Breakpad: Support DWARF CFI-driven stack walking on ARM.

This patch allows the Breakpad minidump processor to use data from
STACK CFI records to generate stack traces for the ARM processor.

In the symbol dumper, we need a table mapping DWARF CFI register
numbers to their names: STACK CFI records refer to registers by name.

In the processor, we expand StackwalkerARM::GetCallerFrame to see if
there are STACK CFI records covering the callee, and then use those to
recover the caller's register values.

There's no good reason the ARM walker couldn't use the SimpleCFIWalker
interface declared in cfi_frame_info.h. Unfortunately, that interface
assumes that one can map register names to member pointers of the raw
context type, while MDRawContextARM uses an array to hold the
registers' values: C++ pointer-to-member types can't refer to elements
of member arrays. So we have to write out SimpleCFIWalker::FindCallerRegisters
in StackwalkerARM::GetCallerFrame.

We define enum MDARMRegisterNumbers in minidump_cpu_arm.h, for
convenience in referring to certain ARM registers with dedicated
purposes, like the stack pointer and the PC.

We define validity flags in StackFrameARM for all the registers, since
CFI could theoretically recover any of them. In the same vein, we
expand minidump_stackwalk.cc to print the values of all valid
callee-saves registers in the context --- and use the proper names for
special-purpose registers.

We provide unit tests that give full code and branch coverage (with
minor exceptions). We add a testing interface to StackwalkerARM that
allows us to create context frames that lack some register values.

a=jimblandy, r=mmentovai


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@553 4c0a9323-5329-0410-9bdc-e9ce6186880e

7 files changed, 641 insertions, 34 deletions

diff --git a/src/common/linux/dump_symbols.cc b/src/common/linux/dump_symbols.cc
index 3df31372..3d3e75e0 100644
--- a/src/common/linux/dump_symbols.cc
+++ b/src/common/linux/dump_symbols.cc
@@ -255,16 +255,19 @@ static bool DwarfCFIRegisterNames(const ElfW(Ehdr) *elf_header,
     NULL
   };
 
+  static const char *const arm_names[] = {
+    "r0",  "r1",  "r2",  "r3",  "r4",  "r5",  "r6",  "r7",
+    "r8",  "r9",  "r10", "r11", "r12", "sp",  "lr",  "pc",
+    "f0",  "f1",  "f2",  "f3",  "f4",  "f5",  "f6",  "f7",
+    "fps", "cpsr",
+    NULL
+  };
+
   const char * const *name_table;
   switch (elf_header->e_machine) {
-    case EM_386:
-      name_table = i386_names;
-      break;
-
-    case EM_X86_64:
-      name_table = x86_64_names;
-      break;
-
+    case EM_386:    name_table = i386_names;   break;
+    case EM_ARM:    name_table = arm_names;    break;
+    case EM_X86_64: name_table = x86_64_names; break;
     default:
       return false;
   }
diff --git a/src/google_breakpad/common/minidump_cpu_arm.h b/src/google_breakpad/common/minidump_cpu_arm.h
index bd3d934f..14d81460 100644
--- a/src/google_breakpad/common/minidump_cpu_arm.h
+++ b/src/google_breakpad/common/minidump_cpu_arm.h
@@ -116,6 +116,16 @@ typedef struct {
 
 } MDRawContextARM;
 
+/* Indices into iregs for registers with a dedicated or conventional 
+ * purpose.
+ */
+enum MDARMRegisterNumbers {
+  MD_CONTEXT_ARM_REG_FP = 11,
+  MD_CONTEXT_ARM_REG_SP = 13,
+  MD_CONTEXT_ARM_REG_LR = 14,
+  MD_CONTEXT_ARM_REG_PC = 15
+};
+
 /* For (MDRawContextARM).context_flags.  These values indicate the type of
  * context stored in the structure. */
 #define MD_CONTEXT_ARM_INTEGER           (MD_CONTEXT_ARM | 0x00000002)
diff --git a/src/google_breakpad/processor/stack_frame_cpu.h b/src/google_breakpad/processor/stack_frame_cpu.h
index 8b88fdc6..c5bddffb 100644
--- a/src/google_breakpad/processor/stack_frame_cpu.h
+++ b/src/google_breakpad/processor/stack_frame_cpu.h
@@ -189,29 +189,54 @@ struct StackFrameSPARC : public StackFrame {
 };
 
