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[Bug binutils/4959] New: Multiple relocs for the same symbol are not han

From: atgraham at gmail dot com
Subject: [Bug binutils/4959] New: Multiple relocs for the same symbol are not handled correctly
Date: 24 Aug 2007 16:29:16 -0000

Sample code:

#include "pthread.h"
static __typeof(pthread_mutex_lock) __gthrw_pthread_mutex_lock __attribute__
int main() {

This (mipsel-linux-gcc-4.2.1 -Os -c) generates the following object code:

00000000 <main>:
   0:   3c1c0000        lui     gp,0x0
                        0: R_MIPS_HI16  _gp_disp
   4:   279c0000        addiu   gp,gp,0
                        4: R_MIPS_LO16  _gp_disp
   8:   0399e021        addu    gp,gp,t9
   c:   27bdffe0        addiu   sp,sp,-32
  10:   afbf0018        sw      ra,24(sp)
  14:   afbc0010        sw      gp,16(sp)
  18:   8f990000        lw      t9,0(gp)
                        18: R_MIPS_GOT16        pthread_mutex_lock
  1c:   27390000        addiu   t9,t9,0
                        1c: R_MIPS_LO16 pthread_mutex_lock
  20:   0320f809        jalr    t9
  24:   00002021        move    a0,zero
  28:   8fbc0010        lw      gp,16(sp)
  2c:   8f990000        lw      t9,0(gp)
                        2c: R_MIPS_CALL16       pthread_mutex_lock
  30:   0320f809        jalr    t9
  34:   00002021        move    a0,zero
  38:   8fbc0010        lw      gp,16(sp)
  3c:   8fbf0018        lw      ra,24(sp)
  40:   00001021        move    v0,zero
  44:   03e00008        jr      ra
  48:   27bd0020        addiu   sp,sp,32
  4c:   00000000        nop

Which gets linked (using binutils 2.17) to this:

00400750 <main>:
  400750:       3c1c0005        lui     gp,0x5
  400754:       279c8290        addiu   gp,gp,-32112
  400758:       0399e021        addu    gp,gp,t9
  40075c:       27bdffe0        addiu   sp,sp,-32
  400760:       afbf0018        sw      ra,24(sp)
  400764:       afbc0010        sw      gp,16(sp)
  400768:       8f998058        lw      t9,-32680(gp)
  40076c:       27393c60        addiu   t9,t9,15456 <-- adds offset to GOT
  400770:       0320f809        jalr    t9
  400774:       00002021        move    a0,zero
  400778:       8fbc0010        lw      gp,16(sp)
  40077c:       8f998058        lw      t9,-32680(gp)
  400780:       0320f809        jalr    t9          <-- no additional offset
  400784:       00002021        move    a0,zero
  400788:       8fbc0010        lw      gp,16(sp)
  40078c:       8fbf0018        lw      ra,24(sp)
  400790:       00001021        move    v0,zero
  400794:       03e00008        jr      ra
  400798:       27bd0020        addiu   sp,sp,32
  40079c:       00000000        nop

Using a weak symbol appears to force gcc to use a GOT16 reloc.  A CALL16 reloc
is used otherwise.  If either one or the other are used, the linker functions
correctly, but if they are both used, the linker only functions correctly in the
case of the CALL16 reloc.

A bug was submitted to GCC bugzilla (#33169) and it was determined that
generating two different relocs for the same symbol is not a bug.  The linker
should be able to handle it by using 0 for the LO16 offset.

This problem will occur if pthreads are used directly in a C++ program that uses
weak pthread linkage indirectly through libstdc++ (not very rare, I think).

           Summary: Multiple relocs for the same symbol are not handled
           Product: binutils
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: binutils
        AssignedTo: unassigned at sources dot redhat dot com
        ReportedBy: atgraham at gmail dot com
                CC: bug-binutils at gnu dot org
  GCC host triplet: i386-pc-linux-gnu
GCC target triplet: mipsel-linux


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