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Question

I have a module of code compiled for ARM with -fpie, and I'd like to clear the BSS when it is first executed. However, when I reference the linker script symbols for the start and end of the BSS section, I get code that contains absolute addresses in local variables.

Here is my assembly entry point:

.section entry_point

code_entry:
    # Save arguments to the stack
    push    {r0-r2}

    # Loop over the BSS section, set it all to 0
    mov     r0, #0
    ldr     r1, =__bss_start
    ldr     r2, =__bss_end
loop_bss:
    str     r0, [r1], #4
    cmp     r1, r2
    blt     loop_bss

    # Pop arguments from the stack
    pop     {r0-r2}

    # Branch to C code (which will return to caller on its own)
    b       startPICode

The output is the following:

Disassembly of section .text:

00180000 <code_entry>:
  180000:   e92d0007    push    {r0, r1, r2}
  180004:   e3a00000    mov r0, #0
  180008:   e59f1014    ldr r1, [pc, #20]   ; 180024 <loop_bss+0x14>
  18000c:   e59f2014    ldr r2, [pc, #20]   ; 180028 <loop_bss+0x18>

00180010 <loop_bss>:
  180010:   e4810004    str r0, [r1], #4
  180014:   e1510002    cmp r1, r2
  180018:   bafffffc    blt 180010 <loop_bss>
  18001c:   e8bd0007    pop {r0, r1, r2}
  180020:   ea00189f    b   1862a4 <startPICode>
  180024:   00194360    andseq  r4, r9, r0, ror #6
  180028:   001950e8    andseq  r5, r9, r8, ror #1

Readelf indicates that my ELF file has no relocations.

Am I correct in thinking that the linker symbols are just integers that will always be absolute addresses? If so, is there a way to fix up the addresses at load or execution time? Or is there some other correct way to clear the BSS for this code?

Edit: Adding some readelf -s output. Here's the relevant .S file and everything pertaining to the BSS.

Symbol table '.symtab' contains 812 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00180000     0 SECTION LOCAL  DEFAULT    1 
     2: 0019297c     0 SECTION LOCAL  DEFAULT    2 
     3: 00192994     0 SECTION LOCAL  DEFAULT    3 
     4: 001929b0     0 SECTION LOCAL  DEFAULT    4 
     5: 00193808     0 SECTION LOCAL  DEFAULT    5 
     6: 001942dc     0 SECTION LOCAL  DEFAULT    6 
     7: 00194350     0 SECTION LOCAL  DEFAULT    7 
     8: 00194358     0 SECTION LOCAL  DEFAULT    8 
     9: 0019435c     0 SECTION LOCAL  DEFAULT    9 
    10: 00194360     0 SECTION LOCAL  DEFAULT   10 
    11: 00194360     0 SECTION LOCAL  DEFAULT   11 
    12: 001950e8     0 SECTION LOCAL  DEFAULT   12 
    13: 001970f0     0 SECTION LOCAL  DEFAULT   13 
    14: 00000000     0 SECTION LOCAL  DEFAULT   14 
    15: 00000000     0 SECTION LOCAL  DEFAULT   15 
    16: 00000000     0 SECTION LOCAL  DEFAULT   16 
    17: 00000000     0 SECTION LOCAL  DEFAULT   17 
    18: 00000000     0 SECTION LOCAL  DEFAULT   18 
    19: 00000000     0 SECTION LOCAL  DEFAULT   19 
    20: 00000000     0 SECTION LOCAL  DEFAULT   20 
    21: 00000000     0 SECTION LOCAL  DEFAULT   21 
    22: 00000000     0 SECTION LOCAL  DEFAULT   22 
    23: 00000000     0 SECTION LOCAL  DEFAULT   23 
    24: 00000000     0 FILE    LOCAL  DEFAULT  ABS ./src/entry.o
    25: 00180000     0 NOTYPE  LOCAL  DEFAULT    1 code_entry
    26: 00180000     0 NOTYPE  LOCAL  DEFAULT    1 $a
    27: 00180010     0 NOTYPE  LOCAL  DEFAULT    1 loop_bss
    28: 00180024     0 NOTYPE  LOCAL  DEFAULT    1 $d
    29: 00000124     0 NOTYPE  LOCAL  DEFAULT   17 $d

   484: 00194360     0 NOTYPE  GLOBAL DEFAULT   10 __sbss_start

   525: 00193804     0 NOTYPE  GLOBAL DEFAULT    5 __sbss2_end

   555: 001950e8     0 NOTYPE  GLOBAL DEFAULT   11 __bss_end

   565: 00194360     0 NOTYPE  GLOBAL DEFAULT   10 __tbss_start

   581: 00194360     0 NOTYPE  GLOBAL DEFAULT   10 __sbss_end

   599: 00193804     0 NOTYPE  GLOBAL DEFAULT    5 __sbss2_start

   667: 00194360     0 NOTYPE  GLOBAL DEFAULT   11 __bss_start

   753: 00194360     0 NOTYPE  GLOBAL DEFAULT   10 __tbss_end

Answer 1

Your right, the code that you've produced isn't relocatable. In order to make it relocatable you're going to explicitly make the addresses of __bss_start and __bss_end relative. For example:

.section entry_point

bss_start_rel:
    .word __bss_start__ - bss_start_rel
bss_end_rel:
    .word __bss_end__ - bss_start_rel

.global code_entry
code_entry:
    # Save arguments to the stack
    push    {r0-r2}

    # Loop over the BSS section, set it all to 0
    mov     r0, #0
    adr     r3, bss_start_rel 
    ldr     r1, bss_start_rel
    ldr     r2, bss_end_rel
    add     r1, r1, r3
    add     r2, r2, r3

    ...

The ADR pseudo-instruction results in R3 being loaded with the address of bss_start_rel using PC relative arithmetic (eg. sub r3, pc, #24 ). That's used as base to calculate the location of __bss_start__ and __bss_end__ , who's relative position is stored at bss_start_rel and bss_start_end .