[llvm] Remove empty SHT_GROUP sections. (PR #97141)

[Dmitriy Chestnykh via llvm-commits]

https://github.com/chestnykh created https://github.com/llvm/llvm-project/pull/97141

Currently `llvm-strip` in `--strip-debug` mode doesn't remove such sections. This behavior can lead to incompatibilities with GNU binutils (for examples ld.bfd cannot process the object file contains empty .group section).
The ELF object that contains group section with `.debug_*` sections inside can be obtained by `gcc -g3`.
Fix #97139

>From 5b0c32b9910f54ec2d0dafe86dcc74055b783d09 Mon Sep 17 00:00:00 2001
From: Dmitry Chestnykh <dm.chestnykh at gmail.com>
Date: Sat, 29 Jun 2024 09:24:11 +0300
Subject: [PATCH] Remove empty SHT_GROUP sections.

Currently `llvm-strip` in `--strip-debug` mode doesn't remove
such sections. This behavior can lead to incompatibilities
with GNU binutils (for examples ld.bfd cannot process
the object file contains empty .group section).
The ELF object that contains group section with `.debug_*`
sections inside can be obtained by `gcc -g3`.
Fix #97139
---
 llvm/lib/ObjCopy/ELF/ELFObject.cpp            | 10 +++++++-
 llvm/lib/ObjCopy/ELF/ELFObject.h              |  2 ++
 .../ELF/strip-debug-empty-group.test          | 25 +++++++++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/tools/llvm-objcopy/ELF/strip-debug-empty-group.test

diff --git a/llvm/lib/ObjCopy/ELF/ELFObject.cpp b/llvm/lib/ObjCopy/ELF/ELFObject.cpp
index 02591e6f987c2..44181ccd328bf 100644
--- a/llvm/lib/ObjCopy/ELF/ELFObject.cpp
+++ b/llvm/lib/ObjCopy/ELF/ELFObject.cpp
@@ -2206,8 +2206,16 @@ Error Object::removeSections(
   // Transfer removed sections into the Object RemovedSections container for use
   // later.
   std::move(Iter, Sections.end(), std::back_inserter(RemovedSections));
-  // Now finally get rid of them all together.
+  // Now get rid of them all together.
   Sections.erase(Iter, std::end(Sections));
+
+  // Finally iterate over all sections and erase empty SHT_GROUP sections.
+  for (auto Iter = Sections.begin(); Iter != Sections.end(); ++Iter) {
+    if (auto GroupSec = dyn_cast<GroupSection>(Iter->get())) {
+      if (GroupSec->getMembersCount() == 0)
+        Sections.erase(Iter);
+    }
+  }
   return Error::success();
 }
 
diff --git a/llvm/lib/ObjCopy/ELF/ELFObject.h b/llvm/lib/ObjCopy/ELF/ELFObject.h
index 2b1895a30b41e..9bbef268017ea 100644
--- a/llvm/lib/ObjCopy/ELF/ELFObject.h
+++ b/llvm/lib/ObjCopy/ELF/ELFObject.h
@@ -963,6 +963,8 @@ class GroupSection : public SectionBase {
       const DenseMap<SectionBase *, SectionBase *> &FromTo) override;
   void onRemove() override;
 
+  size_t getMembersCount() {return GroupMembers.size(); }
+
   static bool classof(const SectionBase *S) {
     return S->OriginalType == ELF::SHT_GROUP;
   }
diff --git a/llvm/test/tools/llvm-objcopy/ELF/strip-debug-empty-group.test b/llvm/test/tools/llvm-objcopy/ELF/strip-debug-empty-group.test
new file mode 100644
index 0000000000000..26bbde0c603bb
--- /dev/null
+++ b/llvm/test/tools/llvm-objcopy/ELF/strip-debug-empty-group.test
@@ -0,0 +1,25 @@
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-strip --strip-debug %t -o %t1
+# RUN: llvm-readelf --section-groups %t1 | FileCheck %s
+
+--- !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:  .debug_macro
+  - Name:     .debug_macro
+    Type:     SHT_PROGBITS
+    Flags:    [ SHF_GROUP ]
+Symbols:
+  - Name:     foo_grp
+    Section:  .group
+
+# CHECK: There are no section groups in this file.