 struct StackFrameARM : public StackFrame {
-  // ContextValidity should eventually contain entries for the validity of
-  // other nonvolatile (callee-save) registers as in
-  // StackFrameX86::ContextValidity. I suspect this list is sufficient
-  // for arm stackwalking.
+  // A flag for each register we might know.
   enum ContextValidity {
     CONTEXT_VALID_NONE = 0,
-    CONTEXT_VALID_R13 = 1 << 0,
-    CONTEXT_VALID_R14 = 1 << 1,
-    CONTEXT_VALID_R15 = 1 << 2,
-    CONTEXT_VALID_ALL = -1
+    CONTEXT_VALID_R0   = 1 << 0,
+    CONTEXT_VALID_R1   = 1 << 1,
+    CONTEXT_VALID_R2   = 1 << 2,
+    CONTEXT_VALID_R3   = 1 << 3,
+    CONTEXT_VALID_R4   = 1 << 4,
+    CONTEXT_VALID_R5   = 1 << 5,
+    CONTEXT_VALID_R6   = 1 << 6,
+    CONTEXT_VALID_R7   = 1 << 7,
+    CONTEXT_VALID_R8   = 1 << 8,
+    CONTEXT_VALID_R9   = 1 << 9,
+    CONTEXT_VALID_R10  = 1 << 10,
+    CONTEXT_VALID_R11  = 1 << 11,
+    CONTEXT_VALID_R12  = 1 << 12,
+    CONTEXT_VALID_R13  = 1 << 13,
+    CONTEXT_VALID_R14  = 1 << 14,
+    CONTEXT_VALID_R15  = 1 << 15,
+    CONTEXT_VALID_ALL  = ~CONTEXT_VALID_NONE,
+
+    // Aliases for registers with dedicated or conventional roles.
+    CONTEXT_VALID_FP   = CONTEXT_VALID_R11,
+    CONTEXT_VALID_SP   = CONTEXT_VALID_R13,
+    CONTEXT_VALID_LR   = CONTEXT_VALID_R14,
+    CONTEXT_VALID_PC   = CONTEXT_VALID_R15
   };
 
   StackFrameARM() : context(), context_validity(CONTEXT_VALID_NONE) {}
 
+  // Return the ContextValidity flag for register rN.
+  static ContextValidity RegisterValidFlag(int n) {
+    return ContextValidity(1 << n);
+  } 
+
   // Register state.  This is only fully valid for the topmost frame in a
   // stack.  In other frames, the values of nonvolatile registers may be
   // present, given sufficient debugging information.  Refer to
   // context_validity.
   MDRawContextARM context;
 
-  // context_validity is actually ContextValidity, but int is used because
-  // the OR operator doesn't work well with enumerated types.  This indicates
-  // which fields in context are valid.
+  // For each register in context whose value has been recovered, we set
+  // the corresponding CONTEXT_VALID_ bit in context_validity.
+  //
+  // context_validity's type should actually be ContextValidity, but
+  // we use int instead because the bitwise inclusive or operator
+  // yields an int when applied to enum values, and C++ doesn't
+  // silently convert from ints to enums.
   int context_validity;
 };
 
diff --git a/src/processor/minidump_stackwalk.cc b/src/processor/minidump_stackwalk.cc
index 1d0a913a..9fe3b547 100644
--- a/src/processor/minidump_stackwalk.cc
+++ b/src/processor/minidump_stackwalk.cc
@@ -216,12 +216,31 @@ static void PrintStack(const CallStack *stack, const string &cpu) {
       const StackFrameARM *frame_arm =
           reinterpret_cast<const StackFrameARM*>(frame);
 
-      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R13)
-        sequence = PrintRegister("r13", frame_arm->context.iregs[13], sequence);
-      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R14)
-        sequence = PrintRegister("r14", frame_arm->context.iregs[14], sequence);
-      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R15)
-        sequence = PrintRegister("r15", frame_arm->context.iregs[15], sequence);
+      // General-purpose callee-saves registers.
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R4)
+        sequence = PrintRegister("r4", frame_arm->context.iregs[4], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R5)
+        sequence = PrintRegister("r5", frame_arm->context.iregs[5], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R6)
+        sequence = PrintRegister("r6", frame_arm->context.iregs[6], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R7)
+        sequence = PrintRegister("r7", frame_arm->context.iregs[7], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R8)
+        sequence = PrintRegister("r8", frame_arm->context.iregs[8], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R9)
+        sequence = PrintRegister("r9", frame_arm->context.iregs[9], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_R10)
+        sequence = PrintRegister("r10", frame_arm->context.iregs[10], sequence);
+
+      // Registers with a dedicated or conventional purpose.
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_FP)
+        sequence = PrintRegister("fp", frame_arm->context.iregs[11], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_SP)
+        sequence = PrintRegister("sp", frame_arm->context.iregs[13], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_LR)
+        sequence = PrintRegister("lr", frame_arm->context.iregs[14], sequence);
+      if (frame_arm->context_validity & StackFrameARM::CONTEXT_VALID_PC)
+        sequence = PrintRegister("pc", frame_arm->context.iregs[15], sequence);
     }
     printf("\n");
   }
diff --git a/src/processor/stackwalker_arm.cc b/src/processor/stackwalker_arm.cc
index 4b15593d..0af27a32 100644
--- a/src/processor/stackwalker_arm.cc
+++ b/src/processor/stackwalker_arm.cc
@@ -31,14 +31,17 @@
 //
 // See stackwalker_arm.h for documentation.
 //
-// Author: Mark Mentovai, Ted Mielczarek
+// Author: Mark Mentovai, Ted Mielczarek, Jim Blandy
 
