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Re: What is R_X86_64_GOTPLT64 used for?


On Thu, 13 Nov 2014, H.J. Lu wrote:

> Linker does:
> ... code that looks like it might create just one GOT slot ...
> So if  a symbol is accessed by both @GOT and @PLTOFF, its
> needs_plt will be true and its got.plt entry will be used for
> both @GOT and @GOTPLT.  @GOTPLT has no advantage
> over @GOT, but potentially wastes a PLT entry.

The above is not correct.  Had you tried you'd see this:

% cat x.c
extern void foo (void);
void main (void)
  void (*f)(void) = foo;
% gcc -fPIE -mcmodel=large -S x.c; cat x.s
        movabsq $foo@GOT, %rax
        movabsq $foo@PLTOFF, %rax

So, foo is access via @GOT offset and @PLTOFF.  Then,

% cat y.c
void foo (void) {}
% gcc -o -shared -fPIC y.c
% gcc -fPIE -mcmodel=large x.s
% readelf -r a.out
000000600ff8  000400000006 R_X86_64_GLOB_DAT 0000000000000000 foo + 0
000000601028  000400000007 R_X86_64_JUMP_SLO 0000000000000000 foo + 0

The first one (to 600ff8) is the normal GOT slot, the second one the GOT 
slot for the PLT entry.  Both are actually used:

00000000004005f0 <foo@plt>:
  4005f0:       ff 25 32 0a 20 00       jmpq   *0x200a32(%rip)        # 601028 <_GLOBAL_OFFSET_TABLE_+0x28>

That uses the second GOT slot, and:

00000000004006ec <main>:
  4006ec:       55                      push   %rbp
  4006ed:       48 89 e5                mov    %rsp,%rbp
  4006f0:       53                      push   %rbx
  4006f1:       48 83 ec 18             sub    $0x18,%rsp
  4006f5:       48 8d 1d f9 ff ff ff    lea    -0x7(%rip),%rbx        # 4006f5 <main+0x9>
  4006fc:       49 bb 0b 09 20 00 00    movabs $0x20090b,%r11
  400703:       00 00 00 
  400706:       4c 01 db                add    %r11,%rbx
  400709:       48 b8 f8 ff ff ff ff    movabs $0xfffffffffffffff8,%rax
  400710:       ff ff ff 
  400713:       48 8b 04 03             mov    (%rbx,%rax,1),%rax

This uses the first slot at 0x600ff8.

So, no, currently GOT and GOTPLT (at least how it's supposed to be 
implemented) are not equivalent.

> Here is a patch to mark relocation 30 (R_X86_64_GOTPLT64) as reserved.  
> I pushed updated x86-64 psABI changes to
> I will update linker to keep accepting relocation 30 and treat it the 
> same as R_X86_64_GOT64.

That seems a bit premature given the above.


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