[ELF] Do not corrupt compressed debug sections on i386

Previously, if an input debug section is compressed, mold produced
corrupted output debug section on i386.

Fixes #361

elf/input-sections.cc

@@ -39,16 +39,31 @@ InputSection<E>::InputSection(Context<E> &ctx, ObjectFile<E> &file,
   if (section_idx < file.elf_sections.size())
     contents = {(char *)file.mf->data + shdr().sh_offset, shdr().sh_size};
 
+  bool compressed;
+
   if (name.starts_with(".zdebug")) {
     sh_size = *(ubig64 *)&contents[4];
     p2align = to_p2align(shdr().sh_addralign);
+    compressed = true;
   } else if (shdr().sh_flags & SHF_COMPRESSED) {
     ElfChdr<E> &chdr = *(ElfChdr<E> *)&contents[0];
     sh_size = chdr.ch_size;
     p2align = to_p2align(chdr.ch_addralign);
+    compressed = true;
   } else {
     sh_size = shdr().sh_size;
     p2align = to_p2align(shdr().sh_addralign);
+    compressed = false;
+  }
+
+  // Uncompress early if the relocation is REL-type so that we can read
+  // addends from section contents. If RELA-type, we don't need to do this
+  // because addends are in relocations.
+  if (compressed && E::is_rel) {
+    u8 *buf = new u8[sh_size];
+    uncompress(ctx, buf);
+    contents = {(char *)buf, sh_size};
+    ctx.string_pool.emplace_back(buf);
   }
 
   output_section =
@@ -57,7 +72,8 @@ InputSection<E>::InputSection(Context<E> &ctx, ObjectFile<E> &file,
 
 template <typename E>
 bool InputSection<E>::is_compressed() {
-  return name().starts_with(".zdebug") || (shdr().sh_flags & SHF_COMPRESSED);
+  return !E::is_rel &&
+         (name().starts_with(".zdebug") || (shdr().sh_flags & SHF_COMPRESSED));
 }
 
 template <typename E>

test/elf/compressed-debug-info-i386.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+export LANG=
+set -e
+CC="${CC:-cc}"
+CXX="${CXX:-c++}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+mold="$(pwd)/mold"
+t=out/test/elf/$testname
+mkdir -p $t
+
+[ "$(uname -m)" = x86_64 ] || { echo skipped; exit; }
+
+which dwarfdump >& /dev/null || { echo skipped; exit; }
+
+cat <<EOF | $CXX -c -o $t/a.o -g -gz=zlib-gnu -xc++ - -m32
+int main() {
+  return 0;
+}
+EOF
+
+cat <<EOF | $CXX -c -o $t/b.o -g -gz=zlib -xc++ - -m32
+int foo() {
+  return 0;
+}
+EOF
+
+$CC -B. -o $t/exe $t/a.o $t/b.o -m32
+dwarfdump $t/exe > /dev/null
+readelf --sections $t/exe | fgrep -q .debug_info
+
+echo ' OK'

test/elf/compressed-debug-info.sh

@@ -12,13 +12,13 @@ mkdir -p $t
 
 which dwarfdump >& /dev/null || { echo skipped; exit; }
 
-cat <<EOF | g++ -c -o $t/a.o -g -gz=zlib-gnu -xc++ -
+cat <<EOF | $CXX -c -o $t/a.o -g -gz=zlib-gnu -xc++ -
 int main() {
   return 0;
 }
 EOF
 
-cat <<EOF | g++ -c -o $t/b.o -g -gz=zlib -xc++ -
+cat <<EOF | $CXX -c -o $t/b.o -g -gz=zlib -xc++ -
 int foo() {
   return 0;
 }