 
-#include "processor/stackwalker_arm.h"
 #include "google_breakpad/processor/call_stack.h"
 #include "google_breakpad/processor/memory_region.h"
+#include "google_breakpad/processor/source_line_resolver_interface.h"
 #include "google_breakpad/processor/stack_frame_cpu.h"
+#include "processor/cfi_frame_info.h"
 #include "processor/logging.h"
+#include "processor/scoped_ptr.h"
+#include "processor/stackwalker_arm.h"
 
 namespace google_breakpad {
 
@@ -50,8 +53,8 @@ StackwalkerARM::StackwalkerARM(const SystemInfo *system_info,
                                SymbolSupplier *supplier,
                                SourceLineResolverInterface *resolver)
     : Stackwalker(system_info, memory, modules, supplier, resolver),
-      context_(context) {
-}
+      context_(context), 
+      context_frame_validity_(StackFrameARM::CONTEXT_VALID_ALL) { }
 
 
 StackFrame* StackwalkerARM::GetContextFrame() {
@@ -65,7 +68,7 @@ StackFrame* StackwalkerARM::GetContextFrame() {
   // The instruction pointer is stored directly in a register (r15), so pull it
   // straight out of the CPU context structure.
   frame->context = *context_;
-  frame->context_validity = StackFrameARM::CONTEXT_VALID_ALL;
+  frame->context_validity = context_frame_validity_;
   frame->instruction = frame->context.iregs[15];
 
   return frame;
@@ -78,9 +81,104 @@ StackFrame* StackwalkerARM::GetCallerFrame(const CallStack *stack) {
     return NULL;
   }
 
-  // TODO: Can't actually walk the stack on ARM without the CFI data.
-  // Implement this when the CFI symbol dumper changes have landed.
-  return NULL;
+  const vector<StackFrame *> &frames = *stack->frames();
+  StackFrameARM *last_frame = static_cast<StackFrameARM *>(frames.back());
+
+  // See if we have DWARF call frame information covering this address.
+  scoped_ptr<CFIFrameInfo> cfi_frame_info(resolver_
+                                          ->FindCFIFrameInfo(last_frame));
+  if (cfi_frame_info == NULL)
+    // Unfortunately, CFI is our only option on the ARM for now. If we
+    // add a second strategy, we should put each one in its own function.
+    return NULL;
+
+  static const char *register_names[] = {
+    "r0",  "r1",  "r2",  "r3",  "r4",  "r5",  "r6",  "r7",
+    "r8",  "r9",  "r10", "r11", "r12", "sp",  "lr",  "pc",
+    "f0",  "f1",  "f2",  "f3",  "f4",  "f5",  "f6",  "f7",
+    "fps", "cpsr",
+    NULL
+  };
+
+  // Populate a dictionary with the valid register values in last_frame.
+  CFIFrameInfo::RegisterValueMap<u_int32_t> callee_registers;
+  for (int i = 0; register_names[i]; i++)
+    if (last_frame->context_validity & StackFrameARM::RegisterValidFlag(i))
+      callee_registers[register_names[i]] = last_frame->context.iregs[i];
+
+  // Use the STACK CFI data to recover the caller's register values.
+  CFIFrameInfo::RegisterValueMap<u_int32_t> caller_registers;
+  if (!cfi_frame_info->FindCallerRegs(callee_registers, *memory_,
+                                      &caller_registers))
+    return NULL;
+
+  // Construct a new stack frame given the values the CFI recovered.
+  scoped_ptr<StackFrameARM> frame(new StackFrameARM());
+  for (int i = 0; register_names[i]; i++) {
+    CFIFrameInfo::RegisterValueMap<u_int32_t>::iterator entry =
+      caller_registers.find(register_names[i]);
+    if (entry != caller_registers.end()) {
+      // We recovered the value of this register; fill the context with the
+      // value from caller_registers.
+      frame->context_validity |= StackFrameARM::RegisterValidFlag(i);
+      frame->context.iregs[i] = entry->second;
+    } else if (4 <= i && i <= 11 && (last_frame->context_validity &
+                                     StackFrameARM::RegisterValidFlag(i))) {
+      // If the STACK CFI data doesn't mention some callee-saves register, and
+      // it is valid in the callee, assume the callee has not yet changed it.
+      // Registers r4 through r11 are callee-saves, according to the Procedure
+      // Call Standard for the ARM Architecture, which the Linux ABI follows.
+      frame->context_validity |= StackFrameARM::RegisterValidFlag(i);
+      frame->context.iregs[i] = last_frame->context.iregs[i];
+    }
+  }
+  // If the CFI doesn't recover the PC explicitly, then use .ra.
+  if (! (frame->context_validity & StackFrameARM::CONTEXT_VALID_PC)) {
+    CFIFrameInfo::RegisterValueMap<u_int32_t>::iterator entry =
+      caller_registers.find(".ra");
+    if (entry != caller_registers.end()) {
+      frame->context_validity |= StackFrameARM::CONTEXT_VALID_PC;
+      frame->context.iregs[MD_CONTEXT_ARM_REG_PC] = entry->second;
+    }
+  }
+  // If the CFI doesn't recover the SP explicitly, then use .cfa.
+  if (! (frame->context_validity & StackFrameARM::CONTEXT_VALID_SP)) {
+    CFIFrameInfo::RegisterValueMap<u_int32_t>::iterator entry =
+      caller_registers.find(".cfa");
+    if (entry != caller_registers.end()) {
+      frame->context_validity |= StackFrameARM::CONTEXT_VALID_SP;
+      frame->context.iregs[MD_CONTEXT_ARM_REG_SP] = entry->second;
+    }
+  }
+
+  // If we didn't recover the PC and the SP, then the frame isn't very useful.
+  static const int essentials = (StackFrameARM::CONTEXT_VALID_SP
+                                 | StackFrameARM::CONTEXT_VALID_PC);
+  if ((frame->context_validity & essentials) != essentials)
+    return NULL;
+
+  // An instruction address of zero marks the end of the stack.
+  if (frame->context.iregs[MD_CONTEXT_ARM_REG_PC] == 0)
+    return NULL;
+
+  // If the new stack pointer is at a lower address than the old, then
+  // that's clearly incorrect. Treat this as end-of-stack to enforce
+  // progress and avoid infinite loops.
+  if (frame->context.iregs[MD_CONTEXT_ARM_REG_SP]
+      < last_frame->context.iregs[MD_CONTEXT_ARM_REG_SP])
+    return NULL;
+
+  // The new frame's context's PC is the return address, which is one
+  // instruction past the instruction that caused us to arrive at the
+  // callee. Set new_frame->instruction to one less than the PC. This won't
+  // reference the beginning of the call instruction, but it's at least
+  // within it, which is sufficient to get the source line information to
+  // match up with the line that contains the function call. Callers that
+  // require the exact return address value may access
+  // frame->context.iregs[MD_CONTEXT_ARM_REG_PC].
+  frame->instruction = frame->context.iregs[MD_CONTEXT_ARM_REG_PC] - 1;
+
+  return frame.release();
 }
 
