[Mach-O] Add -add_empty_section

macho/cmdline.cc

@@ -20,6 +20,8 @@ Options:
   -U <SYMBOL>                 Allow a symbol to be undefined
   -Z                          Do not search the standard directories when
                               searching for libraries and frameworks
+  -add_empty_section <SEGNAME> <SECTNAME>
+                              Add an empty section
   -adhoc_codesign             Add ad-hoc code signature to the output file
     -no_adhoc_codesign
   -all_load                   Include all objects from static archives
@@ -194,6 +196,18 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       return false;
     };
 
+    auto read_arg2 = [&](std::string name) {
+      if (args[i] == name) {
+        if (args.size() <= i + 2)
+          Fatal(ctx) << "option -" << name << ": argument missing";
+        arg = args[i + 1];
+        arg2 = args[i + 2];
+        i += 3;
+        return true;
+      }
+      return false;
+    };
+
     auto read_arg3 = [&](std::string name) {
       if (args[i] == name) {
         if (args.size() <= i + 3)
@@ -261,6 +275,8 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       ctx.arg.ObjC = true;
     } else if (read_arg("-U")) {
       ctx.arg.U.push_back(std::string(arg));
+    } else if (read_arg2("-add_empty_section")) {
+      ctx.arg.add_empty_section.push_back({arg, arg2});
     } else if (read_flag("-adhoc_codesign")) {
       ctx.arg.adhoc_codesign = true;
     } else if (read_flag("-no_adhoc_codesign")) {

macho/main.cc

@@ -392,7 +392,14 @@ template <typename E>
 static void create_synthetic_chunks(Context<E> &ctx) {
   Timer t(ctx, "create_synthetic_chunks");
 
-  // First, we add subsections specified by -order_file to output sections.
+  // Handle -sectcreate
+  for (SectCreateOption arg : ctx.arg.sectcreate) {
+    MappedFile<Context<E>> *mf =
+      MappedFile<Context<E>>::must_open(ctx, std::string(arg.filename));
+    new SectCreateSection<E>(ctx, arg.segname, arg.sectname, mf->get_contents());
+  }
+
+  // We add subsections specified by -order_file to output sections.
   for (std::string_view name : ctx.arg.order_file)
     if (Symbol<E> *sym = get_symbol(ctx, name); sym->file)
       if (Subsection<E> *subsec = sym->subsec)
@@ -416,6 +423,15 @@ static void create_synthetic_chunks(Context<E> &ctx) {
     seg->chunks.push_back(chunk);
   }
 
+  // Handle -add_empty_section
+  for (AddEmptySectionOption &opt : ctx.arg.add_empty_section) {
+    if (!section_exists(ctx, opt.segname, opt.sectname)) {
+      OutputSegment<E> *seg = OutputSegment<E>::get_instance(ctx, opt.segname);
+      Chunk<E> *sec = new SectCreateSection<E>(ctx, opt.segname, opt.sectname, {});
+      seg->chunks.push_back(sec);
+    }
+  }
+
   // Sort segments and output sections.
   sort(ctx.segments, compare_segments<E>);
 
@@ -423,6 +439,15 @@ static void create_synthetic_chunks(Context<E> &ctx) {
     sort(seg->chunks, compare_chunks<E>);
 }
 
+template <typename E>
+static bool section_exists(Context<E> &ctx, std::string_view segname,
+                           std::string_view sectname) {
+  for (Chunk<E> *chunk : ctx.chunks)
+    if (chunk->hdr.match(segname, sectname))
+      return true;
+  return false;
+}
+
 template <typename E>
 static void scan_unwind_info(Context<E> &ctx) {
   tbb::parallel_for_each(ctx.objs, [&](ObjectFile<E> *file) {
@@ -822,15 +847,6 @@ static int do_main(int argc, char **argv) {
   tbb::global_control tbb_cont(tbb::global_control::max_allowed_parallelism,
                                ctx.arg.thread_count);
 
-  // Handle -sectcreate
-  for (SectCreateOption arg : ctx.arg.sectcreate) {
-    MappedFile<Context<E>> *mf =
-      MappedFile<Context<E>>::must_open(ctx, std::string(arg.filename));
-    SectCreateSection<E> *sec =
-      new SectCreateSection<E>(ctx, arg.segname, arg.sectname, mf->get_contents());
-    ctx.chunk_pool.emplace_back(sec);
-  }
-
   if (ctx.arg.adhoc_codesign)
     ctx.code_sig.reset(new CodeSignatureSection<E>(ctx));
 

macho/mold.h

@@ -707,10 +707,7 @@ template <typename E>
 class SectCreateSection : public Chunk<E> {
 public:
   SectCreateSection(Context<E> &ctx, std::string_view seg, std::string_view sect,
-                    std::string_view contents)
+                    std::string_view contents);
-    : Chunk<E>(ctx, seg, sect), contents(contents) {
-    this->hdr.size = contents.size();
-  }
 
   void copy_buf(Context<E> &ctx) override;
 
@@ -795,6 +792,11 @@ void create_range_extension_thunks(Context<ARM64> &ctx, OutputSection<ARM64> &os
 
 enum UuidKind { UUID_NONE, UUID_HASH, UUID_RANDOM };
 
+struct AddEmptySectionOption {
+  std::string_view segname;
+  std::string_view sectname;
+};
+
 struct SectCreateOption {
   std::string_view segname;
   std::string_view sectname;
@@ -870,6 +872,7 @@ struct Context {
     std::string map;
     std::string object_path_lto;
     std::string output = "a.out";
+    std::vector<AddEmptySectionOption> add_empty_section;
     std::vector<SectCreateOption> sectcreate;
     std::vector<std::string> U;
     std::vector<std::string> framework_paths;

macho/output-chunks.cc

@@ -1530,6 +1530,15 @@ void ThreadPtrsSection<E>::copy_buf(Context<E> &ctx) {
       buf[i] = sym.get_addr(ctx);
 }
 
+template <typename E>
+SectCreateSection<E>::SectCreateSection(Context<E> &ctx, std::string_view seg,
+                                        std::string_view sect,
+                                        std::string_view contents)
+  : Chunk<E>(ctx, seg, sect), contents(contents) {
+  this->hdr.size = contents.size();
+  ctx.chunk_pool.emplace_back(this);
+}
+
 template <typename E>
 void SectCreateSection<E>::copy_buf(Context<E> &ctx) {
   write_string(ctx.buf + this->hdr.offset, contents);

test/macho/add-empty-section.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+export LC_ALL=C
+set -e
+CC="${TEST_CC:-cc}"
+CXX="${TEST_CXX:-c++}"
+GCC="${TEST_GCC:-gcc}"
+GXX="${TEST_GXX:-g++}"
+OBJDUMP="${OBJDUMP:-objdump}"
+MACHINE="${MACHINE:-$(uname -m)}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+t=out/test/macho/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+int main() {}
+EOF
+
+clang --ld-path=./ld64 -o $t/exe $t/a.o -Wl,-add_empty_section,__FOO,__foo
+
+otool -l $t/exe | grep -q 'segname __FOO'
+otool -l $t/exe | grep -q 'sectname __foo'
+
+echo OK