powerpc/boot: Allow building the zImage wrapper as a relocatable ET_DYN

This patch adds code, linker script and makefile support to allow
building the zImage wrapper around the kernel as a position independent
executable.  This results in an ET_DYN instead of an ET_EXEC ELF output
file, which can be loaded at any location by the firmware and will
process its own relocations to work correctly at the loaded address.

This is of interest particularly since the standard ePAPR image format
must be an ET_DYN (although this patch alone is not sufficient to
produce a fully ePAPR compliant boot image).

Note for now we don't enable building with -pie for anything.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

4 files changed, 118 insertions, 70 deletions

diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
index f1c4dfc635be..0f7428a37efb 100644
--- a/arch/powerpc/boot/crt0.S
+++ b/arch/powerpc/boot/crt0.S
@@ -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 +36,65 @@ _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 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 .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 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:     /* skip relocation if we don't have both */
+        cmpwi   r0,0
         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
+        cmpwi   r9,0
+        beq     3f
+
+        add     r9,r9,r11       /* Relocate RELA pointer */
+        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 +104,19 @@ _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:
diff --git a/arch/powerpc/boot/wrapper b/arch/powerpc/boot/wrapper
index cb97e7511d7e..fef527867811 100755
--- a/arch/powerpc/boot/wrapper
+++ b/arch/powerpc/boot/wrapper
@@ -39,6 +39,7 @@ dts=
 cacheit=
 binary=
 gzip=.gz
+pie=
 
 # cross-compilation prefix
 CROSS=
@@ -157,9 +158,10 @@ 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
@@ -208,6 +210,7 @@ ps3)
     ksection=.kernel:vmlinux.bin
     isection=.kernel:initrd
     link_address=''
+    pie=
     ;;
 ep88xc|ep405|ep8248e)
     platformo="$object/fixed-head.o $object/$platform.o"
@@ -310,9 +313,9 @@ 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
diff --git a/arch/powerpc/boot/zImage.coff.lds.S b/arch/powerpc/boot/zImage.coff.lds.S
index 856dc78b14ef..de4c9e3c9344 100644
--- a/arch/powerpc/boot/zImage.coff.lds.S
+++ b/arch/powerpc/boot/zImage.coff.lds.S
@@ -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)
diff --git a/arch/powerpc/boot/zImage.lds.S b/arch/powerpc/boot/zImage.lds.S
index 0962d62bdb50..2bd8731f1365 100644
--- a/arch/powerpc/boot/zImage.lds.S
+++ b/arch/powerpc/boot/zImage.lds.S
@@ -3,49 +3,64 @@ 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)
+    *(COMMON)
+    _end = . ;
   }
-  . = ALIGN(4096);
-  _end = . ;
 }