 
diff --git a/src/processor/stackwalker_arm.h b/src/processor/stackwalker_arm.h
index 5dfde5ab..3768cc2c 100644
--- a/src/processor/stackwalker_arm.h
+++ b/src/processor/stackwalker_arm.h
@@ -1,3 +1,5 @@
+// -*- mode: C++ -*-
+
 // Copyright (c) 2010 Google Inc.
 // All rights reserved.
 //
@@ -60,15 +62,25 @@ class StackwalkerARM : public Stackwalker {
                  SymbolSupplier *supplier,
                  SourceLineResolverInterface *resolver);
 
+  // Change the context validity mask of the frame returned by
+  // GetContextFrame to VALID. This is only for use by unit tests; the
+  // default behavior is correct for all application code.
+  void SetContextFrameValidity(int valid) { context_frame_validity_ = valid; }
+
  private:
   // Implementation of Stackwalker, using arm context and stack conventions.
   // TODO: currently stubbed out, needs CFI symbol dumper support
   virtual StackFrame* GetContextFrame();
   virtual StackFrame* GetCallerFrame(const CallStack *stack);
 
-  // Stores the CPU context corresponding to the innermost stack frame to
+  // Stores the CPU context corresponding to the youngest stack frame, to
   // be returned by GetContextFrame.
   const MDRawContextARM *context_;
+
+  // Validity mask for youngest stack frame. This is always
+  // CONTEXT_VALID_ALL in real use; it is only changeable for the sake of
+  // unit tests.
+  int context_frame_validity_;
 };
 
