[OgShadoww]

Fix a crash during relocatable linking when an SHT_GROUP section has SHF_MERGE set. Before, shouldMerge() could classify the group as a MergeInputSection, losing information needed to update the group section and symbol indices. Fix excludes SHT_GROUP sections from merge classification

AI assisted with understanding and implementing the issue, writing regression tests, and reviewing the changes
Fixes #235240

[llvmorg-github-actions]

@llvm/pr-subscribers-lld

@llvm/pr-subscribers-lld-elf

Author: Orest Halenza (OgShadoww)

Changes

Fix a crash during relocatable linking when an SHT_GROUP section has SHF_MERGE set. Before, shouldMerge() could classify the group as a MergeInputSection, losing information needed to update the group section and symbol indices. Fix excludes SHT_GROUP sections from merge classification

AI assisted with understanding and implementing the issue, writing regression tests, and reviewing the changes
Fixes #235240

---

Full diff: https://github.com/llvm/llvm-project/pull/240069.diff

2 Files Affected:

• (modified) lld/ELF/InputFiles.cpp (+4)
• (added) lld/test/ELF/sht-group-merge.test (+48)

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index b412fd566f93e..678beab20f4b7 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -766,6 +766,10 @@ StringRef ObjFile<ELFT>::getShtGroupSignature(ArrayRef<Elf_Shdr> sections,
 
 template <class ELFT>
 bool ObjFile<ELFT>::shouldMerge(const Elf_Shdr &sec, StringRef name) {
+  // Keep the original group section so copyShtGroup can remap its members.
+  if (sec.sh_type == SHT_GROUP)
+    return false;
+
   // On a regular link we don't merge sections if -O0 (default is -O1). This
   // sometimes makes the linker significantly faster, although the output will
   // be bigger.
diff --git a/lld/test/ELF/sht-group-merge.test b/lld/test/ELF/sht-group-merge.test
new file mode 100644
index 0000000000000..7322ff2188082
--- /dev/null
+++ b/lld/test/ELF/sht-group-merge.test
@@ -0,0 +1,48 @@
+# REQUIRES: x86
+## Remap the group signature and member indices even when the group has merge
+## flags. The two identical constants in the member section should coalesce.
+
+# RUN: yaml2obj %s -o %t.o
+# RUN: ld.lld -r %t.o -o %t.ro
+# RUN: llvm-readelf -Sgs %t.ro | FileCheck %s --check-prefixes=CHECK,PLAIN
+
+# RUN: yaml2obj %s -DFLAGS="[SHF_MERGE, SHF_STRINGS]" -o %t.o
+# RUN: ld.lld -r %t.o -o %t.ro
+# RUN: llvm-readelf -Sgs %t.ro | FileCheck %s --check-prefixes=CHECK,STRINGS
+
+# PLAIN:      .group GROUP {{.*}} 000008 04    [[#SYMTAB:]] [[#SIG:]]
+# STRINGS:    .group GROUP {{.*}} 000008 04 MS [[#SYMTAB:]] [[#SIG:]]
+# CHECK:      [ [[#MEMBER:]]] .rodata.cst4 PROGBITS {{.*}} 000004 04 AMG
+# CHECK:      [ [[#SYMTAB]]] .symtab SYMTAB
+# CHECK:      [[#SIG]]: {{.*}} GLOBAL DEFAULT [[#MEMBER]] sig
+# CHECK:      COMDAT group section {{.*}} `.group' [sig] contains 1 sections:
+# CHECK-NEXT: [Index] Name
+# CHECK-NEXT: [ [[#MEMBER]]] .rodata.cst4
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Sections:
+## Put .symtab first so both sh_link and the member section index change.
+  - Name: .symtab
+    Type: SHT_SYMTAB
+  - Name:    .group
+    Type:    SHT_GROUP
+    Flags:   [[FLAGS=<none>]]
+    Link:    .symtab
+    Info:    sig
+    Members:
+      - SectionOrType: GRP_COMDAT
+      - SectionOrType: .rodata.cst4
+  - Name:    .rodata.cst4
+    Type:    SHT_PROGBITS
+    Flags:   [ SHF_ALLOC, SHF_MERGE, SHF_GROUP ]
+    EntSize: 4
+    Content: "1122334411223344"
+Symbols:
+  - Name:    sig
+    Section: .rodata.cst4
+    Binding: STB_GLOBAL