[ELF] Don't crash in GC mode when linker script is used and .debug_lines section is present

[evgeny777]

Summary

This revision depends on:
https://reviews.llvm.org/D24733

.debug_lines is a special section, which may contain section relocations to member of comdat group. This comdat member
section may be discarded when input file is parsed, and this in its turn may cause a crash in addSection, when we'll try to analyze discarded
section header. This patch fixes this.

[ruiu]

LGTM

[• rafael]

test/ELF/linkerscript/Inputs/comdat-gc.s

4 ↗

(On Diff #72290)

Can you make this test more realistic by having MC create .debug_line?

[• rafael]

To make MC create .debug_line I think you only need to add a .loc directive.

lld/trunk/ELF/MarkLive.cpp

if (D->isSection())
     Offset += getAddend(Sec, Rel);
   return {D->Section->Repl, Offset};
 }
 
 // Calls Fn for each section that Sec refers to via relocations.
 template <class ELFT>
 static void forEachSuccessor(InputSection<ELFT> &Sec,
                              std::function<void(ResolvedReloc<ELFT>)> Fn) {
-  // Skip over discarded sections. This in theory shouldn't happen, because
-  // the ELF spec doesn't allow a relocation to point to a deduplicated
-  // COMDAT section directly. Unfortunately this happens in practice (e.g.
-  // .eh_frame) so we need to add a check.
-  if (&Sec == &InputSection<ELFT>::Discarded)
-    return;
-
   ELFFile<ELFT> &Obj = Sec.getFile()->getObj();
   for (const typename ELFT::Shdr *RelSec : Sec.RelocSections) {
     if (RelSec->sh_type == SHT_RELA) {
       for (const typename ELFT::Rela &Rel : Obj.relas(RelSec))
         Fn(resolveReloc(Sec, Rel));
     } else {
       for (const typename ELFT::Rel &Rel : Obj.rels(RelSec))
         Fn(resolveReloc(Sec, Rel));

 
 // This is the main function of the garbage collector.
 // Starting from GC-root sections, this function visits all reachable
 // sections to set their "Live" bits.
 template <class ELFT> void elf::markLive() {
   SmallVector<InputSection<ELFT> *, 256> Q;
 
   auto Enqueue = [&](ResolvedReloc<ELFT> R) {
-    if (!R.Sec)
+    // Skip over discarded sections. This in theory shouldn't happen, because
+    // the ELF spec doesn't allow a relocation to point to a deduplicated
+    // COMDAT section directly. Unfortunately this happens in practice (e.g.
+    // .eh_frame) so we need to add a check.
+    if (!R.Sec || R.Sec == &InputSection<ELFT>::Discarded)
       return;
 
     // Usually, a whole section is marked as live or dead, but in mergeable
     // (splittable) sections, each piece of data has independent liveness bit.
     // So we explicitly tell it which offset is in use.
     if (auto *MS = dyn_cast<MergeInputSection<ELFT>>(R.Sec))
       MS->markLiveAt(R.Offset);
 

lld/trunk/test/ELF/linkerscript/Inputs/comdat-gc.s

+.file 1 "test/ELF/linkerscript/Inputs/comdat_gc.s"
+
+.section  .text._Z3fooIiEvv,"axG",@progbits,_Z3fooIiEvv,comdat
+.loc 1 5
+  ret

lld/trunk/test/ELF/linkerscript/comdat-gc.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/comdat-gc.s -o %t1
+# RUN: echo "SECTIONS { .text : { *(.text*) } }" > %t.script
+# RUN: ld.lld --gc-sections --script %t.script %t %t1 -o %t2
+# RUN: llvm-readobj -sections -symbols %t2 | FileCheck -check-prefix=GC1 %s
+
+# GC1:     Name: .debug_line
+
+.file 1 "test/ELF/linkerscript/comdat_gc.s"
+.section  .text._Z3fooIiEvv,"axG",@progbits,_Z3fooIiEvv,comdat
+.loc 1 14
+  ret