[ELF] -r: fix crash when processing a SHT_REL[A] that relocates a SHF_MERGE after D67504/r372734

Fix PR43767

In -r mode, when processing a SHT_REL[A] that relocates a SHF_MERGE, sec->getRelocatedSection() is a
MergeInputSection and its parent is an OutputSection but is asserted to
be a SyntheticSection (MergeSyntheticSection) in LinkerScript.cpp:addInputSec().
 ##
The code path is not exercised in non -r mode because the relocated
section changed from MergeInputSection to InputSection.

Reorder the code to make the non -r logic apply to -r as well, thus fix
the crash.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D69364

2 files changed, 33 insertions, 10 deletions

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index fdf935a3085..cd5c018ed9b 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -838,6 +838,16 @@ InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &sec) {
     if (!target)
       return nullptr;
 
+    // ELF spec allows mergeable sections with relocations, but they are
+    // rare, and it is in practice hard to merge such sections by contents,
+    // because applying relocations at end of linking changes section
+    // contents. So, we simply handle such sections as non-mergeable ones.
+    // Degrading like this is acceptable because section merging is optional.
+    if (auto *ms = dyn_cast<MergeInputSection>(target)) {
+      target = toRegularSection(ms);
+      this->sections[sec.sh_info] = target;
+    }
+
     // This section contains relocation information.
     // If -r is given, we do not interpret or apply relocation
     // but just copy relocation sections to output.
@@ -856,16 +866,6 @@ InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &sec) {
       fatal(toString(this) +
             ": multiple relocation sections to one section are not supported");
 
-    // ELF spec allows mergeable sections with relocations, but they are
-    // rare, and it is in practice hard to merge such sections by contents,
-    // because applying relocations at end of linking changes section
-    // contents. So, we simply handle such sections as non-mergeable ones.
-    // Degrading like this is acceptable because section merging is optional.
-    if (auto *ms = dyn_cast<MergeInputSection>(target)) {
-      target = toRegularSection(ms);
-      this->sections[sec.sh_info] = target;
-    }
-
     if (sec.sh_type == SHT_RELA) {
       ArrayRef<Elf_Rela> rels = CHECK(getObj().relas(&sec), this);
       target->firstRelocation = rels.begin();
diff --git a/lld/test/ELF/merge-relocatable.s b/lld/test/ELF/merge-relocatable.s
new file mode 100644
index 00000000000..d376f4ca0b4
--- /dev/null
+++ b/lld/test/ELF/merge-relocatable.s
@@ -0,0 +1,23 @@
+# REQUIRES: x86
+
+## Test that we keep a SHT_REL[A] section which relocates a SHF_MERGE section
+## in -r mode. The relocated SHF_MERGE section is handled as non-mergeable.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld -r %t.o -o %t
+# RUN: llvm-readobj -S %t | FileCheck %s
+
+# CHECK:     Name: .rodata.cst8
+# CHECK-NOT: }
+# CHECK:     Size: 16
+# CHECK:     Name: .rela.rodata.cst8
+# CHECK-NOT: }
+# CHECK:     Size: 48
+
+foo:
+
+.section .rodata.cst8,"aM",@progbits,8,unique,0
+.quad foo
+
+.section .rodata.cst8,"aM",@progbits,8,unique,1
+.quad foo