[lld] [LLD] [COFF] Fix handling of comdat .drectve sections (PR #68116)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-coff

<details>
<summary>Changes</summary>

This can happen when manually emitting strings into .drectve sections with `__attribute__((section(".drectve")))`, which is a way to emulate `#pragma comment(linker, "...")` for mingw compilers, without requiring building with -fms-extensions.

Normally, this doesn't generate any comdat, but if compiled with -fsanitize=address, this section does get turned into a comdat section.

This fixes #<!-- -->67261. This issue can be seen as a regression; a change in the "lli" tool in 17.x triggers this case, if compiled with ASAN enabled, triggering this unsupported corner case in LLD. With this change, LLD can handle it.

---
Full diff: https://github.com/llvm/llvm-project/pull/68116.diff


2 Files Affected:

- (modified) lld/COFF/InputFiles.cpp (+9-4) 
- (added) lld/test/COFF/comdat-drectve.s (+31) 


``````````diff
diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
index 541837a7fceca72..0b0fa4ae3bab868 100644
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -660,10 +660,15 @@ std::optional<Symbol *> ObjFile::createDefined(
 
     if (prevailing) {
       SectionChunk *c = readSection(sectionNumber, def, getName());
-      sparseChunks[sectionNumber] = c;
-      c->sym = cast<DefinedRegular>(leader);
-      c->selection = selection;
-      cast<DefinedRegular>(leader)->data = &c->repl;
+      if (c) {
+        sparseChunks[sectionNumber] = c;
+        c->sym = cast<DefinedRegular>(leader);
+        c->selection = selection;
+        cast<DefinedRegular>(leader)->data = &c->repl;
+      } else {
+        sparseChunks[sectionNumber] = nullptr;
+        return nullptr;
+      }
     } else {
       sparseChunks[sectionNumber] = nullptr;
     }
diff --git a/lld/test/COFF/comdat-drectve.s b/lld/test/COFF/comdat-drectve.s
new file mode 100644
index 000000000000000..6f96a8709fc7703
--- /dev/null
+++ b/lld/test/COFF/comdat-drectve.s
@@ -0,0 +1,31 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.obj
+
+# RUN: lld-link %t.obj -out:%t.exe -debug:symtab -subsystem:console
+# RUN: llvm-readobj --coff-exports %t.exe | FileCheck %s
+
+# CHECK: Name: exportedFunc
+
+## This assembly snippet has been reduced from what Clang generates from
+## this C snippet, with -fsanitize=address. Normally, the .drectve
+## section would be a regular section - but when compiled with
+## -fsanitize=address, it becomes a comdat section.
+##
+# void exportedFunc(void) {}
+# void mainCRTStartup(void) {}
+# static __attribute__((section(".drectve"), used)) const char export_chkstk[] =
+#     "-export:exportedFunc";
+
+	.text
+	.globl	exportedFunc
+exportedFunc:
+	retq
+
+	.globl	mainCRTStartup
+mainCRTStartup:
+	retq
+
+	.section	.drectve,"dr",one_only,export_chkstk
+export_chkstk:
+	.asciz	"-export:exportedFunc"

``````````

</details>


https://github.com/llvm/llvm-project/pull/68116