Support stepping through Darwin "branch islands" (#139301)

When an intra-module jump doesn't fit in the immediate branch slot, the
Darwin linker inserts "branch island" symbols, and emits code to jump
from branch island to branch island till it makes it to the actual
function.

The previous submissions failed because in that environment the linker
was putting the `foo.island` symbol at the same address as the `padding`
symbol we we emitting to make our faked-up large binary. This submission
jams a byte after the padding symbol so that the other symbols can't
overlap it.

Author: jimingham

lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderDarwin.cpp

@@ -26,6 +26,7 @@
 #include "lldb/Target/Thread.h"
 #include "lldb/Target/ThreadPlanCallFunction.h"
 #include "lldb/Target/ThreadPlanRunToAddress.h"
+#include "lldb/Target/ThreadPlanStepInstruction.h"
 #include "lldb/Utility/DataBuffer.h"
 #include "lldb/Utility/DataBufferHeap.h"
 #include "lldb/Utility/LLDBLog.h"
@@ -923,15 +924,15 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
   if (current_symbol != nullptr) {
     std::vector<Address> addresses;
 
+    ConstString current_name =
+        current_symbol->GetMangled().GetName(Mangled::ePreferMangled);
     if (current_symbol->IsTrampoline()) {
-      ConstString trampoline_name =
-          current_symbol->GetMangled().GetName(Mangled::ePreferMangled);
 
-      if (trampoline_name) {
+      if (current_name) {
         const ModuleList &images = target_sp->GetImages();
 
         SymbolContextList code_symbols;
-        images.FindSymbolsWithNameAndType(trampoline_name, eSymbolTypeCode,
+        images.FindSymbolsWithNameAndType(current_name, eSymbolTypeCode,
                                           code_symbols);
         for (const SymbolContext &context : code_symbols) {
           Address addr = context.GetFunctionOrSymbolAddress();
@@ -945,8 +946,8 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
         }
 
         SymbolContextList reexported_symbols;
-        images.FindSymbolsWithNameAndType(
-            trampoline_name, eSymbolTypeReExported, reexported_symbols);
+        images.FindSymbolsWithNameAndType(current_name, eSymbolTypeReExported,
+                                          reexported_symbols);
         for (const SymbolContext &context : reexported_symbols) {
           if (context.symbol) {
             Symbol *actual_symbol =
@@ -968,7 +969,7 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
         }
 
         SymbolContextList indirect_symbols;
-        images.FindSymbolsWithNameAndType(trampoline_name, eSymbolTypeResolver,
+        images.FindSymbolsWithNameAndType(current_name, eSymbolTypeResolver,
                                           indirect_symbols);
 
         for (const SymbolContext &context : indirect_symbols) {
@@ -1028,6 +1029,23 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
       thread_plan_sp = std::make_shared<ThreadPlanRunToAddress>(
           thread, load_addrs, stop_others);
     }
+    // One more case we have to consider is "branch islands".  These are regular
+    // TEXT symbols but their names end in .island plus maybe a .digit suffix.
+    // They are to allow arm64 code to branch further than the size of the
+    // address slot allows.  We just need to single-instruction step in that
+    // case.
+    static const char *g_branch_island_pattern = "\\.island\\.?[0-9]*$";
+    static RegularExpression g_branch_island_regex(g_branch_island_pattern);
+
+    bool is_branch_island = g_branch_island_regex.Execute(current_name);
+    if (!thread_plan_sp && is_branch_island) {
+      thread_plan_sp = std::make_shared<ThreadPlanStepInstruction>(
+          thread,
+          /* step_over= */ false, /* stop_others */ false, eVoteNoOpinion,
+          eVoteNoOpinion);
+      LLDB_LOG(log, "Stepping one instruction over branch island: '{0}'.",
+               current_name);
+    }
   } else {
     LLDB_LOGF(log, "Could not find symbol for step through.");
   }

