[SOLVED] Getting the stratum init ?

ychaouche · 10-22-2019, 12:21 PM

In the installer's archive, in bedrock/sbin/init, around line 371, this block of code

Code:

if [ -n "${def_tuple}" ] && [ -d "/bedrock/strata/${def_stratum}" ]; then
	sproc="/bedrock/strata/${def_stratum}/proc"
	mkdir -p "${sproc}" 2>/dev/null || true
	mount -t proc proc "${sproc}"
	if def_link="$(chroot "/bedrock/strata/${def_stratum}" "/proc/1/root/bedrock/libexec/busybox" realpath "${def_cmd}" 2>/dev/null)"; then
		def_path="/bedrock/strata/${def_stratum}${def_link}"
	fi
	umount "${sproc}"
fi

The difficult part for me was to understand what this assignement does :

Quote:

def_link="$(chroot "/bedrock/strata/${def_stratum}" "/proc/1/root/bedrock/libexec/busybox" realpath "${def_cmd}" 2>/dev/null)";

it seems to be trying to retrive the stratum's init binary but in a somewhat convoluted way. I'm sure it is necessary to do it this way, but I was wondering if anyone would like to explain the details of what's going on there ?

ParadigmComplex · 10-22-2019, 04:06 PM

Consider a distro "foo" which, in its normal non-Bedrock usage, has a symlink at /sbin/init pointing to /usr/lib/systemd/systemd.

If it's added to Bedrock as a new stratum, there will be a symlink at /bedrock/strata/foo/sbin/init, but it will be pointing at /usr/lib/systemd/systemd rather than /bedrock/strata/foo/usr/lib/systemd/systemd. That symlink only works if you're chrooted to /bedrock/strata/foo.

If we know foo contains a "realpath" command, we could run:

Code:

chroot /bedrock/strata/foo realpath /sbin/init

to see where /sbin/init points, then append that to /bedrock/strata/foo to find the actual path we're interested in:

Code:

echo "/bedrock/strata/foo/$(chroot /bedrock/strata/foo realpath /sbin/init)"

However, we don't know if the stratum has realpath in it. We could copy a portable busybox realpath into the stratum:

Code:

cp /bedrock/libexec/busybox /bedrock/strata/foo/busybox
chroot /bedrock/strata/foo /busybox realpath /sbin/init

but I don't want to cause unnecessary disk writes.

Linux's /proc/<PID>/root points to a given process' root directory. In this case, we know PID 1 is running in the bedrock stratum and has /bedrock/libexec/busybox. So instead of copying it, mount proc and go through there:

Code:

mount -t proc proc /bedrock/strata/foo/proc
chroot /bedrock/strata/foo /proc/1/root/bedrock/libexec/busybox realpath /sbin/init

That's what this bit of code is doing. It's definitely convoluted.

There's work towards a proper test suite. Once that's in place with adequate tests to confirm changes here won't break things, I plan on making a small C utility which resolves symlinks as though they were chrooted. I'll then refactor code like this to use them, which should simplify things. Once RESOLVE_IN_ROOT becomes available I can use it and improve performance in some parts of the code as well for people on new enough kernels.

ychaouche · 10-23-2019, 11:26 AM

All clear, thanks a lot paradigmcomplex. If I understand correctly, you are using proc as a backdoor to let commands from the chrooted stratum access the real root filesystem (where busybox is), more or less ?

ParadigmComplex · 10-23-2019, 12:37 PM

Affirmative