[RFC PATCH] Link the bootwrapper as a position-independent executable

@@ -6,16 +6,28 @@
  * as published by the Free Software Foundation; either version
  * 2 of the License, or (at your option) any later version.
  *
- * NOTE: this code runs in 32 bit mode and is packaged as ELF32.
+ * NOTE: this code runs in 32 bit mode, is position-independent,
+ * and is packaged as ELF32.
  */
 
 #include "ppc_asm.h"
 
 	.text
-	/* a procedure descriptor used when booting this as a COFF file */
+	/* A procedure descriptor used when booting this as a COFF file.
+	 * When making COFF, this comes first in the link and we're
+	 * linked at 0x500000.
+	 */
 	.globl	_zimage_start_opd
 _zimage_start_opd:
-	.long	_zimage_start, 0, 0, 0
+	.long	0x500000, 0, 0, 0
+
+p_start:	.long	_start
+p_etext:	.long	_etext
+p_bss_start:	.long	__bss_start
+p_end:		.long	_end
+
+	.weak	_platform_stack_top
+p_pstack:	.long	_platform_stack_top
 
 	.weak	_zimage_start
 	.globl	_zimage_start

@@ -24,37 +36,60 @@ _zimage_start:
 _zimage_start_lib:
 	/* Work out the offset between the address we were linked at
 	   and the address where we're running. */
-	bl	1f
-1:	mflr	r0
-	lis	r9,1b@ha
-	addi	r9,r9,1b@l
-	subf.	r0,r9,r0
-	beq	3f		/* if running at same address as linked */
+	bl	.+4
+p_base:	mflr	r10		/* r10 now points to runtime addr of p_base */
+	/* grab the link address of the dynamic section in r11 */
+	addis	r11,r10,(_GLOBAL_OFFSET_TABLE_-p_base)@ha
+	lwz	r11,(_GLOBAL_OFFSET_TABLE_-p_base)@l(r11)
+	cmpwi	r11,0
+	beq	3f		/* if not linked -pie */
+	/* get the runtime address of the dynamic section in r12 */
+	.weak	__dynamic_start
+	addis	r12,r10,(__dynamic_start-p_base)@ha
+	addi	r12,r12,(__dynamic_start-p_base)@l
+	subf	r11,r11,r12	/* runtime - linktime offset */
 
-	/* The .got2 section contains a list of addresses, so add
-	   the address offset onto each entry. */
-	lis	r9,__got2_start@ha
-	addi	r9,r9,__got2_start@l
-	lis	r8,__got2_end@ha
-	addi	r8,r8,__got2_end@l
-	subf.	r8,r9,r8
+	/* The dynamic section contains a series of tagged entries.
+	 * We need the RELA and RELACOUNT entries. */
+RELA = 7
+RELACOUNT = 0x6ffffff9
+	li	r9,0
+	li	r0,0
+9:	lwz	r8,0(r12)	/* get tag */
+	cmpwi	r8,0
+	beq	10f		/* end of list */
+	cmpwi	r8,RELA
+	bne	11f
+	lwz	r9,4(r12)	/* get RELA pointer in r9 */
+	b	12f
+11:	addis	r8,r8,(-RELACOUNT)@ha
+	cmpwi	r8,RELACOUNT@l
+	bne	12f
+	lwz	r0,4(r12)	/* get RELACOUNT value in r0 */
+12:	addi	r12,r12,8
+	b	9b
+
+	/* The relocation section contains a list of relocations.
+	 * We now do the R_PPC_RELATIVE ones, which point to words
+	 * which need to be initialized with addend + offset.
+	 * The R_PPC_RELATIVE ones come first and there are RELACOUNT
+	 * of them. */
+10:	or.	r8,r0,r9	/* skip relocation if we don't have both */
 	beq	3f
-	srwi.	r8,r8,2
-	mtctr	r8
-	add	r9,r0,r9
-2:	lwz	r8,0(r9)
-	add	r8,r8,r0
-	stw	r8,0(r9)
-	addi	r9,r9,4
+	mtctr	r0
+2:	lbz	r0,4+3(r9)	/* ELF32_R_INFO(reloc->r_info) */
+	cmpwi	r0,22		/* R_PPC_RELATIVE */
+	bne	3f
+	lwz	r12,0(r9)	/* reloc->r_offset */
+	lwz	r0,8(r9)	/* reloc->r_addend */
+	add	r0,r0,r11
+	stwx	r0,r11,r12
+	addi	r9,r9,12
 	bdnz	2b
 
 	/* Do a cache flush for our text, in case the loader didn't */
-3:	lis	r9,_start@ha
-	addi	r9,r9,_start@l
-	add	r9,r0,r9
-	lis	r8,_etext@ha
-	addi	r8,r8,_etext@l
-	add	r8,r0,r8
+3:	lwz	r9,p_start-p_base(r10)	/* note: these are relocated now */
+	lwz	r8,p_etext-p_base(r10)
 4:	dcbf	r0,r9
 	icbi	r0,r9
 	addi	r9,r9,0x20