lldb/test/API/macosx/branch-islands/Makefile

@@ -0,0 +1,16 @@
+C_SOURCES := main.c foo.c
+CFLAGS_EXTRAS := -std=c99
+
+include Makefile.rules
+
+a.out: main.o padding1.o padding2.o padding3.o padding4.o foo.o
+	${CC} ${LDFLAGS} foo.o padding1.o padding2.o padding3.o padding4.o main.o -o a.out
+
+%.o: $(SRCDIR)/%.s
+	${CC} -c $<
+
+#padding1.o: padding1.s
+#	${CC} -c $(SRCDIR)/padding1.s
+
+#padding2.o: padding2.s
+#	${CC} -c $(SRCDIR)/padding2.s

lldb/test/API/macosx/branch-islands/TestBranchIslands.py

@@ -0,0 +1,46 @@
+"""
+Make sure that we can step in across an arm64 branch island
+"""
+
+
+import lldb
+import lldbsuite.test.lldbutil as lldbutil
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.decorators import *
+
+
+class TestBranchIslandStepping(TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
+
+    @skipUnlessAppleSilicon
+    def test_step_in_branch_island(self):
+        """Make sure we can step in across a branch island"""
+        self.build()
+        self.main_source_file = lldb.SBFileSpec("main.c")
+        self.do_test()
+
+    def do_test(self):
+        (target, process, thread, bkpt) = lldbutil.run_to_source_breakpoint(
+            self, "Set a breakpoint here", self.main_source_file
+        )
+
+        # Make sure that we did manage to generate a branch island for foo:
+        syms = target.FindSymbols("foo.island", lldb.eSymbolTypeCode)
+        self.assertEqual(len(syms), 1, "We did generate an island for foo")
+        # There's a bug in the Xcode 15.2 linker, where it did not emit
+        # "function starts" entries for the branch island symbols, which
+        # causes lldb to set the range of the previous non-island symbol to
+        # encompass the range of the branch island symbols.  If we encounter
+        # that bug, then we won't successfully do the step in.  Test for
+        # that here - if the symbol doesn't round-trip from
+        # name->address->name then the rest of the test can't pass.
+        island_sym_ctx = syms[0]
+        sym_addr = island_sym_ctx.symbol.addr
+        resolved_name = sym_addr.symbol.name
+        if resolved_name != "foo.island":
+            self.skipTest("Encountered overlapping symbol linker bug")
+        thread.StepInto()
+        stop_frame = thread.frames[0]
+        self.assertIn("foo", stop_frame.name, "Stepped into foo")
+        var = stop_frame.FindVariable("a_variable_in_foo")
+        self.assertTrue(var.IsValid(), "Found the variable in foo")

lldb/test/API/macosx/branch-islands/foo.c

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+void foo() {
+  int a_variable_in_foo = 10;
+  printf("I am foo: %d.\n", a_variable_in_foo);
+}

lldb/test/API/macosx/branch-islands/main.c

@@ -0,0 +1,6 @@
+extern void foo();
+
+int main() {
+  foo(); // Set a breakpoint here
+  return 0;
+}

lldb/test/API/macosx/branch-islands/padding1.s

@@ -0,0 +1,5 @@
+.text
+_padding1:
+.p2align 2
+.byte 0x10        
+.space 120*1024*1024

lldb/test/API/macosx/branch-islands/padding2.s

@@ -0,0 +1,5 @@
+.text
+_padding2:
+.p2align 2
+.byte 0x10
+.space 120*1024*1024

lldb/test/API/macosx/branch-islands/padding3.s

@@ -0,0 +1,5 @@
+.text
+_padding3:
+.p2align 2
+.byte 0x10
+.space 120*1024*1024

lldb/test/API/macosx/branch-islands/padding4.s

@@ -0,0 +1,5 @@
+.text
+_padding4:
+.p2align 2
+.byte 0x10        
+.space 120*1024*1024