 
diff --git a/src/processor/stackwalker_arm_unittest.cc b/src/processor/stackwalker_arm_unittest.cc
new file mode 100644
index 00000000..37103241
--- /dev/null
+++ b/src/processor/stackwalker_arm_unittest.cc
@@ -0,0 +1,440 @@
+// Copyright (c) 2010, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Original author: Jim Blandy <jimb@mozilla.com> <jimb@red-bean.com>
+
+// stackwalker_arm_unittest.cc: Unit tests for StackwalkerARM class.
+
+#include <string>
+#include <string.h>
+#include <vector>
+
+#include "breakpad_googletest_includes.h"
+#include "google_breakpad/common/minidump_format.h"
+#include "google_breakpad/processor/basic_source_line_resolver.h"
+#include "google_breakpad/processor/call_stack.h"
+#include "google_breakpad/processor/source_line_resolver_interface.h"
+#include "google_breakpad/processor/stack_frame_cpu.h"
+#include "processor/stackwalker_unittest_utils.h"
+#include "processor/stackwalker_arm.h"
+#include "processor/test_assembler.h"
+#include "processor/windows_frame_info.h"
+
+using google_breakpad::BasicSourceLineResolver;
+using google_breakpad::CallStack;
+using google_breakpad::StackFrame;
+using google_breakpad::StackFrameARM;
+using google_breakpad::StackwalkerARM;
+using google_breakpad::SystemInfo;
+using google_breakpad::WindowsFrameInfo;
+using google_breakpad::TestAssembler::kLittleEndian;
+using google_breakpad::TestAssembler::Label;
+using google_breakpad::TestAssembler::Section;
+using std::string;
+using std::vector;
+using testing::_;
+using testing::Return;
+using testing::SetArgumentPointee;
+using testing::Test;
+
+class StackwalkerARMFixture {
+ public:
+  StackwalkerARMFixture()
+    : stack_section(kLittleEndian),
+      // Give the two modules reasonable standard locations and names
+      // for tests to play with.
+      module1(0x40000000, 0x10000, "module1", "version1"),
+      module2(0x50000000, 0x10000, "module2", "version2") {
+    // Identify the system as a Linux system.
+    system_info.os = "Linux";
+    system_info.os_short = "linux";
+    system_info.os_version = "Lugubrious Labrador";
+    system_info.cpu = "arm";
+    system_info.cpu_info = "";
+
+    // Put distinctive values in the raw CPU context.
+    BrandContext(&raw_context);
+
+    // Create some modules with some stock debugging information.
+    modules.Add(&module1);
+    modules.Add(&module2);
+
+    // By default, none of the modules have symbol info; call
+    // SetModuleSymbols to override this.
+    EXPECT_CALL(supplier, GetSymbolFile(_, _, _, _))
+      .WillRepeatedly(Return(MockSymbolSupplier::NOT_FOUND));
+  }
+
+  // Set the Breakpad symbol information that supplier should return for
+  // MODULE to INFO.
+  void SetModuleSymbols(MockCodeModule *module, const string &info) {
+    EXPECT_CALL(supplier, GetSymbolFile(module, &system_info, _, _))
+      .WillRepeatedly(DoAll(SetArgumentPointee<3>(info),
+                            Return(MockSymbolSupplier::FOUND)));
+  }
+
+  // Populate stack_region with the contents of stack_section. Use
+  // stack_section.start() as the region's starting address.
+  void RegionFromSection() {
+    string contents;
+    ASSERT_TRUE(stack_section.GetContents(&contents));
+    stack_region.Init(stack_section.start().Value(), contents);
+  }
+
+  // Fill RAW_CONTEXT with pseudo-random data, for round-trip checking.
+  void BrandContext(MDRawContextARM *raw_context) {
+    u_int8_t x = 173;
+    for (size_t i = 0; i < sizeof(*raw_context); i++)
+      reinterpret_cast<u_int8_t *>(raw_context)[i] = (x += 17);
+  }
+  
+  SystemInfo system_info;
+  MDRawContextARM raw_context;
+  Section stack_section;
+  MockMemoryRegion stack_region;
+  MockCodeModule module1;
+  MockCodeModule module2;
+  MockCodeModules modules;
+  MockSymbolSupplier supplier;
+  BasicSourceLineResolver resolver;
+  CallStack call_stack;
+  const vector<StackFrame *> *frames;
+};
+
+class GetContextFrame: public StackwalkerARMFixture, public Test { };
+
+TEST_F(GetContextFrame, Simple) {
+  // Since we have no call frame information, and all unwinding
+  // requires call frame information, the stack walk will end after
+  // the first frame.
+  StackwalkerARM walker(&system_info, &raw_context, &stack_region, &modules,
+                        &supplier, &resolver);
+  ASSERT_TRUE(walker.Walk(&call_stack));
+  frames = call_stack.frames();
+  ASSERT_EQ(1U, frames->size());
+  StackFrameARM *frame = static_cast<StackFrameARM *>(frames->at(0));
+  // Check that the values from the original raw context made it
+  // through to the context in the stack frame.
+  EXPECT_TRUE(memcmp(&raw_context, &frame->context, sizeof(raw_context)) == 0);
