libc: add support for external vfs file systems

These file systems are provided on-demand by loading a shared library
when the fstab node is traversed. By convention this library is named
after the file system it provides. For example a file system that
provides a 'random' file system node is called 'vfs_random.lib.so'. It
is still possible to give the the node another name in the vfs. The
following code snippts illustrates this matter:

! [...]
! <config>
!   <libc>
!     <vfs>
!       <dir name="dev"> <jitterentropy name="random"/> </dir>
!     </vfs>
!   </libc>
! </config>
! [...]

Here the jitterentropy file system, implemented in
'vfs_jitterentropy.lib.so' provides a file system node named 'random'
in the 'dev' directory. When traversing the vfs section the libc will
try to load 'vfs_jitterentropy.lib.so' but programs may access the
file system only via '/dev/random'.

Fixes #1240.

repos/libports/include/libc-plugin/vfs.h

@@ -0,0 +1,39 @@
+/*
+ * \brief  vfs plugin interface
+ * \author Josef Soentgen
+ * \date   2014-08-19
+ */
+
+/*
+ * Copyright (C) 2014 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU General Public License version 2.
+ */
+
+#ifndef _LIBC_PLUGIN__VFS_H_
+#define _LIBC_PLUGIN__VFS_H_
+
+/* Genode includes */
+#include <vfs/file_system.h>
+#include <util/xml_node.h>
+
+namespace Libc {
+	struct File_system_factory;
+
+	typedef Libc::File_system_factory*(*File_system_factory_func)(void);
+}
+
+struct Libc::File_system_factory
+{
+	char const * const name;
+
+	File_system_factory(char const *name) : name(name) { }
+
+	virtual Vfs::File_system *create(Genode::Xml_node node) = 0;
+
+	virtual bool matches(Genode::Xml_node node) const {
+		return node.has_type(name); }
+};
+
+#endif /* _LIBC_PLUGIN__VFS_H_ */

repos/libports/src/lib/libc/vfs_plugin.cc

@@ -41,10 +41,12 @@
 #include <sys/mman.h>
 #include <sys/ioctl.h>
 #include <sys/disk.h>
+#include <dlfcn.h>
 
 /* libc plugin interface */
 #include <libc-plugin/plugin.h>
 #include <libc-plugin/fd_alloc.h>
+#include <libc-plugin/vfs.h>
 
 /* libc-internal includes */
 #include <libc_mem_alloc.h>
@@ -89,18 +91,10 @@ class Libc_file_system_factory : public Vfs::File_system_factory
 {
 	private:
 
-		struct Entry_base : Genode::List<Entry_base>::Element
+		struct Entry_base : Libc::File_system_factory,
+		                    Genode::List<Entry_base>::Element
 		{
-			char const * const name;
-
-			Entry_base(char const *name) : name(name) { }
-
-			virtual Vfs::File_system *create(Genode::Xml_node node) = 0;
-
-			bool matches(Genode::Xml_node node) const
-			{
-				return node.has_type(name);
-			}
+			Entry_base(char const *name) : File_system_factory(name) { }
 		};
 
 		template <typename FILE_SYSTEM>
@@ -114,6 +108,20 @@ class Libc_file_system_factory : public Vfs::File_system_factory
 			}
 		};
 
+		struct External_entry : Entry_base
+		{
+			Libc::File_system_factory &_fs;
+
+			External_entry(char const *name, Libc::File_system_factory_func func)
+			: Entry_base(name), _fs(*func()) { }
+
+			Vfs::File_system *create(Genode::Xml_node node) override {
+				return _fs.create(node); }
+
+			bool matches(Genode::Xml_node node) const override {
+				return node.has_type(_fs.name); }
+		};
+
 		Genode::List<Entry_base> _list;
 
 		template <typename FILE_SYSTEM>
@@ -131,6 +139,33 @@ class Libc_file_system_factory : public Vfs::File_system_factory
 			return 0;
 		}
 
+		bool _probe_lib(Genode::Xml_node node)
+		{
+			enum { MAX_LIB_NAME = 64, MAX_NAME = 32 };
+			char lib_name[MAX_LIB_NAME];
+			char name[MAX_NAME];
+
+			node.type_name(name, sizeof(name));
+			Genode::snprintf(lib_name, sizeof(lib_name), "vfs_%s.lib.so", name);
+
+			void *lib = dlopen(lib_name, RTLD_LAZY);
+			if (!lib) {
+				PWRN("could not open '%s'", lib_name);
+				return false;
+			}
+
+			Libc::File_system_factory_func func = (Libc::File_system_factory_func)
+			                                      dlsym(lib, "Libc_file_system_factory");
+			if (!func) {
+				PWRN("could not find symbol 'Libc_file_system_factory' in '%s'", lib_name);
+				dlclose(lib);
+				return false;
+			}
+
+			_list.insert(new (Genode::env()->heap()) External_entry(name, func));
+			return true;
+		}
+
 	public:
 
 		Vfs::File_system *create(Genode::Xml_node node) override
@@ -139,11 +174,12 @@ class Libc_file_system_factory : public Vfs::File_system_factory
 			if (Vfs::File_system *fs = _try_create(node))
 				return fs;
 
-			/* XXX probe for file system implementation available as shared lib */
-
-			/* try again with the new file system type loaded */
-			if (Vfs::File_system *fs = _try_create(node))
-				return fs;
+			/* probe for file system implementation available as shared lib */
+			if (_probe_lib(node)) {
+				/* try again with the new file system type loaded */
+				if (Vfs::File_system *fs = _try_create(node))
+					return fs;
+			}
 
 			return 0;
 		}