[Bug ld/12131] New: Using R_ARM_MOV[WT]_PREL to load address of _GLOBAL_OFFSET_TABLE_

[stephen.clarke at st dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12131

           Summary: Using R_ARM_MOV[WT]_PREL to load address of
                    _GLOBAL_OFFSET_TABLE_
           Product: binutils
           Version: 2.20
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
        AssignedTo: address@hidden
        ReportedBy: address@hidden


I'm seeing a problem with ARM GOT relocations, specifically
when the .got.plt section is not placed at the start of its output
section.  I have reproduced the problem with the binutils.weekly.bz2
tarball snapshot dated Oct 12 2010.

Here's my test example (also in the attachment):

$ cat gottest.s
        .text
_start:
        .global _start
        movw r0, :lower16:_GLOBAL_OFFSET_TABLE_-(here+8)
        movt r0, :upper16:_GLOBAL_OFFSET_TABLE_-(here+8)
        ldr  r1, littab
here:
littab:
        .word _GLOBAL_OFFSET_TABLE_-(here+8)

        .section .jcr, "aw"
        .space 1000

$ cat gottest.ld
OUTPUT_FORMAT("elf32-littlearm", "elf32-bigarm",
              "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
 .text           :
 {
    *(.text)
    *(.glue_7t) *(.glue_7) *(.vfp11_veneer)
 }  =0
  .data           : ALIGN (8)
  {
    KEEP (*(.jcr))
    *(.got.plt) *(.got)
    *(.data)
  }
}

I build like this:
$ arm-none-eabi-as -o gottest.o gottest.s
$ arm-none-eabi-ld --script gottest.ld gottest.o

My expectation is that the code will load the same value
into r0 and r1, but here is the disassembly:
$ arm-none-eabi-objdump -d a.out

a.out:     file format elf32-littlearm


Disassembly of section .text:

00000000 <_start>:
   0:   e30003e4        movw    r0, #996        ; 0x3e4
   4:   e3400000        movt    r0, #0
   8:   e51f1004        ldr     r1, [pc, #-4]   ; c <here>

0000000c <here>:
   c:   fffffffc        .word   0xfffffffc


It seems that r0 is being loaded with 996, and r1 is loaded
with -4.  The difference (1000) is exactly the size of the .jcr
section I placed at the start of .data.
Looking at the resolved symbol values:
$ arm-none-eabi-nm a.out 
000003f8 d _GLOBAL_OFFSET_TABLE_
00000000 T _start
0000000c t here
0000000c t littab

it seems the correct value should be 0x3f8-(0xc+0x8) = 0x3e4 = 996, i.e. the
movw/movt sequence is consistent with the symbol values.
However, the movw/movt sequence does not work when I use it in my
compiler in combination with R_ARM_GOT32 to get the address of a GOT entry,
whereas the ldr instruction does work.

Everything is fine and consistent iff the .got sections are
placed right at the start of the .data output section (e.g. if the
.jcr section has size zero).  But as soon as there is something before
them, I get problems.

Steve.

-- 
Configure bugmail: http://sourceware.org/bugzilla/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.