fuse-overlayfs: add plugin system

Add a simple plugin mechanism that will help to expand fuse-overlayfs
functionalities, in particular it allows to load data from a layer on
demand.

A plugin is loaded into fuse-overlayfs using the option:

-o plugins=path/to/plugin.so:path/to/another/plugin.so

A layer can use a plugin with the syntax:

-o lowerdir=//plugin-name/DATA-FOR-THE-PLUGIN/path

Each time a file/directory is looked up, if a plugin is registered for
a layer, the plugin is first notified about the request.

After the callback is invoked, fuse-overlayfs still expects the data
to be accessible at the specified directory.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Makefile.am

@@ -4,14 +4,14 @@ bin_PROGRAMS = fuse-overlayfs
 
 ACLOCAL_AMFLAGS = -Im4
 
-EXTRA_DIST = m4/gnulib-cache.m4 rpm/fuse-overlayfs.spec.template autogen.sh fuse-overlayfs.1.md utils.h NEWS tests/suid-test.c
+EXTRA_DIST = m4/gnulib-cache.m4 rpm/fuse-overlayfs.spec.template autogen.sh fuse-overlayfs.1.md utils.h NEWS tests/suid-test.c plugin.h plugin-manager.h
 
 CLEANFILES = fuse-overlayfs.1
 
 fuse_overlayfs_CFLAGS = -I . -I $(abs_srcdir)/lib $(FUSE_CFLAGS)
 fuse_overlayfs_LDFLAGS =
 fuse_overlayfs_LDADD = lib/libgnu.a $(FUSE_LIBS)
-fuse_overlayfs_SOURCES = main.c direct.c utils.c
+fuse_overlayfs_SOURCES = main.c direct.c utils.c plugin-manager.c
 
 WD := $(shell pwd)
 

configure.ac

@@ -54,6 +54,7 @@ LDFLAGS=$old_LDFLAGS
 
 AC_DEFINE_UNQUOTED([HAVE_FUSE_CACHE_READDIR], $cache_readdir, [Define if libfuse is available])
 
+AC_SEARCH_LIBS([dlopen], [dl], [], [AC_MSG_ERROR([unable to find dlopen()])])
 
 AC_FUNC_ERROR_AT_LINE
 AC_FUNC_MALLOC

direct.c

@@ -130,13 +130,16 @@ direct_readlinkat (struct ovl_layer *l, const char *path, char *buf, size_t bufs
 }
 
 static int
-direct_load_data_source (struct ovl_layer *l, const char *opaque)
+direct_load_data_source (struct ovl_layer *l, const char *opaque, const char *path)
 {
-  l->path = realpath (opaque, NULL);
+  l->path = realpath (path, NULL);
   if (l->path == NULL)
-    return -1;
+    {
+      fprintf (stderr, "cannot resolve path %s\n", path);
+      return -1;
+    }
 
-  l->fd = open (l->path, O_DIRECTORY);
+  l->fd = open (path, O_DIRECTORY);
   if (l->fd < 0)
     {
       free (l->path);

fuse-overlayfs.h

@@ -18,8 +18,12 @@
 #ifndef FUSE_OVERLAYFS_H
 # define FUSE_OVERLAYFS_H
 
-# include <hash.h>
 # include <sys/stat.h>
+# include <plugin-manager.h>
+# include <stdbool.h>
+# include <sys/types.h>
+
+typedef struct hash_table Hash_table;
 
 struct ovl_ino
 {
@@ -76,6 +80,7 @@ struct ovl_data
   char *upperdir;
   char *workdir;
   char *redirect_dir;
+  char *plugins;
   int workdir_fd;
   int debug;
   struct ovl_layer *layers;
@@ -94,6 +99,8 @@ struct ovl_data
   /* current uid/gid*/
   uid_t uid;
   uid_t gid;
+
+  struct ovl_plugin_context *plugins_ctx;
 };
 
 struct ovl_layer
@@ -104,12 +111,14 @@ struct ovl_layer
   char *path;
   int fd;
   bool low;
+
+  void *data_source_private_data;
 };
 