+}
+
+struct CFIFixture: public StackwalkerARMFixture {
+  CFIFixture() {
+    // Provide a bunch of STACK CFI records; we'll walk to the caller
+    // from every point in this series, expecting to find the same set
+    // of register values.
+    SetModuleSymbols(&module1,
+                     // The youngest frame's function.
+                     "FUNC 4000 1000 10 enchiridion\n"
+                     // Initially, nothing has been pushed on the stack,
+                     // and the return address is still in the link register.
+                     "STACK CFI INIT 4000 100 .cfa: sp .ra: lr\n"
+                     // Push r4, the frame pointer, and the link register.
+                     "STACK CFI 4001 .cfa: sp 12 + r4: .cfa 12 - ^"
+                     " r11: .cfa 8 - ^ .ra: .cfa 4 - ^\n"
+                     // Save r4..r7 in r0..r3: verify that we populate
+                     // the youngest frame with all the values we have.
+                     "STACK CFI 4002 r4: r0 r5: r1 r6: r2 r7: r3\n"
+                     // Restore r4..r7. Save the non-callee-saves register r1.
+                     "STACK CFI 4003 .cfa: sp 16 + r1: .cfa 16 - ^"
+                     " r4: r4 r5: r5 r6: r6 r7: r7\n"
+                     // Move the .cfa back four bytes, to point at the return
+                     // address, and restore the sp explicitly.
+                     "STACK CFI 4005 .cfa: sp 12 + r1: .cfa 12 - ^"
+                     " r11: .cfa 4 - ^ .ra: .cfa ^ sp: .cfa 4 +\n"
+                     // Recover the PC explicitly from a new stack slot;
+                     // provide garbage for the .ra.
+                     "STACK CFI 4006 .cfa: sp 16 + pc: .cfa 16 - ^\n"
+
+                     // The calling function.
+                     "FUNC 5000 1000 10 epictetus\n"
+                     // Mark it as end of stack.
+                     "STACK CFI INIT 5000 1000 .cfa: 0 .ra: 0\n"
+
+                     // A function whose CFI makes the stack pointer
+                     // go backwards.
+                     "FUNC 6000 1000 20 palinal\n"
+                     "STACK CFI INIT 6000 1000 .cfa: sp 4 - .ra: lr\n"
+
+                     // A function with CFI expressions that can't be
+                     // evaluated.
+                     "FUNC 7000 1000 20 rhetorical\n"
+                     "STACK CFI INIT 7000 1000 .cfa: moot .ra: ambiguous\n");
+
+    // Provide some distinctive values for the caller's registers.
+    expected.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40005510;
+    expected.iregs[MD_CONTEXT_ARM_REG_SP] = 0x80000000;
+    expected.iregs[4] = 0xb5d55e68;
+    expected.iregs[5] = 0xebd134f3;
+    expected.iregs[6] = 0xa31e74bc;
+    expected.iregs[7] = 0x2dcb16b3;
+    expected.iregs[8] = 0x2ada2137;
+    expected.iregs[9] = 0xbbbb557d;
+    expected.iregs[10] = 0x48bf8ca7;
+    expected.iregs[MD_CONTEXT_ARM_REG_FP] = 0x8112e110;
+
+    // Expect CFI to recover all callee-saves registers. Since CFI is the
+    // only stack frame construction technique we have, aside from the
+    // context frame itself, there's no way for us to have a set of valid
+    // registers smaller than this.
+    expected_validity = (StackFrameARM::CONTEXT_VALID_PC |
+                         StackFrameARM::CONTEXT_VALID_SP |
+                         StackFrameARM::CONTEXT_VALID_R4 |
+                         StackFrameARM::CONTEXT_VALID_R5 |
+                         StackFrameARM::CONTEXT_VALID_R6 |
+                         StackFrameARM::CONTEXT_VALID_R7 |
+                         StackFrameARM::CONTEXT_VALID_R8 |
+                         StackFrameARM::CONTEXT_VALID_R9 |
+                         StackFrameARM::CONTEXT_VALID_R10 |
+                         StackFrameARM::CONTEXT_VALID_FP);
+
+    // By default, context frames provide all registers, as normal.
+    context_frame_validity = StackFrameARM::CONTEXT_VALID_ALL;
+
+    // By default, registers are unchanged.
+    raw_context = expected;
+  }
+
+  // Walk the stack, using stack_section as the contents of the stack
+  // and raw_context as the current register values. (Set the stack
+  // pointer to the stack's starting address.) Expect two stack
+  // frames; in the older frame, expect the callee-saves registers to
+  // have values matching those in 'expected'.
+  void CheckWalk() {
+    RegionFromSection();
+    raw_context.iregs[MD_CONTEXT_ARM_REG_SP] = stack_section.start().Value();
+
+    StackwalkerARM walker(&system_info, &raw_context, &stack_region, &modules,
+                          &supplier, &resolver);
+    walker.SetContextFrameValidity(context_frame_validity);
