[Bug ld/21900] New: [MIPS] linking shared library with -z defs --warn-unresolved-symbols emits null relocation

[james410 at cowgill dot org.uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=21900

            Bug ID: 21900
           Summary: [MIPS] linking shared library with -z defs
                    --warn-unresolved-symbols emits null relocation
           Product: binutils
           Version: 2.29
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: james410 at cowgill dot org.uk
  Target Milestone: ---

Hi,

On MIPS, using "-z defs" and "--warn-unresolved-symbols" while building a
shared library with undefined symbols seems to cause all the relocations which
use those symbols to be filled with null values. The executables then crash
when executed because they load a garbage address into t9 and jump to it. Here
is an example:

> $ cat undef.c 
> int parent(void);
> 
> int child(void)
> {
>     return parent();
> }
> 
> $ mips-linux-gnu-gcc -c -fPIC -O2 undef.c
> $ ../build-mips/ld/ld-new -EB -shared -z defs --warn-unresolved-symbols 
> undef.o -o libundef.so
> undef.o: In function `child':
> undef.c:(.text+0xc): warning: undefined reference to `parent'
> undef.c:(.text+0x10): warning: undefined reference to `parent'
> $ mips-linux-gnu-objdump -dr libundef.so 
> 
> libundef.so:     file format elf32-tradbigmips
> 
> Disassembly of section .text:
> 
> 00000300 <child>:
>  300: 3c1c0002        lui     gp,0x2
>  304: 279c8030        addiu   gp,gp,-32720
>  308: 0399e021        addu    gp,gp,t9
>  30c: 8f990000        lw      t9,0(gp)
>  310: 03200008        jr      t9
>  314: 00000000        nop
>       ...
Note the line:
> lw    t9,0(gp)
I would have expected a different offset than 0. If you remove the "-z defs"
option to ld, a valid library is generated.

This was observed while trying to debug a long standing issue where
gnome-settings-daemon would immediately SIGBUS on startup. I have not
personally tested old versions of binutils, but this bug suggests that it was
broken on 2011 and a similar bug affects (or affected) ia64 as well:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=620874#40

Thanks,
James

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