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-linux/include/base/native_types.h

@@ -20,12 +20,14 @@
 
 /*
  * We cannot just include <semaphore.h> and <pthread.h> here
- * because this would impy the nested inclusion of a myriad
+ * because this would imply the nested inclusion of a myriad
  * of Linux types and would pollute the namespace for everyone
  * who includes this header file. We want to cleanly separate
  * Genode from POSIX.
  */
 
+extern Genode::addr_t _context_area_start;
+
 namespace Genode {
 
 	/**
@@ -129,10 +131,11 @@ namespace Genode {
 		 * Thread-context area configuration.
 		 *
 		 * Please update platform-specific files after changing these
-		 * values, e.g., 'base-linux/src/platform/context_area.*.ld'.
+		 * functions, e.g., 'base-linux/src/ld/context_area.*.ld'.
 		 */
-		static constexpr addr_t context_area_virtual_base() {
-			return 0x40000000UL; }
+		static addr_t context_area_virtual_base() {
+			return align_addr((addr_t)&_context_area_start, 20); }
+
 		static constexpr addr_t context_area_virtual_size() {
 			return 0x10000000UL; }
 

repos/base-linux/src/ld/context_area.nostdlib.ld

@@ -20,6 +20,9 @@ SECTIONS
 {
 	. = 0x40000000;
 	_context_area_start = .;
-	.context_area : { . += 0x10000000; } : context_area
-	_context_area_end = .;
+	/*
+	 * Since Linux loads ldso page aligned, we align the context area after
+	 * loading to a 1 MiB boundary, therefore we reserve one MiB more here.
+	 */
+	.context_area : { . += 0x10100000; } : context_area
 }

repos/base-linux/src/ld/context_area.stdlib.ld

@@ -16,5 +16,4 @@ SECTIONS
 	. = 0x40000000;
 	_context_area_start = .;
 	.context_area : { . += 0x10000000; }
-	_context_area_end = .;
 }

repos/base-linux/src/lib/lx_hybrid/lx_hybrid.cc

@@ -19,6 +19,12 @@
 
 extern "C" int raw_write_str(const char *str);
 
+/**
+ * Define context area
+ */
+Genode::addr_t _context_area_start;
+
+
 enum { verbose_atexit = false };
 
 

repos/base/lib/mk/ldso.inc

@@ -9,11 +9,18 @@ SRC_CC       = main.cc test.cc exception.cc file.cc dependency.cc debug.cc \
 SRC_S        = jmp_slot.s
 INC_DIR     += $(DIR)/include
 LD_OPT      += -Bsymbolic-functions --version-script=$(DIR)/symbol.map
-ENTRY_POINT  = _start
 
 ifneq ($(filter linux, $(SPECS)),)
-LD_OPT += -T$(call select_from_repositories,src/ld/context_area.nostdlib.ld)
+
+ENTRY_POINT = _start_initial_stack
+LD_OPT     += -T$(call select_from_repositories,src/ld/context_area.nostdlib.ld) \
+
+ifneq ($(filter x86_32, $(SPECS)),)
+LD_OPT     += -T$(DIR)/linux-32.ld
+endif
+
 else
+ENTRY_POINT = _start
 LD_OPT     += -T$(DIR)/linker.ld
 endif
 

repos/base/src/base/thread/thread.cc

@@ -42,7 +42,6 @@ void Thread_base::Context::stack_size(size_t const size)
 	/* check if the stack enhancement fits the context region */
 	enum {
 		CONTEXT_SIZE      = Native_config::context_virtual_size(),
-		CONTEXT_AREA_BASE = Native_config::context_area_virtual_base(),
 		UTCB_SIZE         = sizeof(Native_utcb),
 		PAGE_SIZE_LOG2    = 12,
 		PAGE_SIZE         = (1UL << PAGE_SIZE_LOG2),
@@ -52,7 +51,8 @@ void Thread_base::Context::stack_size(size_t const size)
 	if (stack_base - ds_size < context_base) { throw Stack_too_large(); }
 
 	/* allocate and attach backing store for the stack enhancement */
-	addr_t const ds_addr = stack_base - ds_size - CONTEXT_AREA_BASE;
+	addr_t const ds_addr = stack_base - ds_size -
+	                       Native_config::context_area_virtual_base();
 	try {
 		Ram_session * const ram = env_context_area_ram_session();
 		Ram_dataspace_capability const ds_cap = ram->alloc(ds_size);

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);
 		}
@@ -157,7 +189,7 @@ 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; }
+		Elf::Addr reloc_base() const { return _reloc_base; }
 		char const *name() const { return _name; }
 
 		File      const *file() { return _file; }

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);
 	}
@@ -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();

repos/base/src/lib/startup/spec/arm/crt0.s

@@ -22,6 +22,8 @@
 	/* program entry-point */
 	.global _start
 	_start:
+	.global _start_initial_stack
+	_start_initial_stack:
 
 	/* make initial value of some registers available to higher-level code */
 	ldr r4, =__initial_sp

repos/base/src/lib/startup/spec/x86_32/crt0.s

@@ -17,6 +17,26 @@
  ** .text (program code) **
  **************************/
 
+.section ".text.crt0.ld"
+
+	/* ld.lib.so entry point for Linux */
+	.global _start_initial_stack
+	_start_initial_stack:
+
+	/* set GOT */
+	call 1f
+	1:
+	pop %ebx
+	addl $_GLOBAL_OFFSET_TABLE_ + 1, %ebx
+
+	/* init_rtld relocates the linker */
+	call init_rtld
+
+	/* the address of __initial_sp is now correct */
+	movl %esp, __initial_sp@GOTOFF(%ebx)
+
+	jmp 2f
+
 .section ".text.crt0"
 
 	/* program entry-point */
@@ -44,11 +64,16 @@
 	/* if this is the dynamic linker, init_rtld relocates the linker */
 	call init_rtld
 
+	jmp 2f
+
+.section ".text.crt0.common"
+2:
+
 	/* create proper environment for the main thread */
 	call init_main_thread
 
 	/* apply environment that was created by init_main_thread */
-	movl init_main_thread_result, %esp
+	movl init_main_thread_result@GOTOFF(%ebx), %esp
 
 	/* clear the base pointer in order that stack backtraces will work */
 	xor %ebp, %ebp

repos/base/src/lib/startup/spec/x86_64/crt0.s

@@ -19,6 +19,22 @@
 
 .section ".text.crt0"
 
+	/* ld.lib.so entry point for Linux */
+	.global _start_initial_stack
+	_start_initial_stack:
+
+	/* initialize GLOBAL OFFSET TABLE */
+	leaq _GLOBAL_OFFSET_TABLE_(%rip), %r15
+
+	/* init_rtld relocates the linker */
+	call init_rtld
+
+	/* the address of __initial_sp is now correct */
+	movq __initial_sp@GOTPCREL(%rip), %rax
+	movq %rsp, (%rax)
+
+	jmp 1f
+
 	/* program entry-point */
 	.global _start
 	_start:
@@ -41,9 +57,10 @@
 	leaq _stack_high@GOTPCREL(%rip),%rax
 	movq (%rax), %rsp
 
-	/* if this is the dynamic linker, init_rtld relocates the linker */
+	/* init_rtld relocates the linker */
 	call init_rtld
 
+1:
 	/* create proper environment for the main thread */
 	call init_main_thread