+    ASSERT_TRUE(walker.Walk(&call_stack));
+    frames = call_stack.frames();
+    ASSERT_EQ(2U, frames->size());
+
+    StackFrameARM *frame0 = static_cast<StackFrameARM *>(frames->at(0));
+    ASSERT_EQ(context_frame_validity, frame0->context_validity);
+    EXPECT_EQ("enchiridion", frame0->function_name);
+    EXPECT_EQ(0x40004000U, frame0->function_base);
+
+    StackFrameARM *frame1 = static_cast<StackFrameARM *>(frames->at(1));
+    ASSERT_EQ(expected_validity, frame1->context_validity);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R1)
+      EXPECT_EQ(expected.iregs[1], frame1->context.iregs[1]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R4)
+      EXPECT_EQ(expected.iregs[4], frame1->context.iregs[4]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R5)
+      EXPECT_EQ(expected.iregs[5], frame1->context.iregs[5]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R6)
+      EXPECT_EQ(expected.iregs[6], frame1->context.iregs[6]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R7)
+      EXPECT_EQ(expected.iregs[7], frame1->context.iregs[7]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R8)
+      EXPECT_EQ(expected.iregs[8], frame1->context.iregs[8]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R9)
+      EXPECT_EQ(expected.iregs[9], frame1->context.iregs[9]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_R10)
+      EXPECT_EQ(expected.iregs[10], frame1->context.iregs[10]);
+    if (expected_validity & StackFrameARM::CONTEXT_VALID_FP)
+      EXPECT_EQ(expected.iregs[MD_CONTEXT_ARM_REG_FP],
+                frame1->context.iregs[MD_CONTEXT_ARM_REG_FP]);
+
+    // We would never have gotten a frame in the first place if the SP
+    // and PC weren't valid or ->instruction weren't set.
+    EXPECT_EQ(expected.iregs[MD_CONTEXT_ARM_REG_SP],
+              frame1->context.iregs[MD_CONTEXT_ARM_REG_SP]);
+    EXPECT_EQ(expected.iregs[MD_CONTEXT_ARM_REG_PC],
+              frame1->context.iregs[MD_CONTEXT_ARM_REG_PC]);
+    EXPECT_EQ(expected.iregs[MD_CONTEXT_ARM_REG_PC],
+              frame1->instruction + 1);
+    EXPECT_EQ("epictetus", frame1->function_name);
+  }
+
+  // The values we expect to find for the caller's registers.
+  MDRawContextARM expected;
+
+  // The validity mask for expected.
+  int expected_validity;
+
+  // The validity mask to impose on the context frame.
+  int context_frame_validity;
+};
+
+class CFI: public CFIFixture, public Test { };
+
+TEST_F(CFI, At4000) {
+  stack_section.start() = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004000;
+  raw_context.iregs[MD_CONTEXT_ARM_REG_LR] = 0x40005510;
+  CheckWalk();
+}
+
+TEST_F(CFI, At4001) {
+  Label frame1_sp = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  stack_section
+    .D32(0xb5d55e68)            // saved r4
+    .D32(0x8112e110)            // saved fp
+    .D32(0x40005510)            // return address
+    .Mark(&frame1_sp);          // This effectively sets stack_section.start().
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004001;
+  raw_context.iregs[4] = 0x635adc9f;                     // distinct callee r4
+  raw_context.iregs[MD_CONTEXT_ARM_REG_FP] = 0xbe145fc4; // distinct callee fp
+  CheckWalk();
+}
+
+// As above, but unwind from a context that has only the PC and SP.
+TEST_F(CFI, At4001LimitedValidity) {
+  context_frame_validity =
+    StackFrameARM::CONTEXT_VALID_PC | StackFrameARM::CONTEXT_VALID_SP;
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004001;
+  raw_context.iregs[MD_CONTEXT_ARM_REG_FP] = 0xbe145fc4; // distinct callee fp
+  Label frame1_sp = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  stack_section
+    .D32(0xb5d55e68)            // saved r4
+    .D32(0x8112e110)            // saved fp
+    .D32(0x40005510)            // return address
+    .Mark(&frame1_sp);          // This effectively sets stack_section.start().
+  expected_validity = (StackFrameARM::CONTEXT_VALID_PC
+                       | StackFrameARM::CONTEXT_VALID_SP
+                       | StackFrameARM::CONTEXT_VALID_FP
+                       | StackFrameARM::CONTEXT_VALID_R4);
+  CheckWalk();
+}
+
+TEST_F(CFI, At4002) {
+  Label frame1_sp = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  stack_section
+    .D32(0xfb81ff3d)            // no longer saved r4
+    .D32(0x8112e110)            // saved fp
+    .D32(0x40005510)            // return address
+    .Mark(&frame1_sp);          // This effectively sets stack_section.start().
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004002;
+  raw_context.iregs[0] = 0xb5d55e68;  // saved r4
+  raw_context.iregs[1] = 0xebd134f3;  // saved r5
+  raw_context.iregs[2] = 0xa31e74bc;  // saved r6
