[lld][MachO] Support for -interposable (#131813)

As discussed in #53680, add
support for ld64's -interposable flag on Apple platforms to lld.

Author: johnno1962

lld/MachO/Config.h

@@ -183,6 +183,7 @@ struct Configuration {
   bool deadStripDylibs = false;
   bool demangle = false;
   bool deadStrip = false;
+  bool interposable = false;
   bool errorForArchMismatch = false;
   bool ignoreAutoLink = false;
   // ld64 allows invalid auto link options as long as the link succeeds. LLD

lld/MachO/Driver.cpp

@@ -1676,6 +1676,7 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
 
   // Must be set before any InputSections and Symbols are created.
   config->deadStrip = args.hasArg(OPT_dead_strip);
+  config->interposable = args.hasArg(OPT_interposable);
 
   config->systemLibraryRoots = getSystemLibraryRoots(args);
   if (const char *path = getReproduceOption(args)) {

lld/MachO/Options.td

@@ -393,6 +393,9 @@ def grp_opts : OptionGroup<"opts">, HelpText<"OPTIMIZATIONS">;
 def dead_strip : Flag<["-"], "dead_strip">,
     HelpText<"Remove unreachable functions and data">,
     Group<grp_opts>;
+def interposable : Flag<["-"], "interposable">,
+    HelpText<"Indirects access to all exported symbols in an image">,
+    Group<grp_opts>;
 def order_file : Separate<["-"], "order_file">,
     MetaVarName<"<file>">,
     HelpText<"Layout functions and data according to specification in <file>">,
@@ -876,10 +879,6 @@ def setuid_safe : Flag<["-"], "setuid_safe">,
     HelpText<"Set the MH_SETUID_SAFE bit in the mach-o header">,
     Flags<[HelpHidden]>,
     Group<grp_rare>;
-def interposable : Flag<["-"], "interposable">,
-    HelpText<"Indirects access to all to exported symbols in a dylib">,
-    Flags<[HelpHidden]>,
-    Group<grp_rare>;
 def multi_module : Flag<["-"], "multi_module">,
     Alias<interposable>,
     HelpText<"Alias for -interposable">,

lld/MachO/SymbolTable.cpp

@@ -203,9 +203,9 @@ Defined *SymbolTable::addDefined(StringRef name, InputFile *file,
   }
 
   // With -flat_namespace, all extern symbols in dylibs are interposable.
-  // FIXME: Add support for `-interposable` (PR53680).
-  bool interposable = config->namespaceKind == NamespaceKind::flat &&
-                      config->outputType != MachO::MH_EXECUTE &&
+  bool interposable = ((config->namespaceKind == NamespaceKind::flat &&
+                        config->outputType != MachO::MH_EXECUTE) ||
+                       config->interposable) &&
                       !isPrivateExtern;
   Defined *defined = replaceSymbol<Defined>(
       s, name, file, isec, value, size, isWeakDef, /*isExternal=*/true,

lld/test/MachO/interposable.s

@@ -0,0 +1,34 @@
+# REQUIRES: x86
+
+# RUN: rm -rf %t; split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/2.s -o %t/2.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/3.s -o %t/3.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/main.s -o %t/main.o
+
+# RUN: %lld -arch x86_64 -interposable -lSystem -o %t/main %t/main.o %t/2.o %t/3.o
+# RUN: llvm-objdump --macho -d %t/main | FileCheck %s --check-prefix BUNDLE-OBJ
+BUNDLE-OBJ-LABEL: _my_user:
+BUNDLE-OBJ-NEXT:          callq   [[#%#x,]] ## symbol stub for: _my_friend
+
+#--- 2.s
+# my_lib: This contains the exported function
+.globl _my_friend
+_my_friend:
+  retq
+
+#--- 3.s
+# _my_user.s: This is the user/caller of the
+#             exported function
+.text
+_my_user:
+  callq _my_friend()
+  retq
+
+#--- main.s
+# main.s: dummy exec/main loads the exported function.
+# This is basically a way to say `my_user` should get
+# `my_func` from this executable.
+.globl _main
+.text
+ _main:
+  retq