libc: resolve symlinks in execve

This patch complements the commit "libc: execve" with the ability to
execute files stored at arbitrary sub directories of the file system.

Issue #3481
Issue #3500

repos/libports/run/execve.run

@@ -16,9 +16,13 @@ install_config {
 				<arg value="name_of_executeable"/>
 				<arg value="100"/>
 				<libc stdin="/null" stdout="/log" stderr="/log"/>
-				<vfs> <null/> <log/> </vfs>
+				<vfs> <rom name="test-execve"/> <null/> <log/> </vfs>
 			</config>
-			<route> <any-service> <parent/> </any-service> </route>
+			<route>
+				<service name="ROM" label="/test-execve">
+					<parent label="test-execve"/> </service>
+				<any-service> <parent/> </any-service>
+			</route>
 		</start>
 	</config>
 }

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

@@ -25,6 +25,7 @@
 #include <internal/call_func.h>
 #include <internal/init.h>
 #include <internal/errno.h>
+#include <internal/file_operations.h>
 
 using namespace Genode;
 
@@ -161,6 +162,13 @@ extern "C" int execve(char const *filename,
 		return Libc::Errno(EACCES);
 	}
 
+	Libc::Absolute_path resolved_path;
+	try {
+		Libc::resolve_symlinks(filename, resolved_path); }
+	catch (Libc::Symlink_resolve_error) {
+		warning("execve: ", filename, " does not exist");
+		return Libc::Errno(ENOENT); }
+
 	/* capture environment variables and args to libc-internal heap */
 	Libc::String_array *saved_env_vars =
 		new (*_alloc_ptr) Libc::String_array(*_alloc_ptr, envp);
@@ -169,7 +177,7 @@ extern "C" int execve(char const *filename,
 		new (*_alloc_ptr) Libc::String_array(*_alloc_ptr, argv);
 
 	try {
-		_main_ptr = Dynamic_linker::respawn<main_fn_ptr>(*_env_ptr, filename, "main");
+		_main_ptr = Dynamic_linker::respawn<main_fn_ptr>(*_env_ptr, resolved_path.string(), "main");
 	}
 	catch (Dynamic_linker::Invalid_symbol) {
 		error("Dynamic_linker::respawn could not obtain binary entry point");

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

@@ -478,12 +478,21 @@ struct Libc::Forked_child : Child_policy, Child_ready
 	Route resolve_session_request(Service::Name const &name,
 	                              Session_label const &label) override
 	{
+		Session_label rewritten_label = label;
+
 		Service *service_ptr = nullptr;
 		if (_state == State::STARTING_UP && name == Clone_session::service_name())
 			service_ptr = &_local_clone_service.service;
 
 		if (name == Rom_session::service_name()) {
 
+			/*
+			 * Strip off the originating child name to allow the application of
+			 * routing rules based on the leading path elements, regardless
+			 * of which child in the process hierarchy requests a ROM.
+			 */
+			rewritten_label = label.last_element();
+
 			try { service_ptr = &_local_rom_services.matching_service(name, label); }
 			catch (...) { }
 
@@ -496,7 +505,7 @@ struct Libc::Forked_child : Child_policy, Child_ready
 
 		if (service_ptr)
 			return Route { .service = *service_ptr,
-			               .label   = label,
+			               .label   = rewritten_label,
 			               .diag    = Session::Diag() };
 
 		throw Service_denied();

repos/libports/src/lib/libc/internal/file_operations.h

@@ -0,0 +1,35 @@
+/*
+ * \brief  Libc-internal file operations
+ * \author Norman Feske
+ * \date   2019-09-23
+ */
+
+/*
+ * Copyright (C) 2019 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _LIBC__INTERNAL__FILE_OPERATIONS_H_
+#define _LIBC__INTERNAL__FILE_OPERATIONS_H_
+
+/* Genode includes */
+#include <os/path.h>
+
+/* libc includes */
+#include <limits.h>   /* for 'PATH_MAX' */
+
+/* libc-internal includes */
+#include <internal/types.h>
+
+namespace Libc {
+
+	typedef Genode::Path<PATH_MAX> Absolute_path;
+
+	class Symlink_resolve_error : Exception { };
+
+	void resolve_symlinks(char const *path, Absolute_path &resolved_path);
+}
+
+#endif /* _LIBC__INTERNAL__FILE_OPERATIONS_H_ */

repos/ports/run/bash.run

@@ -87,7 +87,7 @@ install_config {
 				</dir>
 			</vfs>
 
-			<policy label_prefix="vfs_rom" root="/bin"/>
+			<policy label_prefix="vfs_rom" root="/"/>
 			<default-policy root="/" writeable="yes"/>
 		</config>
 	</start>
@@ -103,7 +103,7 @@ install_config {
 		</route>
 	</start>
 
-	<start name="bash" caps="1000">
+	<start name="/bin/bash" caps="1000">
 		<resource name="RAM" quantum="30M" />
 		<config>
 			<libc stdin="/dev/terminal" stdout="/dev/terminal"
@@ -115,9 +115,9 @@ install_config {
 		</config>
 		<route>
 			<service name="File_system"> <child name="vfs"/> </service>
-			<service name="ROM" label_last="bash"> <child name="vfs_rom"/> </service>
 			<service name="ROM" label_suffix=".lib.so"> <parent/> </service>
-			<service name="ROM" label_prefix=""> <child name="vfs_rom"/> </service>
+			<service name="ROM" label_last="/bin/bash"> <child name="vfs_rom"/> </service>
+			<service name="ROM" label_prefix="/bin"> <child name="vfs_rom"/> </service>
 			<any-service> <parent/> <any-child/> </any-service>
 		</route>
 	</start>