+  raw_context.iregs[3] = 0x2dcb16b3;  // saved r7
+  raw_context.iregs[4] = 0xfdd35466;  // distinct callee r4
+  raw_context.iregs[5] = 0xf18c946c;  // distinct callee r5
+  raw_context.iregs[6] = 0xac2079e8;  // distinct callee r6
+  raw_context.iregs[7] = 0xa449829f;  // distinct callee r7
+  raw_context.iregs[MD_CONTEXT_ARM_REG_FP] = 0xbe145fc4; // distinct callee fp
+  CheckWalk();
+}
+
+TEST_F(CFI, At4003) {
+  Label frame1_sp = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  stack_section
+    .D32(0x48c8dd5a)            // saved r1 (even though it's not callee-saves)
+    .D32(0xcb78040e)            // no longer saved r4
+    .D32(0x8112e110)            // saved fp
+    .D32(0x40005510)            // return address
+    .Mark(&frame1_sp);          // This effectively sets stack_section.start().
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004003;
+  raw_context.iregs[1] = 0xfb756319;                     // distinct callee r1
+  raw_context.iregs[MD_CONTEXT_ARM_REG_FP] = 0x0a2857ea; // distinct callee fp
+  expected.iregs[1] = 0x48c8dd5a;    // caller's r1
+  expected_validity |= StackFrameARM::CONTEXT_VALID_R1;
+  CheckWalk();
+}
+
+// We have no new rule at module offset 0x4004, so the results here should
+// be the same as those at module offset 0x4003.
+TEST_F(CFI, At4004) {
+  Label frame1_sp = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  stack_section
+    .D32(0x48c8dd5a)            // saved r1 (even though it's not callee-saves)
+    .D32(0xcb78040e)            // no longer saved r4
+    .D32(0x8112e110)            // saved fp
+    .D32(0x40005510)            // return address
+    .Mark(&frame1_sp);          // This effectively sets stack_section.start().
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004004;
+  raw_context.iregs[1] = 0xfb756319; // distinct callee r1
+  expected.iregs[1] = 0x48c8dd5a; // caller's r1
+  expected_validity |= StackFrameARM::CONTEXT_VALID_R1;
+  CheckWalk();
+}
+
+// Here we move the .cfa, but provide an explicit rule to recover the SP,
+// so again there should be no change in the registers recovered.
+TEST_F(CFI, At4005) {
+  Label frame1_sp = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  stack_section
+    .D32(0x48c8dd5a)            // saved r1 (even though it's not callee-saves)
+    .D32(0xf013f841)            // no longer saved r4
+    .D32(0x8112e110)            // saved fp
+    .D32(0x40005510)            // return address
+    .Mark(&frame1_sp);          // This effectively sets stack_section.start().
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004005;
+  raw_context.iregs[1] = 0xfb756319; // distinct callee r1
+  expected.iregs[1] = 0x48c8dd5a; // caller's r1
+  expected_validity |= StackFrameARM::CONTEXT_VALID_R1;
+  CheckWalk();
+}
+
+// Here we provide an explicit rule for the PC, and have the saved .ra be
+// bogus.
+TEST_F(CFI, At4006) {
+  Label frame1_sp = expected.iregs[MD_CONTEXT_ARM_REG_SP];
+  stack_section
+    .D32(0x40005510)            // saved pc
+    .D32(0x48c8dd5a)            // saved r1 (even though it's not callee-saves)
+    .D32(0xf013f841)            // no longer saved r4
+    .D32(0x8112e110)            // saved fp
+    .D32(0xf8d15783)            // .ra rule recovers this, which is garbage
+    .Mark(&frame1_sp);          // This effectively sets stack_section.start().
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40004006;
+  raw_context.iregs[1] = 0xfb756319; // callee's r1, different from caller's
+  expected.iregs[1] = 0x48c8dd5a; // caller's r1
+  expected_validity |= StackFrameARM::CONTEXT_VALID_R1;
+  CheckWalk();
+}
+
+// Check that we reject rules that would cause the stack pointer to
+// move in the wrong direction.
+TEST_F(CFI, RejectBackwards) {
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40006000;
+  raw_context.iregs[MD_CONTEXT_ARM_REG_SP] = 0x80000000;  
+  raw_context.iregs[MD_CONTEXT_ARM_REG_LR] = 0x40005510;
+  StackwalkerARM walker(&system_info, &raw_context, &stack_region, &modules,
+                        &supplier, &resolver);
+  ASSERT_TRUE(walker.Walk(&call_stack));
+  frames = call_stack.frames();
+  ASSERT_EQ(1U, frames->size());
+}
+
+// Check that we reject rules whose expressions' evaluation fails.
+TEST_F(CFI, RejectBadExpressions) {
+  raw_context.iregs[MD_CONTEXT_ARM_REG_PC] = 0x40007000;
+  raw_context.iregs[MD_CONTEXT_ARM_REG_SP] = 0x80000000;  
+  StackwalkerARM walker(&system_info, &raw_context, &stack_region, &modules,
+                        &supplier, &resolver);
+  ASSERT_TRUE(walker.Walk(&call_stack));
+  frames = call_stack.frames();
+  ASSERT_EQ(1U, frames->size());
+}
+