[ELF] Ignore comdat groups GCC emits for .debug_macro

GCC creates a .debug_macro section if -g3 is given. That section
is in a comdat group, but the section is directly referred by another
section, which is a violation of the ELF spec.

If a section is deduplicated, we handle any references to that section
as if they had value 0. But that causes a mysterious gdb slowdown.
So we can't set 0 for a dead .debug_macro section.

In this commit, we simply stop deduplicating .debug_macro sections.
This will bloat up debug info, but that's better than producing an
effectively non-debuggable binary.

Fixes https://github.com/rui314/mold/issues/357
Fixes https://github.com/rui314/mold/issues/438

elf/input-files.cc

@@ -133,6 +133,11 @@ void ObjectFile<E>::initialize_sections(Context<E> &ctx) {
       const ElfSym<E> &sym = this->elf_syms[shdr.sh_info];
       std::string_view signature = symbol_strtab.data() + sym.st_name;
 
+      // Ignore a broken comdat group GCC emits for .debug_macros.
+      // https://github.com/rui314/mold/issues/438
+      if (signature.starts_with("wm4."))
+        continue;
+
       // Get comdat group members.
       std::span<u32> entries = this->template get_data<u32>(ctx, shdr);
 

test/elf/debug-macro-section.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+export LC_ALL=C
+set -e
+CC="${CC:-cc}"
+CXX="${CXX:-c++}"
+GCC="${GCC:-gcc}"
+GXX="${GXX:-g++}"
+OBJDUMP="${OBJDUMP:-objdump}"
+MACHINE="${MACHINE:-$(uname -m)}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+mold="$(pwd)/mold"
+t=out/test/elf/$testname
+mkdir -p $t
+
+cat <<EOF > $t/a.h
+#define A 23
+#define B 99
+EOF
+
+cat <<EOF | $GCC -o $t/b.o -c -xc - -I$t -g3
+#include "a.h"
+extern int z();
+int main () { return z() - 122; }
+EOF
+
+cat <<EOF | $GCC -o $t/c.o -c -xc - -I$t -g3
+#include "a.h"
+int z()  { return A + B; }
+EOF
+
+$GCC -B. -o $t/exe $t/b.o $t/c.o
+$OBJDUMP --dwarf=macro $t/exe > $t/log
+! grep 'DW_MACRO_import -.* 0x0$' $t/log || false
+
+echo OK