libmount: make rootfs lookup by parent-id more robust

The root FS id really does not have to be the smallest one.

[kzak@redhat.com: - backported to stable branch without reg.test]

Reported-by: Michal Soltys <soltys@ziu.info>
Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/libmount/src/tab.c b/libmount/src/tab.c
index 85fd427b54d85daba075105c86de60b1e911d571..0eeefe76d69fae779346827319dc12c7f1b76160 100644 (file)
--- a/libmount/src/tab.c
+++ b/libmount/src/tab.c
@@ -438,15 +438,31 @@ int mnt_table_remove_fs(struct libmnt_table *tb, struct libmnt_fs *fs)
        return 0;
 }
 
+static inline struct libmnt_fs *get_parent_fs(struct libmnt_table *tb, struct libmnt_fs *fs)
+{
+       struct libmnt_iter itr;
+       struct libmnt_fs *x;
+       int parent_id = mnt_fs_get_parent_id(fs);
+
+       mnt_reset_iter(&itr, MNT_ITER_FORWARD);
+       while (mnt_table_next_fs(tb, &itr, &x) == 0) {
+               if (mnt_fs_get_id(x) == parent_id)
+                       return x;
+       }
+
+       return NULL;
+}
+
 /**
  * mnt_table_get_root_fs:
  * @tb: mountinfo file (/proc/self/mountinfo)
  * @root: returns pointer to the root filesystem (/)
  *
- * The function uses the parent ID from the mountinfo file to determine the root filesystem
- * (the filesystem with the smallest ID). The function is designed mostly for
- * applications where it is necessary to sort mountpoints by IDs to get the tree
- * of the mountpoints (e.g. findmnt default output).
+ * The function uses the parent ID from the mountinfo file to determine the
+ * root filesystem (the filesystem with the smallest ID with parent ID missing
+ * in the table). The function is designed mostly for applications where it is
+ * necessary to sort mountpoints by IDs to get the tree of the mountpoints
+ * (e.g. findmnt default output).
  *
  * If you're not sure, then use
  *
@@ -469,6 +485,7 @@ int mnt_table_get_root_fs(struct libmnt_table *tb, struct libmnt_fs **root)
 
        *root = NULL;
 
+       /* get smallest possible ID from the table */
        mnt_reset_iter(&itr, MNT_ITER_FORWARD);
        while(mnt_table_next_fs(tb, &itr, &fs) == 0) {
                int id = mnt_fs_get_parent_id(fs);
@@ -479,6 +496,15 @@ int mnt_table_get_root_fs(struct libmnt_table *tb, struct libmnt_fs **root)
                }
        }
 
+       /* go to the root node by "parent_id -> id" relation */
+       while (*root) {
+               struct libmnt_fs *x = get_parent_fs(tb, *root);
+               if (!x || x == *root)
+                       break;
+               DBG(TAB, ul_debugobj(tb, " messy mountinfo, walk to %s", mnt_fs_get_target(x)));
+               *root = x;
+       }
+
        return *root ? 0 : -EINVAL;
 }