 /* a data_source defines the methods for accessing a lower layer.  */
 struct data_source
 {
-  int (*load_data_source)(struct ovl_layer *l, const char *opaque);
+  int (*load_data_source)(struct ovl_layer *l, const char *opaque, const char *path);
   int (*cleanup)(struct ovl_layer *l);
   int (*file_exists)(struct ovl_layer *l, const char *pathname);
   int (*statat)(struct ovl_layer *l, const char *path, struct stat *st, int flags);

main.c

@@ -81,6 +81,8 @@
 
 #include <pthread.h>
 
+#include <plugin.h>
+
 #ifndef TEMP_FAILURE_RETRY
 #define TEMP_FAILURE_RETRY(expression) \
   (__extension__                                                              \
@@ -211,6 +213,8 @@ static const struct fuse_opt ovl_opts[] = {
    offsetof (struct ovl_data, writeback), 1},
   {"noxattrs=%d",
    offsetof (struct ovl_data, disable_xattrs), 1},
+  {"plugins=%s",
+   offsetof (struct ovl_data, plugins), 0},
   FUSE_OPT_END
 };
 
@@ -1540,21 +1544,80 @@ read_dirs (struct ovl_data *lo, char *path, bool low, struct ovl_layer *layers)
 
   for (it = strtok_r (buf, ":", &saveptr); it; it = strtok_r (NULL, ":", &saveptr))
     {
+      char *name, *data;
+      char *it_path = it;
       cleanup_layer struct ovl_layer *l = NULL;
 
       l = calloc (1, sizeof (*l));
       if (l == NULL)
         return NULL;
 
-      l->ds = &direct_access_ds;
+      if (it[0] != '/' || it[1] != '/')
+        {
+          /* By default use the direct access data store.  */
+          l->ds = &direct_access_ds;
+
+          data = NULL;
+          path = it_path;
+        }
+      else
+        {
+          struct ovl_plugin *p;
+          char *plugin_data_sep, *plugin_sep;
+
+          if (! low)
+            {
+              fprintf (stderr, "plugins are supported only with lower layers\n");
+              return NULL;
+            }
+
+          plugin_sep = strchr (it + 2, '/');
+          if (! plugin_sep)
+            {
+              fprintf (stderr, "invalid separator for plugin\n");
+              return NULL;
+            }
+
+          *plugin_sep = '\0';
+
+          name = it + 2;
+          data = plugin_sep + 1;
+
+          plugin_data_sep = strchr (data, '/');
+          if (! plugin_data_sep)
+            {
+              fprintf (stderr, "invalid separator for plugin\n");
+              return NULL;
+            }
+
+          *plugin_data_sep = '\0';
+          path = plugin_data_sep + 1;
+
+          p = plugin_find (lo->plugins_ctx, name);
+          if (! p)
+            {
+              fprintf (stderr, "cannot find plugin %s\n", name);
+              return NULL;
+            }
+
+          l->ds = p->load (l, data, path);
+          if (l->ds == NULL)
+            {
+              fprintf (stderr, "cannot load plugin %s\n", name);
+              return NULL;
+            }
+        }
 
       l->ovl_data = lo;
 
       l->path = NULL;
       l->fd = -1;
 
-      if (l->ds->load_data_source (l, it) < 0)
-        return NULL;
+      if (l->ds->load_data_source (l, data, path) < 0)
+        {
+          fprintf (stderr, "cannot load store %s at %s\n", data, path);
+          return NULL;
+        }
 
       l->low = low;
       if (low)
@@ -4950,6 +5013,7 @@ main (int argc, char *argv[])
       fprintf (stderr, "workdir=%s\n", lo.workdir ? lo.workdir : "NOT USED");
       fprintf (stderr, "lowerdir=%s\n", lo.lowerdir);
       fprintf (stderr, "mountpoint=%s\n", lo.mountpoint);
+      fprintf (stderr, "plugins=%s\n", lo.plugins ? lo.plugins : "<none>");
     }
 
   lo.uid_mappings = lo.uid_str ? read_mappings (lo.uid_str) : NULL;
@@ -4963,6 +5027,9 @@ main (int argc, char *argv[])
         error (EXIT_FAILURE, errno, "cannot convert %s", lo.timeout_str);
     }
 
+  if (lo.plugins)
+    lo.plugins_ctx = load_plugins (lo.plugins);
+
   layers = read_dirs (&lo, lo.lowerdir, true, NULL);
   if (layers == NULL)
     {
@@ -5051,6 +5118,9 @@ err_out1:
 
   hash_free (lo.inodes);
 
