base: Add Linux and GOT relative startup code on ARM

* Linux specific startup code for base-linux ARM
* load symbols global offset table relative in order to avoid text
  relocations

issue #3466

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

@@ -6,7 +6,7 @@
  */
 
 /*
- * Copyright (C) 2007-2017 Genode Labs GmbH
+ * Copyright (C) 2007-2019 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.
@@ -17,39 +17,88 @@
  ** .text (program code) **
  **************************/
 
+/**
+ * Set pointer to global offset table in given register
+ */
+.macro _got reg
+1:
+	ldr \reg, 2f
+	add \reg, pc, \reg
+	b 3f
+2:
+	.word _GLOBAL_OFFSET_TABLE_ - (1b + 12)
+3:
+.endm
+
 .section ".text.crt0"
 
+	/* ld.lib.so entry for Linux */
+	.global _start_initial_stack
+	_start_initial_stack:
+
+	/* on Linux we have a valid stack pointer */
+	bl init_rtld
+
+	/* save stack pointer in __intial_sp  GOT relative */
+	_got sl
+	ldr r4, .LGOT
+	ldr r4, [sl, r4]
+	str sp, [r4]
+
+	/* load stack pointer _stack_high GOT relative */
+	ldr r4, .LGOT + 8
+	ldr sp, [sl, r4]
+
+	b 4f
+
 	/* 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
+	_got sl
+
+	/* save __initial_sp */
+	ldr r4, .LGOT
+	ldr r4, [sl, r4]
 	str sp, [r4]
-	ldr r4, =__initial_r0
+
+	/* r0 contains the boot info on SeL4 save in __intial_r0 */
+	ldr r4, .LGOT + 4
+	ldr r4, [sl, r4]
 	str r0, [r4]
 
 	/*
 	 * Install initial temporary environment that is replaced later by the
-	 * environment that init_main_thread creates.
+	 * environment that init_main_thread creates, load __stack_high
 	 */
-	ldr sp, =_stack_high
+	ldr r4, .LGOT + 8
+	ldr sp, [sl, r4]
 
 	/* if this is the dynamic linker, init_rtld relocates the linker */
 	bl init_rtld
 
+4:
+
 	/* create proper environment for main thread */
 	bl init_main_thread
 
-	/* apply environment that was created by init_main_thread */
-	ldr sp, =init_main_thread_result
+	/*
+	 * Apply environment that was created by init_main_thread from
+	 * init_main_thread_result
+	 */
+	ldr r4, .LGOT + 12
+	ldr sp, [sl, r4]
 	ldr sp, [sp]
 
 	/* jump into init C code instead of calling it as it should never return */
 	b _main
 
+.LGOT:
+	.word __initial_sp(GOT)
+	.word __initial_r0(GOT)
+	.word _stack_high(GOT)
+	.word init_main_thread_result(GOT)
+
 
 /*********************************
  ** .bss (non-initialized data) **
@@ -62,6 +111,7 @@
 	.global __initial_stack_base
 	__initial_stack_base:
 	.space 4*1024
+	.global _stack_high
 	_stack_high:
 
 	/* initial value of the SP register */