmt_misc/register

You will also need to register qemu-arm-static-ld-so for certain file types.

echo ":qemu-arm-static-ld-so:M::\x7fELF\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x03\x00\x28\x00::/usr/bin/qemu-arm-static:" > /proc/sys/fs/binfmt_misc/register

Then make sure that qemu-arm-static and friends also exist in your chroot in the correct locations.

# This is what I wrote down many years ago. I hope
# that if I find myself needing to do this again,
# I will understand what I meant and where to copy things to.
Copy libc.so.*
Copy ld-linux-x86-64.*
Copy /usr/x86-64-unknown-linux-gnu/arm-lin.... *