From: dongshengyuan <545258830@qq.com>
Date: Wed, 22 Jul 2026 11:55:32 +0000 (+0800)
Subject: machine-id-setup: follow symlink target on --commit
X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=aa776aefba2f6af4acf00c86067b19e5a6761a95;p=thirdparty%2Fsystemd.git

machine-id-setup: follow symlink target on --commit

Resolve /etc/machine-id before checking the mount state and before
writing back, so symlinked targets are committed correctly.

Fixes: #43104
Follow-up for: efabf4e04466c85183c969af27de1ff0ecda8e79
---

diff --git a/src/shared/machine-id-setup.c b/src/shared/machine-id-setup.c
index 0b637757481..1ae55ae2e91 100644
--- a/src/shared/machine-id-setup.c
+++ b/src/shared/machine-id-setup.c
@@ -318,17 +318,37 @@ int machine_id_commit(const char *root) {
          * in a mount namespace, a new file is created at the right place. Afterwards the mount is also removed in the
          * original mount namespace, thus revealing the file that was just created. */
 
-        _cleanup_close_ int etc_fd = -EBADF;
-        _cleanup_free_ char *etc = NULL;
-        r = chase("/etc/", root, CHASE_PREFIX_ROOT|CHASE_MUST_BE_DIRECTORY, &etc, &etc_fd);
-        if (r < 0)
-                return log_error_errno(r, "Failed to open %s: %m", "/etc/");
+        _cleanup_close_ int etc_machine_id_fd = -EBADF;
+        _cleanup_free_ char *etc_machine_id = NULL;
+        r = chase("/etc/machine-id", root, CHASE_PREFIX_ROOT|CHASE_MUST_BE_REGULAR,
+                  &etc_machine_id, &etc_machine_id_fd);
+        if (r == -ENOENT) {
+                _cleanup_close_ int etc_fd = -EBADF;
+                _cleanup_free_ char *etc = NULL, *target = NULL;
+
+                r = chase("/etc/", root, CHASE_PREFIX_ROOT|CHASE_MUST_BE_DIRECTORY, &etc, &etc_fd);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to open %s: %m", "/etc/");
+
+                etc_machine_id = path_join(etc, "machine-id");
+                if (!etc_machine_id)
+                        return log_oom();
 
-        _cleanup_free_ char *etc_machine_id = path_join(etc, "machine-id");
-        if (!etc_machine_id)
-                return log_oom();
+                r = readlinkat_malloc(etc_fd, "machine-id", &target);
+                if (r == -ENOENT) {
+                        log_debug_errno(r, "%s does not exist. Nothing to do.", etc_machine_id);
+                        return 0;
+                }
+                if (r < 0)
+                        return log_error_errno(r, "Failed to read symlink target of %s: %m", etc_machine_id);
 
-        r = is_mount_point_at(etc_fd, "machine-id", /* flags= */ 0);
+                log_debug("%s is a dangling symlink to %s. Nothing to do.", etc_machine_id, target);
+                return 0;
+        }
+        if (r < 0)
+                return log_error_errno(r, "Failed to open %s: %m", "/etc/machine-id");
+
+        r = is_mount_point_at(etc_machine_id_fd, /* path= */ NULL, /* flags= */ 0);
         if (r < 0)
                 return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", etc_machine_id);
         if (r == 0) {
@@ -338,11 +358,12 @@ int machine_id_commit(const char *root) {
 
         /* Read existing machine-id */
 
-        _cleanup_close_ int fd = xopenat_full(etc_fd, "machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW, XO_REGULAR, MODE_INVALID);
+        _cleanup_close_ int fd = xopenat_full(etc_machine_id_fd, /* path= */ NULL,
+                                              O_RDONLY|O_CLOEXEC|O_NOCTTY, XO_REGULAR, MODE_INVALID);
         if (fd < 0)
                 return log_error_errno(fd, "Cannot open %s: %m", etc_machine_id);
 
-        etc_fd = safe_close(etc_fd);
+        etc_machine_id_fd = safe_close(etc_machine_id_fd);
 
         r = fd_is_temporary_fs(fd);
         if (r < 0)
@@ -366,22 +387,34 @@ int machine_id_commit(const char *root) {
         if (r < 0)
                 return log_error_errno(r, "Failed to set up new mount namespace: %m");
 
