[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 bbf8c659aa7180722ad12421add6016125c51f12 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            |  9 ++++-
 llvm/lib/ObjCopy/ELF/ELFObject.h              |  2 ++
 .../ELF/remove-section-in-group.test          | 35 ++++++++++++++++++-
 .../ELF/strip-debug-empty-group.test          | 31 ++++++++++++++++
 4 files changed, 75 insertions(+), 2 deletions(-)
 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..dc86078af079c 100644
--- a/llvm/lib/ObjCopy/ELF/ELFObject.cpp
+++ b/llvm/lib/ObjCopy/ELF/ELFObject.cpp
@@ -2206,8 +2206,15 @@ 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..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..bfe3a44a9e721 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 +35,36 @@ Symbols:
   - Name:     foo_bar_grp
     Section:  .group
     Binding:  STB_GLOBAL
+
+# RUN: yaml2obj --docnum=2 %s -o %t
+# RUN: llvm-strip --strip-debug %t -o %t1
+# RUN: llvm-readelf --section-groups %t1 | FileCheck %s --check-prefix=GROUP-REMOVED
+# RUN: llvm-strip --strip-unneeded %t -o %t2
+# RUN: llvm-readelf --section-groups %t2 | FileCheck %s --check-prefix=GROUP-REMOVED
+# RUN: llvm-strip --remove-section=.debug_macro %t -o %t3
+# RUN: llvm-readelf --section-groups %t3 | FileCheck %s --check-prefix=GROUP-REMOVED
+# RUN: llvm-strip --remove-section=.debug_* %t -o %t4
+# RUN: llvm-readelf --section-groups %t4 | 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.
+
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..3e09674e2288d
--- /dev/null
+++ b/llvm/test/tools/llvm-objcopy/ELF/strip-debug-empty-group.test
@@ -0,0 +1,31 @@
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-strip --strip-debug %t -o %t1
+# RUN: llvm-readelf --section-groups %t1 | FileCheck %s
+# RUN: llvm-strip --strip-unneeded %t -o %t2
+# RUN: llvm-readelf --section-groups %t2 | FileCheck %s
+# RUN: llvm-strip --remove-section=.debug_macro %t -o %t3
+# RUN: llvm-readelf --section-groups %t3 | FileCheck %s
+# RUN: llvm-strip --remove-section=.debug_* %t -o %t4
+# RUN: llvm-readelf --section-groups %t4 | 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.