[PATCH] D68094: ELF: Don't merge SHF_LINK_ORDER sections for different output sections in relocatable links.

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

For the record, the original approach does not work for the `ld.lld exidx.o exidx2.o -r -o exidxr.o -T exidx.lds` example at https://reviews.llvm.org/D68094?id=221994#1685161:

  [ 1] .foo              PROGBITS        00000000 000034 000010 00  AX  0   0  1
  [ 2] .text             PROGBITS        00000000 000044 000004 00  AX  0   0  4
  [ 3] .ARM.exidx.text.f1 ARM_EXIDX       00000000 000048 000008 00  AL  1   0  4               
  [ 4] .rel.ARM.exidx.text.f1 REL             00000000 000050 000008 08   I 13   3  4                
  [ 5] .ARM.exidx.text.f2 ARM_EXIDX       00000000 000058 000008 00  AL  1   0  4
  [ 6] .rel.ARM.exidx.text.f2 REL             00000000 000060 000010 08   I 13   5  4
  [ 7] .ARM.attributes   ARM_ATTRIBUTES  00000000 000070 00001e 00      0   0  1
  [ 8] .ARM.exidx.text.f1 ARM_EXIDX       00000000 000090 000008 00  AL  1   0  4
  [ 9] .rel.ARM.exidx.text.f1 REL             00000000 000098 000010 08   I 13   8  4
  [10] .ARM.exidx.text.f2 ARM_EXIDX       00000000 0000a8 000008 00  AL  1   0  4
  [11] .rel.ARM.exidx.text.f2 REL             00000000 0000b0 000008 08   I 13  10  4

The new approach can cause a segfault. The following 3 sections are consecutive in inputSections:

  .text.f1               # `add` returned early because parent(.text.f1) is set (it is specified by a SECTIONS command)
  .ARM.exidx.text.f1     # skipped because it has the SHF_LINK_ORDER flag
  .rel.ARM.exidx.text.f1 # SIGSEGV when dereferencing `OutputSection *out = sec->getRelocatedSection()->getOutputSection();` because out is null.

One probably fix is:

  -  auto add = [&](InputSectionBase *s) {
  +  std::function<void(InputSectionBase *)> add;
  +  add = [&](InputSectionBase *s) {
  +    auto onExit = make_scope_exit([&]() {
  +      if (config->relocatable)
  +        for (InputSectionBase *depSec : s->dependentSections)
  +          if (depSec->flags & SHF_LINK_ORDER)
  +            add(depSec);
  +    });

We may need a proper topological sort when the ordering requirement gets more complicated.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D68094/new/

https://reviews.llvm.org/D68094