[lld] [LLD] [COFF] Don't add pseudo relocs for dangling references (PR #88487)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-coff

Author: Martin Storsjö (mstorsjo)

<details>
<summary>Changes</summary>

When doing GC, we normally won't have dangling references, because such a reference would keep the other section alive, keeping it from being eliminated.

However, references within DWARF sections are ignored for the purposes of GC (because otherwise, they would essentially keep everything alive, defeating the point of the GC), see c579a5b1d92a9bc2046d00ee2d427832e0f5ddec for more context.

Therefore, dangling relocations against discarded symbols are ignored within DWARF sections (see maybeReportRelocationToDiscarded in Chunks.cpp). Consequently, we also shouldn't create any pseudo relocations for these cases, as we run into a null pointer dereference when trying to generate the pseudo relocation info for it.

This fixes the downstream bug
https://github.com/mstorsjo/llvm-mingw/issues/418, fixing crashes on combinations with -ffunction-sections, -fdata-sections, -Wl,--gc-sections and debug info.

---
Full diff: https://github.com/llvm/llvm-project/pull/88487.diff


2 Files Affected:

- (modified) lld/COFF/Chunks.cpp (+7) 
- (added) lld/test/COFF/autoimport-gc.s (+41) 


``````````diff
diff --git a/lld/COFF/Chunks.cpp b/lld/COFF/Chunks.cpp
index 004d71097387a2..0cae52785a0002 100644
--- a/lld/COFF/Chunks.cpp
+++ b/lld/COFF/Chunks.cpp
@@ -648,6 +648,13 @@ void SectionChunk::getRuntimePseudoRelocs(
         dyn_cast_or_null<Defined>(file->getSymbol(rel.SymbolTableIndex));
     if (!target || !target->isRuntimePseudoReloc)
       continue;
+    // If the target doesn't have a chunk allocated, it may be a
+    // DefinedImportData symbol which ended up unnecessary after GC.
+    // Normally we wouldn't eliminate section chunks that are referenced, but
+    // references within DWARF sections don't count for keeping section chunks
+    // alive. Thus such dangling references in DWARF sections are expected.
+    if (!target->getChunk())
+      continue;
     int sizeInBits =
         getRuntimePseudoRelocSize(rel.Type, file->ctx.config.machine);
     if (sizeInBits == 0) {
diff --git a/lld/test/COFF/autoimport-gc.s b/lld/test/COFF/autoimport-gc.s
new file mode 100644
index 00000000000000..fef6c02eba82f9
--- /dev/null
+++ b/lld/test/COFF/autoimport-gc.s
@@ -0,0 +1,41 @@
+# REQUIRES: x86
+# RUN: split-file %s %t.dir
+
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.dir/lib.s -filetype=obj -o %t.dir/lib.obj
+# RUN: lld-link -out:%t.dir/lib.dll -dll -entry:DllMainCRTStartup %t.dir/lib.obj -lldmingw -implib:%t.dir/lib.lib
+
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.dir/main.s -filetype=obj -o %t.dir/main.obj
+# RUN: lld-link -lldmingw -out:%t.dir/main.exe -entry:main %t.dir/main.obj %t.dir/lib.lib -opt:ref -debug:dwarf
+
+#--- main.s
+    .global main
+    .section .text$main,"xr",one_only,main
+main:
+    ret
+
+    .global other
+    .section .text$other,"xr",one_only,other
+other:
+    movq .refptr.variable(%rip), %rax
+    movl (%rax), %eax
+    ret
+
+    .section .rdata$.refptr.variable,"dr",discard,.refptr.variable
+    .global .refptr.variable
+.refptr.variable:
+    .quad   variable
+
+    .section .debug_info
+    .long 1
+    .quad variable
+    .long 2
+
+#--- lib.s
+    .global variable
+    .global DllMainCRTStartup
+    .text
+DllMainCRTStartup:
+    ret
+    .data
+variable:
+    .long 42

``````````

</details>


https://github.com/llvm/llvm-project/pull/88487