[lld-macho] Remove symbols to `__mod_init_func` with `-init_offsets` (#97156)

When `-fixup_chains`/`-init_offsets` is used, a different section,
`__init_offsets` is synthesized from `__mod_init_func`. If there are any
symbols defined inside `__mod_init_func`, they are added to the symbol
table unconditionally while processing the input files. Later, when
querying these symbols' addresses (when constructing the symtab or
exports trie), we crash with a null deref, as there is no output section
assigned to them.

Just making the symbols point to `__init_offsets` is a bad idea, as the
new section stores 32-bit integers instead of 64-bit pointers; accessing
the symbols would not do what the programmer intended. We should
entirely omit them from the output. This is what ld64 and ld-prime do.

This patch uses the same mechanism as dead-stripping to mark these
symbols as not needed in the output. There might be nicer fixes than the
workaround, this is discussed in #97155.

Fixes https://github.com/llvm/llvm-project/pull/79894#issuecomment-1944092892
Fixes #94716

1 files changed, 11 insertions, 0 deletions

diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index 9dddabcf3680..83c92d214de3 100644
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1393,6 +1393,12 @@ static void handleExplicitExports() {
   }
 }
 
+static void eraseInitializerSymbols() {
+  for (ConcatInputSection *isec : in.initOffsets->inputs())
+    for (Defined *sym : isec->symbols)
+      sym->used = false;
+}
+
 namespace lld {
 namespace macho {
 bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
@@ -1971,6 +1977,11 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
     if (config->deadStrip)
       markLive();
 
+    // Ensure that no symbols point inside __mod_init_func sections if they are
+    // removed due to -init_offsets. This must run after dead stripping.
+    if (config->emitInitOffsets)
+      eraseInitializerSymbols();
+
     // Categories are not subject to dead-strip. The __objc_catlist section is
     // marked as NO_DEAD_STRIP and that propagates into all category data.
     if (args.hasArg(OPT_check_category_conflicts))