[Bug ld/20828] New: [MIPS] produces invalid dynamic symbol table when --gc-sections is used since PR ld/13177 fix

[james410 at cowgill dot org.uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=20828

            Bug ID: 20828
           Summary: [MIPS] produces invalid dynamic symbol table when
                    --gc-sections is used since PR ld/13177 fix
           Product: binutils
           Version: 2.27
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: james410 at cowgill dot org.uk
  Target Milestone: ---

Originally from Debian: https://bugs.debian.org/844357
Related: https://bugs.debian.org/844227

On MIPS, it is possible for ld to generate an invalid symbol table if
--gc-sections is used when linking. This can be seen in all versions of
Debian's libGL.so.1 since jessie.

Steps to reproduce:
$ cat libstubs.c
void alpha(void) {}

$ cat stubs.c
void psi(void) {}
void omega(void) {}

$ gcc -shared libstubs.c -o libstubs.so
$ gcc -c stubs.c
$ ld -shared --gc-sections -o liboutput.so stubs.o libstubs.so

After doing the above, the dynamic symbol table contains:
$ readelf --dyn-sym liboutput.so
Symbol table '.dynsym' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000430     0 SECTION LOCAL  DEFAULT    6
     2: 0000000000010490     0 NOTYPE  LOCAL  DEFAULT    7 _end
     3: 0000000000010490     0 NOTYPE  LOCAL  DEFAULT    7 __bss_start
     4: 0000000000000430     0 NOTYPE  LOCAL  DEFAULT    6 _ftext
     5: 0000000000010490     0 NOTYPE  LOCAL  DEFAULT    7 _edata
     6: 0000000000000454    36 FUNC    GLOBAL DEFAULT    6 omega
     7: 0000000000010490     0 NOTYPE  LOCAL  DEFAULT    7 _fbss
     8: 0000000000000430    36 FUNC    GLOBAL DEFAULT    6 psi
     9: 0000000000010480     0 NOTYPE  LOCAL  DEFAULT    7 _fdata

This is clearly invalid because there are LOCAL symbols which appear after
GLOBAL symbols. The bug only appears if --gc-sections is used - without this
flag the "good" symbol table looks like this:

Symbol table '.dynsym' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000450     0 SECTION LOCAL  DEFAULT    7
     2: 00000000000104b0     0 NOTYPE  GLOBAL DEFAULT    8 _end
     3: 00000000000104b0     0 NOTYPE  GLOBAL DEFAULT    8 __bss_start
     4: 0000000000000450     0 NOTYPE  GLOBAL DEFAULT    7 _ftext
     5: 00000000000104b0     0 NOTYPE  GLOBAL DEFAULT    8 _edata
     6: 0000000000000474    36 FUNC    GLOBAL DEFAULT    7 omega
     7: 00000000000104b0     0 NOTYPE  GLOBAL DEFAULT    8 _fbss
     8: 0000000000000450    36 FUNC    GLOBAL DEFAULT    7 psi
     9: 00000000000104a0     0 NOTYPE  GLOBAL DEFAULT    8 _fdata

I have bisected this bug to the following commit. Binutils 2.22 in Debian
wheezy was also OK because the commit was reverted in the binutils 2.22 branch.

commit 1a766c6843ce0005fee23633b3b30c98e81a7b07
Author: H.J. Lu <address@hidden>
Date:   Fri Sep 16 01:17:16 2011 +0000

    Also hide symbols without PLT nor GOT references.

    bfd/

    2011-09-15  H.J. Lu  <address@hidden>

            PR ld/13177
            * elflink.c (elf_gc_sweep_symbol): Also hide symbols without PLT
            nor GOT references.

----

While this bug is quite old, it was only found very recently due to a change in
the calculation of the .dynsym sh_info value in:

commit 90ac242072dc68ad454aaaa228868b0f1c8e10f9
Author: Alan Modra <address@hidden>
Date:   Sat Aug 13 00:03:43 2016 +0930

    Correct .dynsym sh_info

The combination of this commit along with the above bug is causing lots of
undefined references when linking to libGL in Debian (which is compiled using
--gc-sections).

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