-        /* Open /etc/ again after we transitioned into our own private mount namespace */
-        _cleanup_close_ int etc_fd_again = -EBADF;
-        r = chase("/etc/", root, CHASE_PREFIX_ROOT|CHASE_MUST_BE_DIRECTORY, /* ret_path= */ NULL, &etc_fd_again);
+        _cleanup_free_ char *etc_machine_id_filename = NULL;
+        _cleanup_close_ int etc_machine_id_dir_fd =
+                chase_and_open_parent("/etc/machine-id", root, CHASE_PREFIX_ROOT, &etc_machine_id_filename);
+        if (etc_machine_id_dir_fd < 0)
+                return log_error_errno(etc_machine_id_dir_fd,
+                                       "Failed to open parent directory of %s: %m",
+                                       etc_machine_id);
+
+        r = inode_same_at(fd, /* filea= */ NULL,
+                          etc_machine_id_dir_fd, etc_machine_id_filename, AT_EMPTY_PATH);
         if (r < 0)
-                return log_error_errno(r, "Failed to open %s: %m", "/etc/");
+                return log_error_errno(r, "Failed to verify %s before unmounting: %m", etc_machine_id);
+        if (r == 0)
+                return log_error_errno(SYNTHETIC_ERRNO(ESTALE),
+                                       "%s changed while preparing to commit.",
+                                       etc_machine_id);
 
-        r = umountat_detach_verbose(LOG_ERR, etc_fd_again, "machine-id");
+        r = umountat_detach_verbose(LOG_ERR, etc_machine_id_dir_fd, etc_machine_id_filename);
         if (r < 0)
                 return r;
 
         /* Update a persistent version of etc_machine_id */
-        r = id128_write_at(etc_fd_again, "machine-id", ID128_FORMAT_PLAIN | ID128_SYNC_ON_WRITE, id);
+        r = id128_write_at(etc_machine_id_dir_fd, etc_machine_id_filename,
+                           ID128_FORMAT_PLAIN | ID128_SYNC_ON_WRITE, id);
         if (r < 0)
                 return log_error_errno(r, "Cannot write %s. This is mandatory to get a persistent machine ID: %m", etc_machine_id);
 
-        etc_fd_again = safe_close(etc_fd_again);
+        etc_machine_id_dir_fd = safe_close(etc_machine_id_dir_fd);
 
         /* Return to initial namespace and proceed a lazy tmpfs unmount */
         r = namespace_enter(/* pidns_fd= */ -EBADF,
diff --git a/test/units/TEST-74-AUX-UTILS.machine-id-setup.sh b/test/units/TEST-74-AUX-UTILS.machine-id-setup.sh
index bcd2f315713..072a6aeaa9a 100755
--- a/test/units/TEST-74-AUX-UTILS.machine-id-setup.sh
+++ b/test/units/TEST-74-AUX-UTILS.machine-id-setup.sh
@@ -69,6 +69,27 @@ testcase_transient() {
     diff "$root/etc/machine-id" "$root/run/machine-id"
 }
 
+testcase_transient_symlink() {
+    local root transient_id committed_id
+
+    root="$(mktemp -d)"
+    trap "root_cleanup $root" RETURN
+    root_mock "$root"
+
+    mkdir -p "$root/persist/etc"
+    echo abc >>"$root/persist/etc/machine-id"
+    ln -s /persist/etc/machine-id "$root/etc/machine-id"
+    mount -o remount,ro "$root"
+    mount -t tmpfs tmpfs "$root/run"
+    transient_id="$(systemd-machine-id-setup --print --root "$root")"
+    mountpoint "$root/persist/etc/machine-id"
+    mount -o remount,rw "$root"
+    committed_id="$(systemd-machine-id-setup --print --commit --root "$root")"
+    [[ "$transient_id" == "$committed_id" ]]
+    (! mountpoint "$root/persist/etc/machine-id")
+    diff "$root/persist/etc/machine-id" "$root/run/machine-id"
+}
+
 # Check if we correctly processed the invalid machine ID we set up in the respective
 # test.sh file
 systemctl --state=failed --no-legend --no-pager | tee /failed