Fix gpt-auto-generator on usr-only root in presence of sysexts (#43591)

This adds a fallback to `blockdev_get_root`, to handle the case where
`/usr` is a sysext overlay. Additinal changes are:

- move `extension_overlay_block` to `extension-util.h`, and make it
generic to work in both confext and sysext cases (though only sysext is
actually used currently)
- add a usr-only integration test to exercise the failure

Closes #43565.

src/shared/blockdev-util.c

@@ -17,9 +17,12 @@
 #include "devnum-util.h"
 #include "dirent-util.h"
 #include "errno-util.h"
+#include "extension-util.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "fs-util.h"
+#include "macro.h"
+#include "os-util.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "string-util.h"
@@ -875,6 +878,13 @@ int blockdev_get_root(int level, dev_t *ret) {
                                 return btrfs_log_dev_root(level, r, "/usr");
                         if (r < 0)
                                 return log_full_errno(level, r, "Failed to determine block device of /usr/ file system: %m");
+                        if (r == 0) { /* Not backed by a single block device, we might be looking at a sysext overlay */
+                                r = extension_overlay_block("/usr", IMAGE_SYSEXT, &devno);
+                                if (IN_SET(r, -ENOENT, -ENOTTY))
+                                        r = 0; /* no sysext overlay metadata */
+                                else if (r < 0)
+                                        return log_full_errno(level, r, "Failed to determine backing device of /usr/ extension overlay: %m");
+                        }
                         if (r == 0) { /* /usr/ not backed by single block device, either. */
                                 log_debug("Neither root nor /usr/ file system are on a (single) block device.");
 

src/shared/extension-util.c

@@ -1,12 +1,21 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <linux/magic.h>
+
 #include "alloc-util.h"
 #include "architecture.h"
+#include "blockdev-util.h"
 #include "chase.h"
+#include "devnum-util.h"
 #include "env-util.h"
 #include "extension-util.h"
+#include "fd-util.h"
+#include "fileio.h"
 #include "log.h"
+#include "macro.h"
 #include "os-util.h"
+#include "path-util.h"
+#include "stat-util.h"
 #include "string-util.h"
 #include "strv.h"
 
@@ -178,3 +187,46 @@ int extension_has_forbidden_content(const char *root) {
 
         return 0;
 }
+
+int extension_overlay_block(const char *p, ImageClass image_class, dev_t *ret) {
+        int r;
+
+        assert(p);
+        assert(ret);
+        assert(IN_SET(image_class, IMAGE_SYSEXT, IMAGE_CONFEXT));
+
+        /* Tries to read the backing device information systemd-sysext puts in
+         * the virtual file .systemd-sysext/.systemd-confext */
+
+        _cleanup_free_ char *j = path_join(p, image_class == IMAGE_CONFEXT ? ".systemd-confext" : ".systemd-sysext");
+        if (!j)
+                return log_oom_debug();
+
+        _cleanup_close_ int fd = open(j, O_RDONLY|O_DIRECTORY);
+        if (fd < 0)
+                return log_debug_errno(errno, "Failed to open '%s': %m", j);
+
+        r = fd_is_fs_type(fd, OVERLAYFS_SUPER_MAGIC);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to determine backing file system of '%s': %m", j);
+        if (r == 0)
+                return log_debug_errno(
+                                SYNTHETIC_ERRNO(ENOTTY), "Backing file system of '%s' is not an overlayfs.", j);
+
+        _cleanup_free_ char *buf = NULL;
+        r = read_one_line_file_at(fd, "backing", &buf);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to read contents of '%s/backing': %m", j);
+
+        r = parse_devnum(buf, ret);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to parse contents of '%s/backing': %m", j);
+
+        if (major(*ret) == 0) { /* not a block device? */
+                *ret = 0;
+                return 0;
+        }
+
+        (void) block_get_originating(*ret, ret, /* recursive= */ false);
+        return 1;
+}

src/shared/extension-util.h

@@ -22,3 +22,6 @@ int parse_env_extension_hierarchies(char ***ret_hierarchies, const char *hierarc
 /* Insist that extension images do not overwrite the underlying OS release file (it's fine if they place one
  * in /etc/os-release, i.e. where things don't matter, as they aren't merged.) */
 int extension_has_forbidden_content(const char *root);
+
+/* Find the backing device. Returns >0 on success, 0 if the volume is not an extension, and <0 on error. */
+int extension_overlay_block(const char *p, ImageClass image_class, dev_t *ret);

src/sysupdate/sysupdate-resource.c

@@ -1,7 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <fcntl.h>
-#include <linux/magic.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
@@ -10,10 +9,10 @@
 #include "build-path.h"
 #include "chase.h"
 #include "device-util.h"
-#include "devnum-util.h"
 #include "dirent-util.h"
 #include "env-util.h"
 #include "errno-util.h"
+#include "extension-util.h"
 #include "fd-util.h"
 #include "fdisk-util.h"
 #include "fileio.h"
@@ -23,6 +22,7 @@
 #include "hexdecoct.h"
 #include "import-util.h"
 #include "iovec-util.h"
+#include "os-util.h"
 #include "path-util.h"
 #include "pidref.h"
 #include "process-util.h"
@@ -766,47 +766,6 @@ Instance* resource_find_instance(Resource *rr, const char *version) {
         return *found;
 }
 
