[Mach-O] Support -dylib

macho/cmdline.cc

@@ -23,8 +23,10 @@ Options:
   -arch <ARCH_NAME>           Specify target architecture
   -dead_strip                 Remove unreachable functions and data
   -demangle                   Demangle C++ symbols in log messages (default)
+  -dylib                      Produce a dynamic library
   -dynamic                    Link against dylibs (default)
   -e <SYMBOL>                 Specify the entry point of a main executable
+  -execute                    Produce an executable (default)
   -filelist <FILE>[,<DIR>]    Specify the list of input file names
   -framework <NAME>,[,<SUFFIX>]
                               Search for a given framework
@@ -176,6 +178,8 @@ void parse_nonpositional_args(Context &ctx,
       ctx.arg.dead_strip = true;
     } else if (read_flag("-demangle")) {
       ctx.arg.demangle = true;
+    } else if (read_flag("-dylib")) {
+      ctx.output_type = MH_DYLIB;
     } else if (read_arg("-headerpad")) {
       size_t pos;
       ctx.arg.headerpad = std::stoi(std::string(arg), &pos, 16);
@@ -185,6 +189,8 @@ void parse_nonpositional_args(Context &ctx,
       ctx.arg.dynamic = true;
     } else if (read_arg("-e")) {
       ctx.arg.entry = arg;
+    } else if (read_flag("-execute")) {
+      ctx.output_type = MH_EXECUTE;
     } else if (read_arg("-fatal_warnings")) {
     } else if (read_arg("-filelist")) {
       remaining.push_back("-filelist");

macho/dead-strip.cc

@@ -5,9 +5,19 @@ namespace mold::macho {
 static std::vector<Subsection *> collect_root_set(Context &ctx) {
   std::vector<Subsection *> rootset;
 
-  if (Symbol *sym = intern(ctx, ctx.arg.entry))
-    if (sym->subsec)
+  auto mark = [&](Symbol *sym) {
+    if (sym && sym->subsec)
       rootset.push_back(sym->subsec);
+  };
+
+  mark(intern(ctx, ctx.arg.entry));
+
+  if (ctx.output_type == MH_DYLIB)
+    for (ObjectFile *file : ctx.objs)
+      for (Symbol *sym : file->syms)
+        if (sym->file == file && sym->is_extern)
+          mark(sym);
+
   return rootset;
 }
 

macho/main.cc

@@ -34,10 +34,20 @@ static void create_internal_file(Context &ctx) {
 
   add("__dyld_private");
 
-  Symbol *sym = add("__mh_execute_header");
-  sym->is_extern = true;
-  sym->referenced_dynamically = true;
-  sym->value = ctx.arg.pagezero_size;
+  switch (ctx.output_type) {
+  case MH_EXECUTE: {
+    Symbol *sym = add("__mh_execute_header");
+    sym->is_extern = true;
+    sym->referenced_dynamically = true;
+    sym->value = ctx.arg.pagezero_size;
+    break;
+  }
+  case MH_DYLIB:
+    add("__mh_dylib_header");
+    break;
+  default:
+    unreachable();
+  }
 }
 
 static bool compare_segments(const std::unique_ptr<OutputSegment> &a,
@@ -193,6 +203,7 @@ static i64 assign_offsets(Context &ctx) {
 
 static void fix_synthetic_symbol_values(Context &ctx) {
   intern(ctx, "__dyld_private")->value = ctx.data->hdr.addr;
+  intern(ctx, "__mh_dylib_header")->value = ctx.data->hdr.addr;
 }
 
 MappedFile<Context> *find_framework(Context &ctx, std::string name) {
@@ -366,7 +377,7 @@ int main(int argc, char **argv) {
   for (DylibFile *dylib : ctx.dylibs)
     dylib->resolve_symbols(ctx);
 
-  if (!intern(ctx, ctx.arg.entry)->file)
+  if (ctx.output_type == MH_EXECUTE && !intern(ctx, ctx.arg.entry)->file)
     Error(ctx) << "undefined entry point symbol: " << ctx.arg.entry;
 
   create_internal_file(ctx);

macho/mold.h

@@ -723,14 +723,15 @@ struct Context {
     bool dead_strip = true;
     bool deduplicate = true;
     bool demangle = false;
+    bool dylib = false;
     bool dynamic = true;
     bool fatal_warnings = false;
     bool trace = false;
+    i64 headerpad = 256;
+    i64 pagezero_size = 0x100000000;
     i64 platform = PLATFORM_MACOS;
     i64 platform_min_version = 0;
     i64 platform_sdk_version = 0;
-    i64 headerpad = 256;
-    i64 pagezero_size = 0x100000000;
     std::string chroot;
     std::string entry = "_main";
     std::string map;
@@ -742,6 +743,7 @@ struct Context {
   } arg;
 
   std::vector<std::string_view> cmdline_args;
+  u32 output_type = MH_EXECUTE;
 
   bool has_error = false;
 

macho/output-chunks.cc

@@ -193,6 +193,17 @@ static std::vector<u8> create_data_in_code_cmd(Context &ctx) {
   return buf;
 }
 
