[llvm-branch-commits] [lld] release/18.x: [LLD] [COFF] Don't add pseudo relocs for dangling references (#88487) (PR #88759)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/88759

Backport 9c970d5ecd6a85188cd2b0a941fcd4d60063ef81

Requested by: @mstorsjo

>From e7e348ade66017e29e6982e508ec3f9088a1044b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin at martin.st>
Date: Mon, 15 Apr 2024 20:14:07 +0300
Subject: [PATCH] [LLD] [COFF] Don't add pseudo relocs for dangling references
 (#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.

(cherry picked from commit 9c970d5ecd6a85188cd2b0a941fcd4d60063ef81)
---
 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 39f4575031be54..e2074932bc466e 100644
--- a/lld/COFF/Chunks.cpp
+++ b/lld/COFF/Chunks.cpp
@@ -652,6 +652,13 @@ 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