[Bug ld/19704] New: Missing dynamic relocation against undefined weak symbol

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19704

            Bug ID: 19704
           Summary: Missing dynamic relocation against undefined weak
                    symbol
           Product: binutils
           Version: 2.27 (HEAD)
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: hjl.tools at gmail dot com
  Target Milestone: ---

Currently when building executable, undefined weak symbol is resolved
differently, depending on if the relocatable input is compiled with PIC or
not:

address@hidden weak-1]$ cat x.c
extern void __attribute__ ((weak)) fun (void);
int main (void)
{
  if (&fun != 0)
    fun ();
  return 0;
}
address@hidden weak-1]$ make
gcc -B./ -O2 -o x x.c
gcc -B./ -O2 -o y x.c -fPIC
readelf -r x

Relocation section '.rela.dyn' at offset 0x328 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
000000600870  000200000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ +
0

Relocation section '.rela.plt' at offset 0x340 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
000000600890  000100000007 R_X86_64_JUMP_SLO 0000000000000000
address@hidden + 0

There is no dynamic relocation against fun, whose value is
resolved to zero at link-time.

readelf -r y

Relocation section '.rela.dyn' at offset 0x348 contains 2 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
0000006008a0  000100000006 R_X86_64_GLOB_DAT 0000000000000000 fun + 0
0000006008a8  000300000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ +
0

Relocation section '.rela.plt' at offset 0x378 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
0000006008c8  000200000007 R_X86_64_JUMP_SLO 0000000000000000
address@hidden + 0
address@hidden weak-1]$

There is dynamic relocation against fun, whose value is resolved
at runt-time. We should make it consistent, regardless if input
relocatable file is compiled with PIC or not.  GNU ABI should be
extended to:

When creating executable, if dynamic relocation is available at
run-time, the link editor should generate dynamic relocations
against unresolved weak symbols so that their values will be
resolved at run-time.

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