+static std::vector<u8> create_id_dylib_cmd(Context &ctx) {
+  std::vector<u8> buf(sizeof(DylibCommand) + ctx.arg.output.size() + 1);
+  DylibCommand &cmd = *(DylibCommand *)buf.data();
+
+  cmd.cmd = LC_ID_DYLIB;
+  cmd.cmdsize = buf.size();
+  cmd.nameoff = sizeof(cmd);
+  write_string(buf.data() + sizeof(cmd), ctx.arg.output);
+  return buf;
+}
+
 static std::vector<u8> create_code_signature_cmd(Context &ctx) {
   std::vector<u8> buf(sizeof(LinkEditDataCommand));
   LinkEditDataCommand &cmd = *(LinkEditDataCommand *)buf.data();
@@ -239,11 +250,9 @@ static std::pair<i64, std::vector<u8>> create_load_commands(Context &ctx) {
   vec.push_back(create_dyld_info_only_cmd(ctx));
   vec.push_back(create_symtab_cmd(ctx));
   vec.push_back(create_dysymtab_cmd(ctx));
-  vec.push_back(create_dylinker_cmd(ctx));
   vec.push_back(create_uuid_cmd(ctx));
   vec.push_back(create_build_version_cmd(ctx));
   vec.push_back(create_source_version_cmd(ctx));
-  vec.push_back(create_main_cmd(ctx));
   vec.push_back(create_function_starts_cmd(ctx));
 
   for (DylibFile *dylib : ctx.dylibs)
@@ -255,6 +264,18 @@ static std::pair<i64, std::vector<u8>> create_load_commands(Context &ctx) {
   if (!ctx.data_in_code.contents.empty())
     vec.push_back(create_data_in_code_cmd(ctx));
 
+  switch (ctx.output_type) {
+  case MH_EXECUTE:
+    vec.push_back(create_dylinker_cmd(ctx));
+    vec.push_back(create_main_cmd(ctx));
+    break;
+  case MH_DYLIB:
+    vec.push_back(create_id_dylib_cmd(ctx));
+    break;
+  default:
+    unreachable();
+  }
+
   if (ctx.arg.adhoc_codesign)
     vec.push_back(create_code_signature_cmd(ctx));
 
@@ -286,7 +307,7 @@ void OutputMachHeader::copy_buf(Context &ctx) {
   mhdr.magic = 0xfeedfacf;
   mhdr.cputype = CPU_TYPE_X86_64;
   mhdr.cpusubtype = CPU_SUBTYPE_X86_64_ALL;
-  mhdr.filetype = MH_EXECUTE;
+  mhdr.filetype = ctx.output_type;
   mhdr.ncmds = ncmds;
   mhdr.sizeofcmds = cmds.size();
   mhdr.flags = MH_TWOLEVEL | MH_NOUNDEFS | MH_DYLDLINK | MH_PIE;
@@ -294,6 +315,9 @@ void OutputMachHeader::copy_buf(Context &ctx) {
   if (has_tlv(ctx))
     mhdr.flags |= MH_HAS_TLV_DESCRIPTORS;
 
+  if (ctx.output_type == MH_DYLIB)
+    mhdr.flags |= MH_NO_REEXPORTED_DYLIBS;
+
   write_vector(buf + sizeof(mhdr), cmds);
 }
 
@@ -659,7 +683,12 @@ i64 ExportEncoder::finish() {
     return a.name < b.name;
   });
 
-  root = construct_trie(entries, 0);
+  TrieNode trie = construct_trie(entries, 0);
+
+  if (trie.prefix.empty())
+    root = std::move(trie);
+  else
+    root.children.push_back(std::move(trie));
 
   i64 size = set_offset(root, 0);
   for (;;) {

test/macho/dylib.sh

@@ -6,7 +6,7 @@ echo -n "Testing $(basename -s .sh $0) ... "
 t=$(pwd)/../../out/test/macho/$(basename -s .sh $0)
 mkdir -p $t
 
-cat <<EOF | cc -shared -o $t/a.dylib -xc -
+cat <<EOF | cc -c -o $t/a.o -xc -
 #include <stdio.h>
 char world[] = "world";
 
@@ -15,7 +15,9 @@ char *hello() {
 }
 EOF
 
-cat <<EOF | cc -o $t/b.o -c -xc -
+clang -fuse-ld=$mold -o $t/b.dylib -shared $t/a.o
+
+cat <<EOF | cc -o $t/c.o -c -xc -
 #include <stdio.h>
 
 char *hello();
@@ -26,7 +28,7 @@ int main() {
 }
 EOF
 
-clang -fuse-ld=$mold -o $t/exe $t/b.o $t/a.dylib
+clang -fuse-ld=$mold -o $t/exe $t/c.o $t/b.dylib
 $t/exe | grep -q 'Hello world'
 
 echo OK