nspawn: Add support for sysroot pivoting (#5258)

Add a new --pivot-root argument to systemd-nspawn, which specifies a
directory to pivot to / inside the container; while the original / is
pivoted to another specified directory (if provided). This adds
support for booting container images which may contain several bootable
sysroots, as is common with OSTree disk images. When these disk images
are booted on real hardware, ostree-prepare-root is run in conjunction
with sysroot.mount in the initramfs to achieve the same results.

1 files changed, 113 insertions, 0 deletions

diff --git a/src/nspawn/nspawn-mount.c b/src/nspawn/nspawn-mount.c
index 72c007f204..4b2838b752 100644
--- a/src/nspawn/nspawn-mount.c
+++ b/src/nspawn/nspawn-mount.c
@@ -1349,3 +1349,116 @@ fail:
         (void) rmdir(template);
         return r;
 }
+
+/* Expects *pivot_root_new and *pivot_root_old to be initialised to allocated memory or NULL. */
+int pivot_root_parse(char **pivot_root_new, char **pivot_root_old, const char *s) {
+        _cleanup_free_ char *root_new = NULL, *root_old = NULL;
+        const char *p = s;
+        int r;
+
+        assert(pivot_root_new);
+        assert(pivot_root_old);
+
+        r = extract_first_word(&p, &root_new, ":", EXTRACT_DONT_COALESCE_SEPARATORS);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return -EINVAL;
+
+        if (isempty(p))
+                root_old = NULL;
+        else {
+                root_old = strdup(p);
+                if (!root_old)
+                        return -ENOMEM;
+        }
+
+        if (!path_is_absolute(root_new))
+                return -EINVAL;
+        if (root_old && !path_is_absolute(root_old))
+                return -EINVAL;
+
+        free_and_replace(*pivot_root_new, root_new);
+        free_and_replace(*pivot_root_old, root_old);
+
+        return 0;
+}
+
+int setup_pivot_root(const char *directory, const char *pivot_root_new, const char *pivot_root_old) {
+        _cleanup_free_ char *directory_pivot_root_new = NULL;
+        _cleanup_free_ char *pivot_tmp_pivot_root_old = NULL;
+        char pivot_tmp[] = "/tmp/nspawn-pivot-XXXXXX";
+        bool remove_pivot_tmp = false;
+        int r;
+
+        assert(directory);
+
+        if (!pivot_root_new)
+                return 0;
+
+        /* Pivot pivot_root_new to / and the existing / to pivot_root_old.
+         * If pivot_root_old is NULL, the existing / disappears.
+         * This requires a temporary directory, pivot_tmp, which is
+         * not a child of either.
+         *
+         * This is typically used for OSTree-style containers, where
+         * the root partition contains several sysroots which could be
+         * run. Normally, one would be chosen by the bootloader and
+         * pivoted to / by initramfs.
+         *
+         * For example, for an OSTree deployment, pivot_root_new
+         * would be: /ostree/deploy/$os/deploy/$checksum. Note that this
+         * code doesn’t do the /var mount which OSTree expects: use
+         * --bind +/sysroot/ostree/deploy/$os/var:/var for that.
+         *
+         * So in the OSTree case, we’ll end up with something like:
+         *  - directory = /tmp/nspawn-root-123456
+         *  - pivot_root_new = /ostree/deploy/os/deploy/123abc
+         *  - pivot_root_old = /sysroot
+         *  - directory_pivot_root_new =
+         *       /tmp/nspawn-root-123456/ostree/deploy/os/deploy/123abc
+         *  - pivot_tmp = /tmp/nspawn-pivot-123456
+         *  - pivot_tmp_pivot_root_old = /tmp/nspawn-pivot-123456/sysroot
+         *
+         * Requires all file systems at directory and below to be mounted
+         * MS_PRIVATE or MS_SLAVE so they can be moved.
+         */
+        directory_pivot_root_new = prefix_root(directory, pivot_root_new);
+
+        /* Remount directory_pivot_root_new to make it movable. */
+        r = mount_verbose(LOG_ERR, directory_pivot_root_new, directory_pivot_root_new, NULL, MS_BIND, NULL);
+        if (r < 0)
+                goto done;
+
+        if (pivot_root_old) {
+                if (!mkdtemp(pivot_tmp)) {
+                        r = log_error_errno(errno, "Failed to create temporary directory: %m");
+                        goto done;
+                }
+
+                remove_pivot_tmp = true;
+                pivot_tmp_pivot_root_old = prefix_root(pivot_tmp, pivot_root_old);
+
+                r = mount_verbose(LOG_ERR, directory_pivot_root_new, pivot_tmp, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+
+                r = mount_verbose(LOG_ERR, directory, pivot_tmp_pivot_root_old, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+
+                r = mount_verbose(LOG_ERR, pivot_tmp, directory, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+        } else {
+                r = mount_verbose(LOG_ERR, directory_pivot_root_new, directory, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+        }
+
+done:
+        if (remove_pivot_tmp)
+                (void) rmdir(pivot_tmp);
+
+        return r;
+}