catalyst: Restore root and cwd after exiting mount namespace

This commit saves the file descriptor of /proc/self/{root,cwd}
before entering into the new mount namespace. When restoring the
previous mount namespace, it restores /proc/self/{root,cwd}
based on the saved file descriptors.

Without this change, catalyst cannot be run in a chroot when
using the recent changes regarding mount namespaces: After the
mount namespace has been exited, /proc/self/root points to the "/"
of the host system, not the "/" of the chroot. Therefore, the
cleanup phase of catalyst runs outside of the chroot.

The code is similar to how nsenter(1) sets root and cwd:
https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-utils/nsenter.c#n452

Tested in a Gentoo chroot and in Gentoo VM (non-chroot).

Signed-off-by: Felix Bier <felix.bier@rohde-schwarz.com>
Signed-off-by: Matt Turner <mattst88@gentoo.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/catalyst/context.py b/catalyst/context.py
index 8a58f33d..01a6d930 100644
--- a/catalyst/context.py
+++ b/catalyst/context.py
@@ -16,11 +16,21 @@ def namespace(mount=False, uts=False, ipc=False, net=False, pid=False,
         (user,  "user"): None,
     }
 
+    dirs = {
+        "root": None,
+        "cwd":  None,
+    }
+
     # Save fds of current namespaces
     for ns in [ns for ns in namespaces if ns[0]]:
         fp = open(f"/proc/self/ns/{ns[1]}")
         namespaces[ns] = fp
 
+    # Save fds of current directories
+    if mount:
+        for d in dirs:
+            dirs[d] = os.open(f"/proc/self/{d}", os.O_RDONLY)
+
     simple_unshare(mount=mount, uts=uts, ipc=ipc, net=net, pid=pid, user=user,
                    hostname=hostname)
     try:
@@ -30,3 +40,15 @@ def namespace(mount=False, uts=False, ipc=False, net=False, pid=False,
             fp = namespaces[ns]
             setns(fp.fileno(), 0)
             fp.close()
+
+        if mount:
+            # Restore original root and cwd. Since we cannot directly chroot to
+            # a fd, first change the current directory to the fd of the
+            # original root, then chroot to "."
+
+            os.fchdir(dirs["root"])
+            os.chroot(".")
+            os.fchdir(dirs["cwd"])
+
+            for fd in dirs.values():
+                os.close(fd)