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

[Martin Storsjö via llvm-commits]

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

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.

>From f6ea86717358f6957c04064faf95aef3231cfe51 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin at martin.st>
Date: Thu, 11 Apr 2024 16:48:39 +0300
Subject: [PATCH] [LLD] [COFF] Don't add pseudo relocs for dangling references

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.
---
 lld/COFF/Chunks.cpp           |  7 ++++++
 lld/test/COFF/autoimport-gc.s | 41 +++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100644 lld/test/COFF/autoimport-gc.s

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