[llvm] [DWARF5][COFF] Fix wrong relocation in .debug_line (PR #83773)

[via llvm-commits]

https://github.com/timoh-ba created https://github.com/llvm/llvm-project/pull/83773

Dwarf 5 allows separating filenames from .debug_line into a separate .debug_line_str section. The strings are referenced relative to the start of the .debug_line_str section. Previously, on COFF, the relocation information instead caused offsets to be relocated to the base address of the COFF-File. This lead to wrong offsets in linked COFF files which caused the debugger to be unable to find the correct source files.

This patch fixes this problem by making the offsets relative to the start of the .debug_line_str section instead. There should be no changes for ELF-Files as everything seems to be working there.

I'm not sure if we should add a test case for this and I don't know how you'd write one - the problem only occurs in linked .dll-files containing dwarf 5 debugging information. The wrong offsets in the files causes missing file names of the source files there. I tested this change manually and it seems to work for dlls and not break elf files, though someone with more experience in llvm should probably check if this the right way of achieving this.

>From d6357eec1683cb75dc49e21cbc0bf8f443b00373 Mon Sep 17 00:00:00 2001
From: Timo Habighorst <t.habighorst at beckhoff.com>
Date: Fri, 23 Feb 2024 10:48:22 +0100
Subject: [PATCH] [DWARF5][COFF] Fix wrong relocation in .debug_line

Dwarf 5 allows separating filenames from .debug_line into a separate
.debug_line_str section. The strings are referenced relative to the
start of the .debug_line_str section. Previously, on COFF, the
relocation information instead caused offsets to be relocated to the
base address of the COFF-File. This lead  to wrong offsets in linked
COFF files which caused the debugger to be unable to find the correct
source files.

This patch fixes this problem by making the offsets relative to the
start of the .debug_line_str section instead. There should be no
changes for ELF-Files as everything seems to be working there.
---
 llvm/lib/MC/MCDwarf.cpp | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index d0face9140de66..2ee0c3eb27b92e 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -360,7 +360,12 @@ void MCDwarfLineStr::emitRef(MCStreamer *MCOS, StringRef Path) {
   size_t Offset = addString(Path);
   if (UseRelocs) {
     MCContext &Ctx = MCOS->getContext();
-    MCOS->emitValue(makeStartPlusIntExpr(Ctx, *LineStrLabel, Offset), RefSize);
+    if (Ctx.getAsmInfo()->needsDwarfSectionOffsetDirective()) {
+      MCOS->emitCOFFSecRel32(LineStrLabel, Offset);
+    } else {
+      MCOS->emitValue(makeStartPlusIntExpr(Ctx, *LineStrLabel, Offset),
+                      RefSize);
+    }
   } else
     MCOS->emitIntValue(Offset, RefSize);
 }