[PATCH] D149817: [llvm-objdump][X86] Add @plt symbols for .plt.got

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
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If a symbol needs both JUMP_SLOT and GLOB_DAT relocations, there is a
minor linker optimization to keep just GLOB_DAT. This optimization
is only implemented by GNU ld's x86 port and mold.
https://maskray.me/blog/2021-08-29-all-about-global-offset-table#combining-.got-and-.got.plt

With the optimizing, the PLT entry is placed in .plt.got and the
associated GOTPLT entry is placed in .got (ld.bfd -z now) or .got.plt (ld.bfd -z lazy).
The relocation is in .rel[a].dyn.

This patch synthesizes `symbol at plt` labels for these .plt.got entries.

For x86-32, with -z now, in the absence of a PLT entry, there will be no
.got.plt entry, and our code cannot synthesize @plt labels. This is
an extreme corner case that can never happen in practice.
To fix it, we can get the `_GLOBAL_OFFSET_TABLE_` symbol value,
but the complexity is not worth it.

Example:

  cat > a.s <<e
  .globl _start; _start:
  mov combined0 at gotpcrel(%rip), %rax; mov combined1 at gotpcrel(%rip), %rax
  call combined0 at plt; call combined1 at plt
  call foo0 at plt; call foo1 at plt
  e
  cat > b.s <<e
  .globl foo0, foo1, combined0, combined1
  foo0: foo1: combined0: combined1:
  e
  gcc -fuse-ld=bfd -shared b.s -o b.so
  gcc -fuse-ld=bfd -pie -nostdlib a.s b.so -o a



  Disassembly of section .plt:
  
  0000000000001000 <.plt>:
      1000: ff 35 ea 1f 00 00             pushq   0x1fea(%rip)            # 0x2ff0 <_GLOBAL_OFFSET_TABLE_+0x8>
      1006: ff 25 ec 1f 00 00             jmpq    *0x1fec(%rip)           # 0x2ff8 <_GLOBAL_OFFSET_TABLE_+0x10>
      100c: 0f 1f 40 00                   nopl    (%rax)
  
  0000000000001010 <foo1 at plt>:
      1010: ff 25 ea 1f 00 00             jmpq    *0x1fea(%rip)           # 0x3000 <_GLOBAL_OFFSET_TABLE_+0x18>
      1016: 68 00 00 00 00                pushq   $0x0
      101b: e9 e0 ff ff ff                jmp     0x1000 <.plt>
  
  0000000000001020 <foo0 at plt>:
      1020: ff 25 e2 1f 00 00             jmpq    *0x1fe2(%rip)           # 0x3008 <_GLOBAL_OFFSET_TABLE_+0x20>
      1026: 68 01 00 00 00                pushq   $0x1
      102b: e9 d0 ff ff ff                jmp     0x1000 <.plt>
  
  Disassembly of section .plt.got:
  
  0000000000001030 <combined0 at plt>:
      1030: ff 25 a2 1f 00 00             jmpq    *0x1fa2(%rip)           # 0x2fd8 <foo1+0x2fd8>
      1036: 66 90                         nop
  
  0000000000001038 <combined1 at plt>:
      1038: ff 25 a2 1f 00 00             jmpq    *0x1fa2(%rip)           # 0x2fe0 <foo1+0x2fe0>
      103e: 66 90                         nop

Close https://github.com/llvm/llvm-project/issues/62537


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149817

Files:
  llvm/include/llvm/Object/ELFObjectFile.h
  llvm/lib/Object/ELFObjectFile.cpp
  llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
  llvm/test/tools/llvm-objdump/X86/plt-got.test
  llvm/tools/llvm-cfi-verify/lib/FileAnalysis.cpp
  llvm/tools/llvm-objdump/llvm-objdump.cpp

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