Disable memtag sanitization for global fnptrs going into .ctors (#70186)

Looks like there's code out there that, instead of using
'__attribute__((constructor(x)))' to add constructor functions, they
just declare a global function pointer and use
'__attribute__((section('.ctors')))' instead.

Problem is, with memtag-globals, we pad the global function pointer to
be 16 bytes large. This of course means we have an 8-byte real function
pointer, then 8 bytes of zero padding, and this trips up the loader when
it processes this section.

Fixes #69939

Author: hctim

clang/test/CodeGen/memtag-globals-asm.cpp

@@ -259,3 +259,23 @@ int f(int x) {
   // CHECK-Q-DAG: ldr   {{.*}}, [[[REG_O2]]]
       function_int;
 }
+
+typedef void (*func_t)(void);
+#define CONSTRUCTOR(section_name) \
+  __attribute__((used)) __attribute__((section(section_name)))
+
+__attribute__((constructor(0))) void func_constructor() {}
+CONSTRUCTOR(".init") func_t func_init = func_constructor;
+CONSTRUCTOR(".fini") func_t func_fini = func_constructor;
+CONSTRUCTOR(".ctors") func_t func_ctors = func_constructor;
+CONSTRUCTOR(".dtors") func_t func_dtors = func_constructor;
+CONSTRUCTOR(".init_array") func_t func_init_array = func_constructor;
+CONSTRUCTOR(".fini_array") func_t func_fini_array = func_constructor;
+
+// CHECK-NOT: .memtag func_constructor
+// CHECK-NOT: .memtag func_init
+// CHECK-NOT: .memtag func_fini
+// CHECK-NOT: .memtag func_ctors
+// CHECK-NOT: .memtag func_dtors
+// CHECK-NOT: .memtag func_init_array
+// CHECK-NOT: .memtag func_fini_array

llvm/lib/Target/AArch64/AArch64GlobalsTagging.cpp

@@ -43,6 +43,18 @@ static bool shouldTagGlobal(GlobalVariable &G) {
     return false;
   }
 
+  // Don't instrument function pointers that are going into various init arrays
+  // via `__attribute__((section(<foo>)))`:
+  // https://github.com/llvm/llvm-project/issues/69939
+  if (G.hasSection() &&
+      (G.getSection() == ".init" || G.getSection() == ".fini" ||
+       G.getSection() == ".init_array" || G.getSection() == ".fini_array" ||
+       G.getSection() == ".ctors" || G.getSection() == ".dtors")) {
+    Meta.Memtag = false;
+    G.setSanitizerMetadata(Meta);
+    return false;
+  }
+
   return true;
 }