+  if (lo.plugins)
+    plugin_free_all (lo.plugins_ctx);
+
   free_mapping (lo.uid_mappings);
   free_mapping (lo.gid_mappings);
 

plugin-manager.c

@@ -0,0 +1,122 @@
+/* fuse-overlayfs: Overlay Filesystem in Userspace
+
+   Copyright (C) 2019 Red Hat Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <config.h>
+#include <plugin.h>
+#include <stdlib.h>
+#include <error.h>
+#include <errno.h>
+#include <string.h>
+
+struct ovl_plugin_context *
+load_plugins (const char *plugins)
+{
+  char *saveptr = NULL, *it;
+  cleanup_free char *buf = NULL;
+  struct ovl_plugin_context *ctx;
+
+  ctx = calloc (1, sizeof (*ctx));
+  if (ctx == NULL)
+    error (EXIT_FAILURE, errno, "cannot allocate context");
+
+  buf = strdup (plugins);
+  if (buf == NULL)
+    error (EXIT_FAILURE, errno, "cannot allocate memory");
+
+  for (it = strtok_r (buf, ":", &saveptr); it; it = strtok_r (NULL, ":", &saveptr))
+    plugin_load_one (ctx, it);
+
+  return ctx;
+}
+
+void
+plugin_load_one (struct ovl_plugin_context *context, const char *path)
+{
+  plugin_name name;
+  struct ovl_plugin *p;
+  plugin_version version;
+  void *handle = dlopen (path, RTLD_NOW|RTLD_LOCAL);
+  if (! handle)
+    error (EXIT_FAILURE, 0, "cannot load plugin %s: %s", path, dlerror());
+
+  p = calloc (1, sizeof (*p));
+  if (p == NULL)
+    error (EXIT_FAILURE, errno, "cannot load plugin %s", path);
+  p->next = context->plugins;
+
+  version = dlsym (handle, "plugin_version");
+  if (version == NULL)
+    error (EXIT_FAILURE, 0, "cannot find symbol `plugin_version` in plugin %s", path);
+
+  if (version () != 1)
+    error (EXIT_FAILURE, 0, "invalid plugin version for %s", path);
+
+  p->handle = handle;
+  name = dlsym (handle, "plugin_name");
+  if (name == NULL)
+    error (EXIT_FAILURE, 0, "cannot find symbol `plugin_name` in plugin %s", path);
+
+  p->name = name ();
+
+  if (plugin_find (context, p->name))
+    error (EXIT_FAILURE, 0, "plugin %s added twice", p->name);
+
+  p->load = dlsym (handle, "plugin_load");
+  if (p->load == NULL)
+    error (EXIT_FAILURE, 0, "cannot find symbol `plugin_load` in plugin %s", path);
+
+  p->release = dlsym (handle, "plugin_release");
+  if (p->release == NULL)
+    error (EXIT_FAILURE, 0, "cannot find symbol `plugin_release` in plugin %s", path);
+
+  context->plugins = p;
+}
+
+struct ovl_plugin *
+plugin_find (struct ovl_plugin_context *context, const char *name)
+{
+  struct ovl_plugin *it;
+
+  for (it = context->plugins; it; it = it->next)
+    {
+      if (strcmp (name, it->name) == 0)
+        return it;
+    }
+  return NULL;
+}
+
+int
+plugin_free_all (struct ovl_plugin_context *context)
+{
+  struct ovl_plugin *it, *next;
+
+  it = context->plugins;
+  while (it)
+    {
+      next = it->next;
+
+      dlclose (it->handle);
+      free (it);
+
+      it = next;
+    }
+
+  free (context);
+
+  return 0;
+}

plugin-manager.h

@@ -0,0 +1,35 @@
+/* fuse-overlayfs: Overlay Filesystem in Userspace
+
+   Copyright (C) 2019 Red Hat Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef PLUGIN_MANAGER_H
+# define PLUGIN_MANAGER_H
+# include <config.h>
+
+# include <dlfcn.h>
+
+struct ovl_plugin_context
+{
+  struct ovl_plugin *plugins;
+};
+
+void plugin_load_one (struct ovl_plugin_context *context, const char *path);
+int plugin_free_all (struct ovl_plugin_context *context);
+struct ovl_plugin *plugin_find (struct ovl_plugin_context *context, const char *name);
+struct ovl_plugin_context *load_plugins (const char *plugins);
+
+#endif

plugin.h

@@ -0,0 +1,41 @@
+/* fuse-overlayfs: Overlay Filesystem in Userspace
+
+   Copyright (C) 2019 Red Hat Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef PLUGIN_H
+# define PLUGIN_H
+# include <config.h>
+
+# include <fuse-overlayfs.h>
+# include <utils.h>
+
+typedef struct data_source *(*plugin_load_data_source)(struct ovl_layer *layer, const char *opaque, const char *path);
+typedef int (*plugin_release)(struct ovl_layer *layer);
+typedef const char *(*plugin_name)();
+typedef int (*plugin_version)();
+
+struct ovl_plugin
+{
+  struct ovl_plugin *next;
+  const char *name;
+  void *handle;
+
+  plugin_load_data_source load;
+  plugin_release release;
+};
+
+#endif