ldso: Make truly self relocatable

On Linux the linker can now be loaded at arbitrary addresses, this became
necessary for newer kernel versions. The 'linux_arm' target is not supported.

Issue #1728

repos/base/src/lib/ldso/include/dynamic.h

@@ -21,13 +21,6 @@ namespace Linker {
 	struct Dynamic;
 }
 
-	/**
-	 * Offset of dynamic section of this ELF. This is filled out during linkage by
-	 * static linker.
-	 */
-	extern Genode::addr_t _DYNAMIC;
-
-
 /**
  * ELF hash table and hash function
  */
@@ -108,7 +101,7 @@ struct Linker::Dynamic
 
 	Dynamic(Dependency const *dep)
 	:
-	  dep(dep), obj(dep->obj), dynamic((Elf::Dyn *)(obj->reloc_base() + &_DYNAMIC))
+	  dep(dep), obj(dep->obj), dynamic((Elf::Dyn *)dynamic_address())
 	{
 		init();
 	}

repos/base/src/lib/ldso/include/linker.h

@@ -34,6 +34,11 @@ constexpr bool verbose_exception  = false;
 constexpr bool verbose_shared     = false;
 constexpr bool verbose_loading    = false;
 
+
+extern const unsigned long _GLOBAL_OFFSET_TABLE_[] __attribute__((visibility("hidden")));
+extern unsigned long       _DYNAMIC[] __attribute__((visibility("hidden")));
+extern Elf::Addr           etext;
+
 /**
  * Forward declartions and helpers
  */
@@ -126,6 +131,32 @@ namespace Linker {
 	 */
 	constexpr char const *binary_name() { return "binary"; }
 	constexpr char const *linker_name() { return "ld.lib.so"; }
+
+	/**
+	 * Address of .dynamic section in GOT
+	 */
+	static inline unsigned long dynamic_address_got()
+	{
+		return _GLOBAL_OFFSET_TABLE_[0];
+	}
+
+	/**
+	 * Address of .dynamic section from symbol
+	 */
+	static inline unsigned long dynamic_address()
+	{
+		return (unsigned long)&_DYNAMIC;
+	}
+
+	/**
+	 * Return the run-time load address of the shared object.
+	 */
+	static inline unsigned long relocation_address(void)
+	{
+		return dynamic_address() < dynamic_address_got() ?
+		       dynamic_address_got() - dynamic_address() :
+		       dynamic_address() - dynamic_address_got();
+	}
 }
 
 
@@ -141,12 +172,13 @@ class Linker::Object : public Genode::Fifo<Object>::Element,
 
 		char        _name[MAX_PATH];
 		File const *_file = nullptr;
+		Elf::Addr   _reloc_base = 0;
 
 	public:
 
-		Object() { }
+		Object(Elf::Addr reloc_base) : _reloc_base(reloc_base) { }
 		Object(char const *path, File const *file)
-		: _file(file)
+		: _file(file), _reloc_base(file->reloc_base)
 		{
 			Genode::strncpy(_name, Linker::file(path), MAX_PATH);
 		}
@@ -157,8 +189,8 @@ class Linker::Object : public Genode::Fifo<Object>::Element,
 				destroy(Genode::env()->heap(), const_cast<File *>(_file));
 		}
 
-		Elf::Addr  reloc_base() const { return _file ? _file->reloc_base : 0; }
-		char const *name()      const { return _name; }
+		Elf::Addr reloc_base() const { return _reloc_base; }
+		char const *name() const { return _name; }
 
 		File      const *file() { return _file; }
 		Elf::Size const  size() const { return _file ? _file->size : 0; }
@@ -168,7 +200,7 @@ class Linker::Object : public Genode::Fifo<Object>::Element,
 
 		virtual void relocate() = 0;
 
-		virtual void load()   = 0;
+		virtual void load() = 0;
 		virtual bool unload() { return false;}
 
 		/**

repos/base/src/lib/ldso/linux-32.ld

@@ -0,0 +1,20 @@
+/**
+ * \brief  Linker script for Linux
+ * \author Sebastian Sumpf
+ * \date   2015-12-08
+ *
+ * On Linux 32 bit, we remove the ".text.crt0" section because it contains a
+ * text relocation and is not used as startup code for the dynamic linker.
+ */
+
+/*
+ * Copyright (C) 2009-2015 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.
+ */
+
+SECTIONS
+{
+  /DISCARD/ : { *(.text.crt0) }
+}

repos/base/src/lib/ldso/main.cc

@@ -64,7 +64,8 @@ struct Linker::Elf_object : Object, Genode::Fifo<Elf_object>::Element
 	unsigned flags     = 0;
 	bool     relocated = false;
 
-	Elf_object(Dependency const *dep) : dyn(dep)
+	Elf_object(Dependency const *dep, Elf::Addr reloc_base)
+	: Object(reloc_base), dyn(dep)
 	{ }
 
 	Elf_object(char const *path, Dependency const *dep, unsigned flags = 0)
@@ -248,7 +249,8 @@ struct Linker::Elf_object : Object, Genode::Fifo<Elf_object>::Element
  */
 struct Linker::Ld : Dependency, Elf_object
 {
-	Ld() : Dependency(this, nullptr), Elf_object(this)
+	Ld()
+	: Dependency(this, nullptr), Elf_object(this, relocation_address())
 	{
 		Genode::strncpy(_name, linker_name(), Object::MAX_PATH);
 	}
@@ -525,7 +527,7 @@ extern "C" void init_rtld()
 	linker_stack.ref_count++;
 
 	/*
-	 * Create actual linker object with different vtable type  and set PLT to new
+	 * Create actual linker object with different vtable type and set PLT to new
 	 * DAG.
 	 */
 	Ld::linker()->dynamic()->plt_setup();
@@ -569,8 +571,12 @@ int main()
 
 	/* print loaded object information */
 	try {
-		if (Genode::config()->xml_node().attribute("ld_verbose").has_value("yes"))
+		if (Genode::config()->xml_node().attribute("ld_verbose").has_value("yes")) {
+			PINF("  %lx .. %lx: context area", Genode::Native_config::context_area_virtual_base(),
+			     Genode::Native_config::context_area_virtual_base() +
+			     Genode::Native_config::context_area_virtual_size() - 1);
 			dump_loaded();
+		}
 	} catch (...) {  }
 
 	Link_map::dump();