[lldb] Fix off-by-one error in the AppleObjCRuntimeV2 utility function

[JDevlieghere]

Summary

Fix an off-by-one error in the utility function used to extract the dynamic class info. This resulted in a buffer overflow in the inferior which interrupted our utility function.

[aprantl]

I'm curious now why there is both count and max_class_infos and if the second is workaround for this bug? Anyway, this *looks* plausible!

[JDevlieghere]

In D128377#3602856, @aprantl wrote:

I'm curious now why there is both count and max_class_infos and if the second is workaround for this bug? Anyway, this *looks* plausible!

count is the actual value reported by the runtime. max_class_infos is a monotonically increasing number that allows us to detect that new classes have been instantiated. The latter is also used to allocate enough space. The guarantee from the runtime is that count < max_class_infos.

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp

__lldb_apple_objc_v2_get_dynamic_class_info2(void *gdb_objc_realized_classes_ptr,
 
     ClassInfo *class_infos = (ClassInfo *)class_infos_ptr;
 
     uint32_t count = 0;
     Class* realized_class_list = objc_copyRealizedClassList_nolock(&count);
     DEBUG_PRINTF ("count = %u\n", count);
 
     uint32_t idx = 0;
-    for (uint32_t i=0; i<=count; ++i)
+    for (uint32_t i=0; i<count; ++i)
     {
         if (idx < max_class_infos)
         {
             Class isa = realized_class_list[i];
             const char *name_ptr = objc_debug_class_getNameRaw(isa);
             if (!name_ptr)
                 continue;
             const char *s = name_ptr;
__lldb_apple_objc_v2_get_dynamic_class_info3(void *gdb_objc_realized_classes_ptr,
 
     Class *realized_class_list = (Class*)class_buffer;
 
     uint32_t count = objc_getRealizedClassList_trylock(realized_class_list,
                                                        class_buffer_len);
     DEBUG_PRINTF ("count = %u\n", count);
 
     uint32_t idx = 0;
-    for (uint32_t i=0; i<=count; ++i)
+    for (uint32_t i=0; i<count; ++i)
     {
         if (idx < max_class_infos)
         {
             Class isa = realized_class_list[i];
             const char *name_ptr = objc_debug_class_getNameRaw(isa);
             if (!name_ptr) {
                class_getName(isa); // Realize name of lazy classes.
                name_ptr = objc_debug_class_getNameRaw(isa);