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pmb: sandbox: split out mount setup

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allow it to be called directly.

Signed-off-by: Casey Connolly <casey@postmarketos.org>
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Casey Connolly 2025-04-23 18:25:11 +02:00
parent 31eaa927c8
commit e4560a9aca
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1 changed files with 59 additions and 55 deletions
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114
pmb/init/sandbox.py
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@ -871,6 +871,64 @@ documentation", for workarounds.\
""" """
def setup_mounts(fsops: list[FSOperation]) -> None:
# We need a workspace to setup the sandbox, the easiest way to do this in a tmpfs, since it's
# automatically cleaned up. We need a mountpoint to put the workspace on and it can't be root,
# so let's use /tmp which is almost guaranteed to exist.
mount("tmpfs", "/tmp", "tmpfs", 0, "")
os.chdir("/tmp")
with umask(~0o755):
# This is where we set up the sandbox rootfs
os.mkdir("newroot")
# This is the old rootfs which is used as the source for mounts in the new rootfs.
os.mkdir("oldroot")
# Make sure that newroot is a mountpoint.
mount("newroot", "newroot", "", MS_BIND | MS_REC, "")
# Make the workspace in /tmp / and put the old rootfs in oldroot.
if libc.pivot_root(b".", b"oldroot") < 0:
# pivot_root() can fail in the initramfs since / isn't a mountpoint there, so let's fall
# back to MS_MOVE if that's the case.
# First we move the old rootfs to oldroot.
mount("/", "oldroot", "", MS_BIND | MS_REC, "")
# Then we move the workspace (/tmp) to /.
mount(".", "/", "", MS_MOVE, "")
# chroot and chdir to fully make the workspace the new root.
os.chroot(".")
os.chdir(".")
# When we use MS_MOVE we have to unmount oldroot/tmp manually to reveal the original /tmp
# again as it might contain stuff that we want to mount into the sandbox.
umount2("oldroot/tmp", MNT_DETACH)
for fsop in fsops:
fsop.execute("oldroot", "newroot")
# Now that we're done setting up the sandbox let's pivot root into newroot to make it the new
# root. We use the pivot_root(".", ".") process described in the pivot_root() man page.
os.chdir("newroot")
# We're guaranteed to have / be a mount when we get here, so pivot_root() won't fail anymore,
# even if we're in the initramfs.
if libc.pivot_root(b".", b".") < 0:
oserror("pivot_root")
# As documented in the pivot_root() man page, this will unmount the old rootfs.
umount2(".", MNT_DETACH)
# Avoid surprises by making sure the sandbox's mount propagation is shared. This doesn't
# actually mean mounts get propagated into the host. Instead, a new mount propagation peer
# group is set up.
mount("", ".", "", MS_SHARED | MS_REC, "")
def main() -> None: def main() -> None:
# We don't use argparse as it takes +- 10ms to import and since this is primarily for internal # We don't use argparse as it takes +- 10ms to import and since this is primarily for internal
# use, it's not necessary to have amazing UX for this CLI interface so it's trivial to write # use, it's not necessary to have amazing UX for this CLI interface so it's trivial to write
@ -1015,61 +1073,7 @@ def main() -> None:
if not userns: if not userns:
mount("", "/", "", MS_SLAVE | MS_REC, "") mount("", "/", "", MS_SLAVE | MS_REC, "")
# We need a workspace to setup the sandbox, the easiest way to do this in a tmpfs, since it's setup_mounts(fsops)
# automatically cleaned up. We need a mountpoint to put the workspace on and it can't be root,
# so let's use /tmp which is almost guaranteed to exist.
mount("tmpfs", "/tmp", "tmpfs", 0, "")
os.chdir("/tmp")
with umask(~0o755):
# This is where we set up the sandbox rootfs
os.mkdir("newroot")
# This is the old rootfs which is used as the source for mounts in the new rootfs.
os.mkdir("oldroot")
# Make sure that newroot is a mountpoint.
mount("newroot", "newroot", "", MS_BIND | MS_REC, "")
# Make the workspace in /tmp / and put the old rootfs in oldroot.
if libc.pivot_root(b".", b"oldroot") < 0:
# pivot_root() can fail in the initramfs since / isn't a mountpoint there, so let's fall
# back to MS_MOVE if that's the case.
# First we move the old rootfs to oldroot.
mount("/", "oldroot", "", MS_BIND | MS_REC, "")
# Then we move the workspace (/tmp) to /.
mount(".", "/", "", MS_MOVE, "")
# chroot and chdir to fully make the workspace the new root.
os.chroot(".")
os.chdir(".")
# When we use MS_MOVE we have to unmount oldroot/tmp manually to reveal the original /tmp
# again as it might contain stuff that we want to mount into the sandbox.
umount2("oldroot/tmp", MNT_DETACH)
for fsop in fsops:
fsop.execute("oldroot", "newroot")
# Now that we're done setting up the sandbox let's pivot root into newroot to make it the new
# root. We use the pivot_root(".", ".") process described in the pivot_root() man page.
os.chdir("newroot")
# We're guaranteed to have / be a mount when we get here, so pivot_root() won't fail anymore,
# even if we're in the initramfs.
if libc.pivot_root(b".", b".") < 0:
oserror("pivot_root")
# As documented in the pivot_root() man page, this will unmount the old rootfs.
umount2(".", MNT_DETACH)
# Avoid surprises by making sure the sandbox's mount propagation is shared. This doesn't
# actually mean mounts get propagated into the host. Instead, a new mount propagation peer
# group is set up.
mount("", ".", "", MS_SHARED | MS_REC, "")
if chdir: if chdir:
os.chdir(chdir) os.chdir(chdir)