-static int get_sysext_overlay_block(const char *p, dev_t *ret) {
-        int r;
-
-        assert(p);
-        assert(ret);
-
-        /* Tries to read the backing device information systemd-sysext puts in the virtual file
-         * /usr/.systemd-sysext/backing */
-
-        _cleanup_free_ char *j = path_join(p, ".systemd-sysext");
-        if (!j)
-                return log_oom_debug();
-
-        _cleanup_close_ int fd = open(j, O_RDONLY|O_DIRECTORY);
-        if (fd < 0)
-                return log_debug_errno(errno, "Failed to open '%s': %m", j);
-
-        r = fd_is_fs_type(fd, OVERLAYFS_SUPER_MAGIC);
-        if (r < 0)
-                return log_debug_errno(r, "Failed to determine backing file system of '%s': %m", j);
-        if (r == 0)
-                return log_debug_errno(SYNTHETIC_ERRNO(ENOTTY), "Backing file system of '%s' is not an overlayfs.", j);
-
-        _cleanup_free_ char *buf = NULL;
-        r = read_one_line_file_at(fd, "backing", &buf);
-        if (r < 0)
-                return log_debug_errno(r, "Failed to read contents of '%s/backing': %m", j);
-
-        r = parse_devnum(buf, ret);
-        if (r < 0)
-                return log_debug_errno(r, "Failed to parse contents of '%s/backing': %m", j);
-
-        if (major(*ret) == 0) { /* not a block device? */
-                *ret = 0;
-                return 0;
-        }
-
-        (void) block_get_originating(*ret, ret, /* recursive= */ false);
-        return 1;
-}
-
 int resource_resolve_path(
                 Resource *rr,
                 const char *root,
@@ -858,7 +817,7 @@ int resource_resolve_path(
                         /* volatile-root not found */
                         r = get_block_device_harder("/usr/", &d);
                         if (r == 0) /* Not backed by a block device? Let's see if this is a sysext overlayfs instance */
-                                r = get_sysext_overlay_block("/usr/", &d);
+                                r = extension_overlay_block("/usr/", IMAGE_SYSEXT, &d);
                         if (r < 0)
                                 return log_error_errno(r, "Failed to determine block device of file system: %m");
                 } else if (!S_ISBLK(orig_root_stats.st_mode)) /* symlink was present but not block device */

test/units/TEST-50-DISSECT.usr-gpt-auto-generator.sh

@@ -0,0 +1,93 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+GENERATOR_BIN="/usr/lib/systemd/system-generators/systemd-gpt-auto-generator"
+WORK_DIR="$(mktemp --directory /var/tmp/test-gpt-auto-generator.XXXXXX)"
+DEFINITIONS="$WORK_DIR/definitions"
+IMAGE="$WORK_DIR/image.raw"
+OUTPUT="$WORK_DIR/output"
+ROOT="$WORK_DIR/root"
+LOOP=""
+
+at_exit() {
+    set +e
+
+    if [[ -n "$LOOP" ]]; then
+        losetup --detach "$LOOP"
+    fi
+
+    rm -rf "$WORK_DIR"
+}
+
+trap at_exit EXIT
+
+test -x "$GENERATOR_BIN"
+mkdir -p "$DEFINITIONS" "$OUTPUT"/{normal,early,late} "$ROOT"
+
+cat >"$DEFINITIONS/10-usr.conf" <<EOF
+[Partition]
+Type=usr
+SizeMinBytes=10M
+SizeMaxBytes=10M
+EOF
+
+cat >"$DEFINITIONS/20-home.conf" <<EOF
+[Partition]
+Type=home
+SizeMinBytes=10M
+SizeMaxBytes=10M
+EOF
+
+systemd-repart \
+    --definitions="$DEFINITIONS" \
+    --empty=create \
+    --size=32M \
+    --dry-run=no \
+    --offline=yes \
+    "$IMAGE"
+
+LOOP="$(losetup --show --find --partscan "$IMAGE")"
+udevadm wait --timeout=60 --settle --initialized=no "$LOOP"p1 "$LOOP"p2
+
+# Recreate a root=tmpfs system whose /usr/ is backed by a GPT partition and overlaid by systemd-sysext.
+# Run the generator as PID 1 because generators deliberately do nothing in containers.
+unshare --mount --pid --fork --mount-proc \
+    bash -euxo pipefail -s -- "$ROOT" "$OUTPUT" "$WORK_DIR" "$LOOP" "$GENERATOR_BIN" <<'EOF'
+mount --make-rprivate /
+
+mount --types tmpfs tmpfs "$1"
+mkdir -p "$1"/{dev,proc,run/generator-output,sys,usr}
+mount --bind /dev "$1/dev"
+mount --bind /proc "$1/proc"
+mount --bind /sys "$1/sys"
+mount --bind "$2" "$1/run/generator-output"
+
+mkdir -p "$3/usr-upper" "$3/usr-work"
+mount --types overlay overlay \
+    --options lowerdir=/usr,upperdir="$3/usr-upper",workdir="$3/usr-work" \
+    "$1/usr"
+
+for directory in bin lib lib64 sbin; do
+    if [[ -e "/$directory" ]]; then
+        mkdir -p "$1/$directory"
+        mount --bind "/$directory" "$1/$directory"
+    fi
+done
+
+mkdir -p "$1/usr/.systemd-sysext"
+cat "/sys/class/block/${4##*/}p1/dev" >"$1/usr/.systemd-sysext/backing"
+
+export container=
+export SYSTEMD_IN_INITRD=0
+export SYSTEMD_PROC_CMDLINE="root=tmpfs systemd.gpt_auto=yes"
+exec chroot "$1" "$5" \
+    /run/generator-output/normal \
+    /run/generator-output/early \
+    /run/generator-output/late
+EOF
+
+test -f "$OUTPUT/late/home.mount"
+WHAT="$(sed -n 's/^What=//p' "$OUTPUT/late/home.mount")"
+test "$(readlink --canonicalize "$WHAT")" = "$(readlink --canonicalize "${LOOP}p2")"