[ELF] --emit-relocs: adjust offsets of .rel[a].eh_frame relocations

.eh_frame pieces may be dropped due to GC/ICF. When --emit-relocs adds
relocations against .eh_frame, the offsets need to be adjusted. Use the same
way as MergeInputSection with a special case handling outSecOff==-1 for an
invalid piece (see eh-frame-marker.s).

This exposes an issue in mips64-eh-abs-reloc.s that we don't reliably
handle anyway. Just add --no-check-dynamic-relocations to paper over it.

Original patch by Ayrton Muñoz

Differential Revision: https://reviews.llvm.org/D122459

lld/ELF/InputSection.cpp

@@ -160,11 +160,15 @@ uint64_t SectionBase::getOffset(uint64_t offset) const {
   case Regular:
   case Synthetic:
     return cast<InputSection>(this)->outSecOff + offset;
-  case EHFrame:
+  case EHFrame: {
     // The file crtbeginT.o has relocations pointing to the start of an empty
     // .eh_frame that is known to be the first in the link. It does that to
     // identify the start of the output .eh_frame.
-    return offset;
+    const EhInputSection *es = cast<EhInputSection>(this);
+    if (InputSection *isec = es->getParent())
+      return isec->outSecOff + es->getParentOffset(offset);
+    return es->getParentOffset(offset);
+  }
   case Merge:
     const MergeInputSection *ms = cast<MergeInputSection>(this);
     if (InputSection *isec = ms->getParent())
@@ -1334,6 +1338,21 @@ void EhInputSection::split(ArrayRef<RelTy> rels) {
                 getObjMsg(d.data() - rawData.data()));
 }
 
+const EhSectionPiece &EhInputSection::getSectionPiece(uint64_t offset) const {
+  if (this->data().size() < offset)
+    fatal(toString(this) + ": offset is outside the section");
+  return partition_point(
+      pieces, [=](EhSectionPiece p) { return p.inputOff <= offset; })[-1];
+}
+
+// Return the offset in an output section for a given input offset.
+uint64_t EhInputSection::getParentOffset(uint64_t offset) const {
+  const EhSectionPiece &piece = getSectionPiece(offset);
+  if (piece.outputOff == -1) // invalid piece
+    return offset - piece.inputOff;
+  return piece.outputOff + (offset - piece.inputOff);
+}
+
 static size_t findNull(StringRef s, size_t entSize) {
   for (unsigned i = 0, n = s.size(); i != n; i += entSize) {
     const char *b = s.begin() + i;

lld/ELF/InputSection.h

@@ -325,6 +325,10 @@ public:
   SmallVector<EhSectionPiece, 0> pieces;
 
   SyntheticSection *getParent() const;
+  uint64_t getParentOffset(uint64_t offset) const;
+
+  // Return the EHSectionPiece at a given input section offset.
+  const EhSectionPiece &getSectionPiece(uint64_t offset) const;
 };
 
 // This is a section that is added directly to an output section

lld/test/ELF/eh-frame-merge.s

@@ -16,9 +16,9 @@
 
 # RELOC:        Offset             Info     Type          Symbol's Value  Symbol's Name + Addend
 # RELOC-NEXT: {{0*}}[[#%x,OFF:]]   [[#%x,]] R_X86_64_PC32 [[#%x,]]        foo + 0
-# RELOC-NEXT: {{0*}}[[#%x,OFF+20]] [[#%x,]] R_X86_64_PC32 [[#%x,]]        bar + 0
-# RELOC-NEXT: {{0*}}[[#OFF]]       [[#%x,]] R_X86_64_PC32 [[#%x,]]        foo + 1
-# RELOC-NEXT: {{0*}}[[#OFF+20]]    [[#%x,]] R_X86_64_NONE 0{{$}}
+# RELOC-NEXT: {{0*}}[[#%x,OFF+24]] [[#%x,]] R_X86_64_PC32 [[#%x,]]        bar + 0
+# RELOC-NEXT: {{0*}}[[#OFF+48]]    [[#%x,]] R_X86_64_PC32 [[#%x,]]        foo + 1
+# RELOC-NEXT: {{0*}}[[#%x,OFF-24]] [[#%x,]] R_X86_64_NONE 0{{$}}
 
 # EH:          Format:                DWARF32
 # EH:        00000018 00000014 0000001c FDE cie=00000000 pc={{0*}}[[#%x,FOO:]]...

lld/test/ELF/mips64-eh-abs-reloc.s

@@ -2,13 +2,13 @@
 # Having an R_MIPS_64 relocation in eh_frame would previously crash LLD
 # RUN: llvm-mc -filetype=obj -triple=mips64-unknown-freebsd %s -o %t.o
 # RUN: llvm-readobj -r %t.o | FileCheck %s -check-prefix OBJ
-# RUN: ld.lld --eh-frame-hdr -shared -z notext -o %t.so %t.o
+# RUN: ld.lld --eh-frame-hdr -shared -z notext -o %t.so %t.o --no-check-dynamic-relocations
 # RUN: llvm-readobj -r %t.so | FileCheck %s -check-prefix PIC-RELOCS
 
 # Linking this as a PIE executable would also previously crash
 # RUN: llvm-mc -filetype=obj -triple=mips64-unknown-freebsd %S/Inputs/archive2.s -o %t-foo.o
 # -pie needs -z notext because of the R_MIPS_64 relocation
-# RUN: ld.lld --eh-frame-hdr -Bdynamic -pie -z notext -o %t-pie-dynamic.exe %t.o %t-foo.o
+# RUN: ld.lld --eh-frame-hdr -Bdynamic -pie -z notext -o %t-pie-dynamic.exe %t.o %t-foo.o --no-check-dynamic-relocations
 # RUN: llvm-readobj -r %t-pie-dynamic.exe | FileCheck %s -check-prefix PIC-RELOCS