[llvm-objcopy][ELF] Fix removing SHT_GROUP sections.

When a SHT_GROUP section is removed, but other sections of the group are
kept, the SHF_GROUP flag of these sections should be dropped, otherwise
the resulting ELF file will be malformed.

Differential Revision: https://reviews.llvm.org/D80511

test/tools/llvm-objcopy/ELF/remove-section-group.test

@@ -0,0 +1,33 @@
+## This checks that when the header section of a group is removed, the tool
+## drops the flag SHF_GROUP for preserved members of that group.
+
+# RUN: yaml2obj %s -o - \
+# RUN:   | llvm-objcopy -R .group - - \
+# RUN:   | llvm-readobj --sections - \
+# RUN:   | FileCheck %s
+
+# CHECK:      Name: .foo
+# CHECK-NEXT: Type: SHT_PROGBITS
+# CHECK-NEXT: Flags [
+# CHECK-NEXT:   SHF_ALLOC
+# CHECK-NEXT: ]
+
+--- !ELF
+FileHeader:
+  Class:      ELFCLASS64
+  Data:       ELFDATA2LSB
+  Type:       ET_REL
+  Machine:    EM_X86_64
+Sections:
+  - Name:     .group
+    Type:     SHT_GROUP
+    Info:     foo_grp
+    Members:
+      - SectionOrType:  GRP_COMDAT
+      - SectionOrType:  .foo
+  - Name:     .foo
+    Type:     SHT_PROGBITS
+    Flags:    [ SHF_ALLOC, SHF_GROUP ]
+Symbols:
+  - Name:     foo_grp
+    Section:  .group

tools/llvm-objcopy/ELF/Object.cpp

@@ -65,6 +65,7 @@ void SectionBase::finalize() {}
 void SectionBase::markSymbols() {}
 void SectionBase::replaceSectionReferences(
     const DenseMap<SectionBase *, SectionBase *> &) {}
+void SectionBase::onRemove() {}
 
 template <class ELFT> void ELFWriter<ELFT>::writeShdr(const SectionBase &Sec) {
   uint8_t *B = Buf.getBufferStart() + Sec.HeaderOffset;
@@ -988,6 +989,13 @@ void GroupSection::replaceSectionReferences(
       Sec = To;
 }
 
+void GroupSection::onRemove() {
+  // As the header section of the group is removed, drop the Group flag in its
+  // former members.
+  for (SectionBase *Sec : GroupMembers)
+    Sec->Flags &= ~SHF_GROUP;
+}
+
 void Section::initialize(SectionTableRef SecTable) {
   if (Link == ELF::SHN_UNDEF)
     return;
@@ -1838,6 +1846,7 @@ Error Object::removeSections(bool AllowBrokenLinks,
   for (auto &RemoveSec : make_range(Iter, std::end(Sections))) {
     for (auto &Segment : Segments)
       Segment->removeSection(RemoveSec.get());
+    RemoveSec->onRemove();
     RemoveSections.insert(RemoveSec.get());
   }
 

tools/llvm-objcopy/ELF/Object.h

@@ -424,6 +424,8 @@ public:
   virtual void markSymbols();
   virtual void
   replaceSectionReferences(const DenseMap<SectionBase *, SectionBase *> &);
+  // Notify the section that it is subject to removal.
+  virtual void onRemove();
 };
 
 class Segment {
@@ -803,6 +805,7 @@ public:
   void markSymbols() override;
   void replaceSectionReferences(
       const DenseMap<SectionBase *, SectionBase *> &FromTo) override;
+  void onRemove() override;
 
   static bool classof(const SectionBase *S) {
     return S->OriginalType == ELF::SHT_GROUP;