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

[Dmitriy Chestnykh via llvm-commits]

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

>From 49172a9b5c5d7563a581334c3574a4d6f5699f67 Mon Sep 17 00:00:00 2001
From: Dmitry Chestnykh <dm.chestnykh at gmail.com>
Date: Sun, 30 Jun 2024 13:21:34 +0300
Subject: [PATCH] [llvm-strip] 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            | 11 +++-
 llvm/lib/ObjCopy/ELF/ELFObject.h              |  2 +
 .../ELF/remove-section-in-group.test          | 50 ++++++++++++++++++-
 3 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/ObjCopy/ELF/ELFObject.cpp b/llvm/lib/ObjCopy/ELF/ELFObject.cpp
index 02591e6f987c2..a5a42424ad6f2 100644
--- a/llvm/lib/ObjCopy/ELF/ELFObject.cpp
+++ b/llvm/lib/ObjCopy/ELF/ELFObject.cpp
@@ -2206,8 +2206,17 @@ 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 altogether.
   Sections.erase(Iter, std::end(Sections));
+
+  // Finally erase empty SHT_GROUP sections.
+  llvm::erase_if(Sections, [](const SecPtr &Sec) {
+    if (auto GroupSec = dyn_cast<GroupSection>(Sec.get()))
+      return GroupSec->getMembersCount() == 0;
+
+    return false;
+  });
+
   return Error::success();
 }
 
diff --git a/llvm/lib/ObjCopy/ELF/ELFObject.h b/llvm/lib/ObjCopy/ELF/ELFObject.h
index 2b1895a30b41e..2adab618ce12f 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/remove-section-in-group.test b/llvm/test/tools/llvm-objcopy/ELF/remove-section-in-group.test
index 9e683b9f68c93..ccc1ede0589c1 100644
--- a/llvm/test/tools/llvm-objcopy/ELF/remove-section-in-group.test
+++ b/llvm/test/tools/llvm-objcopy/ELF/remove-section-in-group.test
@@ -1,6 +1,6 @@
 ## This checks that the group section is shrunk when its member is removed.
 
-# RUN: yaml2obj %s -o - \
+# RUN: yaml2obj --docnum=1 %s -o - \
 # RUN:   | llvm-objcopy -R .foo - - \
 # RUN:   | obj2yaml - \
 # RUN:   | FileCheck %s
@@ -35,3 +35,51 @@ Symbols:
   - Name:     foo_bar_grp
     Section:  .group
     Binding:  STB_GLOBAL
+
+# RUN: yaml2obj --docnum=2 %s -o %t
+# RUN: llvm-objcopy --remove-section=.debug_macro %t
+# RUN: llvm-readelf --section-groups %t | FileCheck %s --check-prefix=GROUP-REMOVED
+
+--- !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
+
+# GROUP-REMOVED: There are no section groups in this file.
+
+# RUN: yaml2obj --docnum=3 %s -o %t
+# RUN: llvm-objcopy --remove-section=.group %t
+# RUN: llvm-readelf --section-groups %t | FileCheck %s --check-prefix=EMPTY-GROUP-REMOVED
+
+--- !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
+Symbols:
+  - Name:     foo_grp
+    Section:  .group
+
+# EMPTY-GROUP-REMOVED: There are no section groups in this file.