[libunwind] Support for leaf function unwinding.

Unwinding leaf function is useful in cases when the backtrace finds a
leaf function for example when it caused a signal.
This patch also add the support for the DW_CFA_undefined because it marks
the end of the frames.

Ryan Prichard provided code for the tests.

Reviewed By: #libunwind, mstorsjo

Differential Revision: https://reviews.llvm.org/D83573

libunwind/src/DwarfInstructions.hpp

@@ -93,7 +93,8 @@ typename A::pint_t DwarfInstructions<A, R>::getSavedRegister(
 
   case CFI_Parser<A>::kRegisterInRegister:
     return registers.getRegister((int)savedReg.value);
-
+  case CFI_Parser<A>::kRegisterUndefined:
+    return 0;
   case CFI_Parser<A>::kRegisterUnused:
   case CFI_Parser<A>::kRegisterOffsetFromCFA:
     // FIX ME
@@ -117,6 +118,7 @@ double DwarfInstructions<A, R>::getSavedFloatRegister(
 
   case CFI_Parser<A>::kRegisterIsExpression:
   case CFI_Parser<A>::kRegisterUnused:
+  case CFI_Parser<A>::kRegisterUndefined:
   case CFI_Parser<A>::kRegisterOffsetFromCFA:
   case CFI_Parser<A>::kRegisterInRegister:
     // FIX ME
@@ -140,6 +142,7 @@ v128 DwarfInstructions<A, R>::getSavedVectorRegister(
 
   case CFI_Parser<A>::kRegisterIsExpression:
   case CFI_Parser<A>::kRegisterUnused:
+  case CFI_Parser<A>::kRegisterUndefined:
   case CFI_Parser<A>::kRegisterOffsetFromCFA:
   case CFI_Parser<A>::kRegisterInRegister:
     // FIX ME
@@ -190,6 +193,10 @@ int DwarfInstructions<A, R>::stepWithDwarf(A &addressSpace, pint_t pc,
                                     prolog.savedRegisters[i]));
           else
             return UNW_EBADREG;
+        } else if (i == (int)cieInfo.returnAddressRegister) {
+            // Leaf function keeps the return address in register and there is no
+            // explicit intructions how to restore it.
+            returnAddress = registers.getRegister(cieInfo.returnAddressRegister);
         }
       }
 

libunwind/src/DwarfParser.hpp

@@ -69,6 +69,7 @@ public:
   };
   enum RegisterSavedWhere {
     kRegisterUnused,
+    kRegisterUndefined,
     kRegisterInCFA,
     kRegisterOffsetFromCFA,
     kRegisterInRegister,
@@ -503,7 +504,7 @@ bool CFI_Parser<A>::parseInstructions(A &addressSpace, pint_t instructions,
                 "malformed DW_CFA_undefined DWARF unwind, reg too big");
         return false;
       }
-      results->setRegisterLocation(reg, kRegisterUnused, initialState);
+      results->setRegisterLocation(reg, kRegisterUndefined, initialState);
       _LIBUNWIND_TRACE_DWARF("DW_CFA_undefined(reg=%" PRIu64 ")\n", reg);
       break;
     case DW_CFA_same_value:

libunwind/test/lit.site.cfg.in

@@ -44,6 +44,10 @@ config.test_source_root = os.path.join(config.libunwind_src_root, 'test')
 # Allow expanding substitutions that are based on other substitutions
 config.recursiveExpansionLimit = 10
 
+# Make symbols available in the tests.
+config.test_compiler_flags += " -funwind-tables "
+config.test_linker_flags += " -Wl,--export-dynamic "
+
 # Infer the test_exec_root from the build directory.
 config.test_exec_root = os.path.join(config.libunwind_obj_root, 'test')
 

libunwind/test/signal_unwind.pass.cpp

@@ -0,0 +1,44 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Ensure that the unwinder can cope with the signal handler.
+
+#include <assert.h>
+#include <dlfcn.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <unwind.h>
+
+_Unwind_Reason_Code frame_handler(struct _Unwind_Context* ctx, void* arg) {
+  (void)arg;
+  Dl_info info = { 0, 0, 0, 0 };
+  assert(dladdr((void*)_Unwind_GetIP(ctx), &info));
+
+  // Unwind util the main is reached, above frames deeped on the platfrom and architecture.
+  if(info.dli_sname && !strcmp("main", info.dli_sname)) {
+    _Exit(0);
+  }
+  return _URC_NO_REASON;
+}
+
+void signal_handler(int signum) {
+  (void)signum;
+  _Unwind_Backtrace(frame_handler, NULL);
+  _Exit(-1);
+}
+
+int main() {
+  signal(SIGUSR1, signal_handler);
+  kill(getpid(), SIGUSR1);
+  return -2;
+}

libunwind/test/unwind_leaffunction.pass.cpp

@@ -0,0 +1,50 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Ensure that leaf function can be unwund.
+
+#include <assert.h>
+#include <dlfcn.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <unwind.h>
+
+_Unwind_Reason_Code frame_handler(struct _Unwind_Context* ctx, void* arg) {
+  (void)arg;
+  Dl_info info = { 0, 0, 0, 0 };
+  assert(dladdr((void*)_Unwind_GetIP(ctx), &info));
+
+  // Unwind util the main is reached, above frames deeped on the platfrom and architecture.
+  if(info.dli_sname && !strcmp("main", info.dli_sname)) {
+    _Exit(0);
+  }
+  return _URC_NO_REASON;
+}
+
+void signal_handler(int signum) {
+  (void)signum;
+  _Unwind_Backtrace(frame_handler, NULL);
+  _Exit(-1);
+}
+
+int* faultyPointer = NULL;
+
+__attribute__((noinline)) void crashing_leaf_func(void) {
+  *faultyPointer = 0;
+}
+
+int main() {
+  signal(SIGSEGV, signal_handler);
+  crashing_leaf_func();
+  return -2;
+}