qemu-user-static works on chroot environment.
Let's see how it works. mock command is good example to see it. mock command uses chroot to enable the mock environment internally. The specified architecture's binary files are installed to the chroot environment when using mock --forcearch $arch (cpu) option. [1]
$ uname -m
x86_64
If you have not installed qemu-user-static RPM, install it.
$ sudo dnf install qemu-user-static
$ cat /proc/sys/fs/binfmt_misc/qemu-aarch64
enabled
interpreter /usr/bin/qemu-aarch64-static
flags: F
offset 0
magic 7f454c460201010000000000000000000200b700
mask ffffffffffffff00fffffffffffffffffeffffff
You can use the detault mock configuratoin files /etc/mock/fedora-rawhide-$cpu.cfg without any change.
$ mock -r fedora-rawhide-aarch64 --forcearch=aarch64 shell
<mock-chroot> sh-5.0# uname -m
aarch64
<mock-chroot> sh-5.0# exit
/var/lib/mock/fedora-rawhide-aarch64/root is the root directory used for the chroot environment.
$ ls /var/lib/mock/fedora-rawhide-aarch64/root
bin@ builddir/ etc/ .initialized lib64@ mnt/ proc/ run/ srv/ tmp/ var/
boot/ dev/ home/ lib@ media/ opt/ root/ sbin@ sys/ usr/
$ ls /var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/uname
/var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/uname*
$ file /var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/uname
/var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/uname: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-aarch64.so.1, BuildID[sha1]=4e21c4458f4d12d79ba73e24c24b40b4d085e70c, for GNU/Linux 3.7.0, stripped
Remember this /proc/sys/fs/binfmt_misc/qemu-aarch64 file again. flags setting is flags: F. That means interpreter is checked when registering binary format entry, but it is not checked at run time.
$ cat /proc/sys/fs/binfmt_misc/qemu-aarch64
enabled
interpreter /usr/bin/qemu-aarch64-static
flags: F
...
$ ls /var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/qemu-aarch64-static
ls: cannot access '/var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/qemu-aarch64-static': No such file or directory
$ sudo chroot /var/lib/mock/fedora-rawhide-aarch64/root /usr/bin/uname -m
aarch64
Now update /proc/sys/fs/binfmt_misc/qemu-aarch64 file from flags: F to flags: (empty flag).
$ cat /proc/sys/fs/binfmt_misc/qemu-aarch64
enabled
interpreter /usr/bin/qemu-aarch64-static
flags: F
offset 0
magic 7f454c460201010000000000000000000200b700
mask ffffffffffffff00fffffffffffffffffeffffff
# echo -1 > /proc/sys/fs/binfmt_misc/qemu-aarch64
# echo ":qemu-aarch64:M::\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xb7\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin/qemu-aarch64-static:" > /proc/sys/fs/binfmt_misc/register
$ cat /proc/sys/fs/binfmt_misc/qemu-aarch64
enabled
interpreter /usr/bin/qemu-aarch64-static
flags:
offset 0
magic 7f454c460201010000000000000000000200b700
mask ffffffffffffff00fffffffffffffffffeffffff
Below command is failed, because when the flag F is not set, the interpreter /usr/bin/qemu-aarch64-static is needed to exist inside of the chroot to run the emulation.
$ sudo chroot /var/lib/mock/fedora-rawhide-aarch64/root /usr/bin/uname -m
chroot: failed to run command ‘/usr/bin/uname’: No such file or directory
qemu-aarch64-static does not exist.
$ ls /var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/qemu-aarch64-static
ls: cannot access '/var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/qemu-aarch64-static': No such file or directory
But after copying qemu-aarch64-static binary to chroot, below sudo chroot ... works.
$ sudo cp -p /usr/bin/qemu-aarch64-static /var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/
$ ls /var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/qemu-aarch64-static
/var/lib/mock/fedora-rawhide-aarch64/root/usr/bin/qemu-aarch64-static*
$ sudo chroot /var/lib/mock/fedora-rawhide-aarch64/root /usr/bin/uname -m
aarch64
As a post procedure, let's restore /proc/sys/fs/binfmt_misc/qemu-aarch64 to default content by running systemctl restart systemd-binfmt.
$ sudo systemctl restart systemd-binfmt
$ cat /proc/sys/fs/binfmt_misc/qemu-aarch64
enabled
interpreter /usr/bin/qemu-aarch64-static
flags: F
offset 0
magic 7f454c460201010000000000000000000200b700
mask ffffffffffffff00fffffffffffffffffeffffff
There is no file under /var/lib/mock/fedora-rawhide-aarch64/root/proc.
$ ls /var/lib/mock/fedora-rawhide-aarch64/root/proc
There is no file under /proc/sys/fs/binfmt_misc/.
<mock-chroot> sh-5.0# ls /proc/sys/fs/binfmt_misc/
I assume that /proc is commonly used by host OS and chroot environment.
But I am not sure. Please tell me if you know something about it. :)
- [1] mock forcearch feature: https://github.com/rpm-software-management/mock/wiki/Feature-forcearch