@@ -64,27 +99,19 @@ _zimage_start_lib:
 	isync
 
 	/* Clear the BSS */
-	lis	r9,__bss_start@ha
-	addi	r9,r9,__bss_start@l
-	add	r9,r0,r9
-	lis	r8,_end@ha
-	addi	r8,r8,_end@l
-	add	r8,r0,r8
-	li	r10,0
-5:	stw	r10,0(r9)
+	lwz	r9,p_bss_start-p_base(r10)
+	lwz	r8,p_end-p_base(r10)
+	li	r0,0
+5:	stw	r0,0(r9)
 	addi	r9,r9,4
 	cmplw	cr0,r9,r8
 	blt	5b
 
 	/* Possibly set up a custom stack */
-.weak	_platform_stack_top
-	lis	r8,_platform_stack_top@ha
-	addi	r8,r8,_platform_stack_top@l
+	lwz	r8,p_pstack-p_base(r10)
 	cmpwi	r8,0
 	beq	6f
-	add	r8,r0,r8
 	lwz	r1,0(r8)
-	add	r1,r0,r1
 	li	r0,0
 	stwu	r0,-16(r1)	/* establish a stack frame */
 6:

@@ -39,6 +39,7 @@ dts=
 cacheit=
 binary=
 gzip=.gz
+pie=-pie
 
 # cross-compilation prefix
 CROSS=

@@ -149,9 +150,10 @@ pmac|chrp)
     platformo=$object/of.o
     ;;
 coff)
-    platformo=$object/of.o
+    platformo=$object/crt0.o $object/of.o
     lds=$object/zImage.coff.lds
     link_address='0x500000'
+    pie=
     ;;
 miboot|uboot)
     # miboot and U-boot want just the bare bits, not an ELF binary

@@ -197,6 +199,7 @@ ps3)
     ksection=.kernel:vmlinux.bin
     isection=.kernel:initrd
     link_address=''
+    pie=
     ;;
 ep88xc|ep405|ep8248e)
     platformo="$object/fixed-head.o $object/$platform.o"

@@ -288,9 +291,9 @@ fi
 
 if [ "$platform" != "miboot" ]; then
     if [ -n "$link_address" ] ; then
-        text_start="-Ttext $link_address --defsym _start=$link_address"
+        text_start="-Ttext $link_address"
     fi
-    ${CROSS}ld -m elf32ppc -T $lds $text_start -o "$ofile" \
+    ${CROSS}ld -m elf32ppc -T $lds $text_start $pie -o "$ofile" \
 	$platformo $tmp $object/wrapper.a
     rm $tmp
 fi

@@ -3,13 +3,13 @@ ENTRY(_zimage_start_opd)
 EXTERN(_zimage_start_opd)
 SECTIONS
 {
-  _start = .;
   .text      :
   {
+    _start = .;
     *(.text)
     *(.fixup)
+    _etext = .;
   }
-  _etext = .;
   . = ALIGN(4096);
   .data    :
   {

@@ -17,9 +17,7 @@ SECTIONS
     *(.data*)
     *(__builtin_*)
     *(.sdata*)
-    __got2_start = .;
     *(.got2)
-    __got2_end = .;
 
     _dtb_start = .;
     *(.kernel:dtb)

@@ -3,49 +3,63 @@ ENTRY(_zimage_start)
 EXTERN(_zimage_start)
 SECTIONS
 {
-  _start = .;
   .text      :
   {
+    _start = .;
     *(.text)
     *(.fixup)
+    _etext = .;
   }
-  _etext = .;
   . = ALIGN(4096);
   .data    :
   {
     *(.rodata*)
     *(.data*)
     *(.sdata*)
-    __got2_start = .;
     *(.got2)
-    __got2_end = .;
   }
+  .dynsym : { *(.dynsym) }
+  .dynstr : { *(.dynstr) }
+  .dynamic :
+  {
+    __dynamic_start = .;
+    *(.dynamic)
+  }
+  .hash : { *(.hash) }
+  .interp : { *(.interp) }
+  .rela.dyn : { *(.rela*) }
 
   . = ALIGN(8);
-  _dtb_start = .;
-  .kernel:dtb : { *(.kernel:dtb) }
-  _dtb_end = .;
-
-  . = ALIGN(4096);
-  _vmlinux_start =  .;
-  .kernel:vmlinux.strip : { *(.kernel:vmlinux.strip) }
-  _vmlinux_end =  .;
+  .kernel:dtb :
+  {
+    _dtb_start = .;
+    *(.kernel:dtb)
+    _dtb_end = .;
+  }
 
   . = ALIGN(4096);
-  _initrd_start =  .;
-  .kernel:initrd : { *(.kernel:initrd) }
-  _initrd_end =  .;
+  .kernel:vmlinux.strip :
+  {
+    _vmlinux_start =  .;
+    *(.kernel:vmlinux.strip)
+    _vmlinux_end =  .;
+  }
 
   . = ALIGN(4096);
-  _edata  =  .;
+  .kernel:initrd :
+  {
+    _initrd_start =  .;
+    *(.kernel:initrd)
+    _initrd_end =  .;
+  }
 
   . = ALIGN(4096);
-  __bss_start = .;
   .bss       :
   {
-   *(.sbss)
-   *(.bss)
+    _edata  =  .;
+    __bss_start = .;
+    *(.sbss)
+    *(.bss)
+    _end = . ;
   }
-  . = ALIGN(4096);
-  _end = . ;
 }