Implement -interposable, -interposable_list

[int3]

https://reviews.llvm.org/D119294 should make this fairly straightforward now. I'm not particularly in a rush to implement these flags though since I don't have an immediate use for them.

[llvmbot]

@llvm/issue-subscribers-lld-macho

[johnno1962]

Hi @int3, I have a project that is dependent on ld64's -interposable flag https://github.com/johnno1962/InjectionIII and uses it to be able to late rebind symbols. If this is a straightforward change now is there any way I can help out to bring it about?

[johnno1962]

I've taken it as far as these changes which it seems nearly works. The entries are there in the binding table but it seems the function calls are not indirected at the call site and can not be redirected using fishhook. Am I missing something simple?

diff --git a/lld/MachO/Config.h b/lld/MachO/Config.h
index f8dcc84e4ee1..629fa2bfd0cb 100644
--- a/lld/MachO/Config.h
+++ b/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
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index 4f6c9b4ddc79..a335b5750b3d 100644
--- a/lld/MachO/Driver.cpp
+++ b/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)) {
diff --git a/lld/MachO/Options.td b/lld/MachO/Options.td
index 9001e85582c1..abc7697fde87 100644
--- a/lld/MachO/Options.td
+++ b/lld/MachO/Options.td
@@ -875,7 +875,6 @@ def setuid_safe : Flag<["-"], "setuid_safe">,
     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>,
diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
index a61e60a944fb..e85815fbcfee 100644
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -205,8 +205,8 @@ 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 &&
-                      !isPrivateExtern;
+                      (config->outputType != MachO::MH_EXECUTE &&
+                      !isPrivateExtern || config->interposable);
   Defined *defined = replaceSymbol<Defined>(
       s, name, file, isec, value, size, isWeakDef, /*isExternal=*/true,
       isPrivateExtern, /*includeInSymtab=*/true, isReferencedDynamically,

[smeenai]

Are you also building with -flat_namespace? Also, I don't think private_extern symbols should be made interposable. Something like this (untested and unformatted) might work:

  bool interposable = (config->namespaceKind == NamespaceKind::flat || config->interposable) &&
                      config->outputType != MachO::MH_EXECUTE &&
                      !isPrivateExtern;

[johnno1962]

Thanks for replying @smeenai. Your version works but I also need to be able to interpose symbols inside executables so I settled on the following which works in testing inside Xcode with InjectionIII:

diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
index a61e60a944fb..f6ac2019533c 100644
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -204,8 +204,9 @@ 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,

How to upstream these minor changes into the project along with the changes above? I'm not set up for llvm.

[johnno1962]

Hi @int3, you can close this issue now as #131813 should have implemented the -interposable flag.