Fix issue with incorrect header struct size

Author: Alex B

lld/MachO/SyntheticSections.cpp

@@ -1990,8 +1990,7 @@ void ObjCMethListSection::setUp() {
   for (const ConcatInputSection *isec : inputs) {
     uint32_t structSizeAndFlags = 0, structCount = 0;
     readMethodListHeader(isec->data.data(), structSizeAndFlags, structCount);
-    uint32_t structSize = structSizeAndFlags & m_structSizeMask;
-
+    uint32_t originalStructSize = structSizeAndFlags & structSizeMask;
     // Method name is immediately after header
     uint32_t methodNameOff = methodListHeaderSize;
 
@@ -2008,7 +2007,7 @@ void ObjCMethListSection::setUp() {
         ObjCSelRefsHelper::makeSelRef(methname);
 
       // Jump to method name offset in next struct
-      methodNameOff += structSize;
+      methodNameOff += originalStructSize;
     }
   }
 }
@@ -2137,8 +2136,15 @@ ObjCMethListSection::writeRelativeMethodList(const ConcatInputSection *isec,
   // value flag
   uint32_t structSizeAndFlags = 0, structCount = 0;
   readMethodListHeader(isec->data.data(), structSizeAndFlags, structCount);
-  structSizeAndFlags |= relMethodHeaderFlag;
-  writeMethodListHeader(buf, structSizeAndFlags, structCount);
+  // Set the struct size for the relative method list
+  uint32_t relativeStructSizeAndFlags =
+      (relativeOffsetSize * pointersPerStruct) & structSizeMask;
+  // Carry over the old flags from the input struct
+  relativeStructSizeAndFlags |= structSizeAndFlags & structFlagsMask;
+  // Set the relative method list flag
+  relativeStructSizeAndFlags |= relMethodHeaderFlag;
+
+  writeMethodListHeader(buf, relativeStructSizeAndFlags, structCount);
 
   assert(methodListHeaderSize +
                  (structCount * pointersPerStruct * target->wordSize) ==

lld/MachO/SyntheticSections.h

@@ -720,7 +720,9 @@ class ObjCMethListSection final : public SyntheticSection {
   // The runtime identifies relative method lists via this magic value
   static constexpr uint32_t relMethodHeaderFlag = 0x80000000;
   // In the method list header, the first 2 bytes are the size of struct
-  static constexpr uint32_t m_structSizeMask = 0xFFFF;
+  static constexpr uint32_t structSizeMask = 0x0000FFFF;
+  // In the method list header, the last 2 bytes are the flags for the struct
+  static constexpr uint32_t structFlagsMask = 0xFFFF0000;
   // Relative method lists have 4 byte alignment as all data in the InputSection
   // is 4 byte
   static constexpr uint32_t relativeOffsetSize = sizeof(uint32_t);

lld/test/MachO/objc-relative-method-lists-simple.s

@@ -16,7 +16,7 @@
 CHK_REL:       Contents of (__DATA_CONST,__objc_classlist) section
 CHK_REL-NEXT:  _OBJC_CLASS_$_MyClass
 CHK_REL:       baseMethods
-CHK_REL-NEXT:  entsize 24 (relative)
+CHK_REL-NEXT:  entsize 12 (relative)
 CHK_REL-NEXT:  count 3
 CHK_REL-NEXT:   name 0x{{[0-9a-f]*}} (0x{{[0-9a-f]*}}) instance_method_00
 CHK_REL-NEXT:  types 0x{{[0-9a-f]*}} (0x{{[0-9a-f]*}}) v16@0:8
@@ -31,7 +31,7 @@ CHK_REL-NEXT:    imp 0x{{[0-9a-f]*}} (0x{{[0-9a-f]*}}) -[MyClass instance_method
 CHK_REL:       Meta Class
 CHK_REL-NEXT:  isa 0x{{[0-9a-f]*}} _OBJC_METACLASS_$_MyClass
 CHK_REL:       baseMethods 0x{{[0-9a-f]*}} (struct method_list_t *)
-CHK_REL-NEXT:  entsize 24 (relative)
+CHK_REL-NEXT:  entsize 12 (relative)
 CHK_REL-NEXT:  count 3
 CHK_REL-NEXT:   name 0x{{[0-9a-f]*}} (0x{{[0-9a-f]*}})  class_method_00
 CHK_REL-NEXT:  types 0x{{[0-9a-f]*}} (0x{{[0-9a-f]*}})  v16@0:8