How to get fullstacktrace using _Unwind_Backtrace on SIGSEGV

Question

I handle SIGSEGV by code:

int C()
{
  int *i = NULL;
  *i = 10; // Crash there
}

int B()
{
  return C();
}

int A()
{
   return B();
}

int main(void)
{
  struct sigaction handler;
  memset(&handler,0,sizeof(handler));
  handler.sa_sigaction = handler_func;
  handler.sa_flags = SA_SIGINFO;
  sigaction(SIGSEGV,&handler,NULL);
  return(C());
}

Where handler code are:

static int handler_func(int signal, siginfo_t info, void* rserved)
{
  const void* stack[MAX_DEPTH];
  StackCrowlState state;
  state.addr = stack;
  state.count = MAX_DEPTH;

  _Unwind_Reason_Code code = _Unwind_Backtrace(trace_func,&state);
  printf("Stack trace count: %d, code: %d\n",MAX_DEPTH - state.count, code);

  kill(getpid(),SIGKILL);
}

static _Unwind_Reason_Code trace_func(void* context, void* arg)
{
  StackCrowlState *state = (StackCrowlState *)arg;
  if(state->count>0)
  {
     void *ip = (void *)_Unwind_GetIP(context);
     if(ip)
     {
       state->addr[0] = ip;
       state->count--;
       state->addr++;
     }
  }
  return(_URC_NO_REASON);
}

But trace_func where called only once, and shows only on _Unwind_Backtrace calls. Is it possible to get stacktrace of code which cause SIGSEGV signal using _Unwind_Backtrace?

thnx

Answer 1

If you want to use particularly _Unwind_Context(), you can do it like this (the code is 32-bit ARM specific):

---

struct BacktraceState {
    const ucontext_t*   signal_ucontext;
    size_t              address_count = 0;
    static const size_t address_count_max = 30;
    uintptr_t           addresses[address_count_max] = {};

    BacktraceState(const ucontext_t* ucontext) : signal_ucontext(ucontext) {}

    bool AddAddress(uintptr_t ip) {
        // No more space in the storage. Fail.
        if (address_count >= address_count_max)
            return false;

        // Reset the Thumb bit, if it is set.
        const uintptr_t thumb_bit = 1;
        ip &= ~thumb_bit;

        // Ignore null addresses.
        // They sometimes happen when using _Unwind_Backtrace()
        // with the compiler optimizations,
        // when the Link Register is overwritten by the inner
        // stack frames.
        if (ip == 0)
            return true;

        // Ignore duplicate addresses.
        // They sometimes happen when using _Unwind_Backtrace()
        // with the compiler optimizations,
        // because we both add the second address from the Link Register
        // in ProcessRegisters() and receive the same address
        // in UnwindBacktraceCallback().
        if (address_count > 0 && ip == addresses[address_count - 1])
            return true;

        // Finally add the address to the storage.
        addresses[address_count++] = ip;
        return true;
    }
};

void ProcessRegisters(
        _Unwind_Context* unwind_context, BacktraceState* state) {
    assert(state);
    assert(unwind_context);

    const ucontext_t* signal_ucontext = state->signal_ucontext;
    assert(signal_ucontext);

    const sigcontext* signal_mcontext = &(signal_ucontext->uc_mcontext);
    assert(signal_mcontext);

    _Unwind_SetGR(unwind_context, REG_R0,  signal_mcontext->arm_r0);
    _Unwind_SetGR(unwind_context, REG_R1,  signal_mcontext->arm_r1);
    _Unwind_SetGR(unwind_context, REG_R2,  signal_mcontext->arm_r2);
    _Unwind_SetGR(unwind_context, REG_R3,  signal_mcontext->arm_r3);
    _Unwind_SetGR(unwind_context, REG_R4,  signal_mcontext->arm_r4);
    _Unwind_SetGR(unwind_context, REG_R5,  signal_mcontext->arm_r5);
    _Unwind_SetGR(unwind_context, REG_R6,  signal_mcontext->arm_r6);
    _Unwind_SetGR(unwind_context, REG_R7,  signal_mcontext->arm_r7);
    _Unwind_SetGR(unwind_context, REG_R8,  signal_mcontext->arm_r8);
    _Unwind_SetGR(unwind_context, REG_R9,  signal_mcontext->arm_r9);
    _Unwind_SetGR(unwind_context, REG_R10, signal_mcontext->arm_r10);
    _Unwind_SetGR(unwind_context, REG_R11, signal_mcontext->arm_fp);
    _Unwind_SetGR(unwind_context, REG_R12, signal_mcontext->arm_ip);
    _Unwind_SetGR(unwind_context, REG_R13, signal_mcontext->arm_sp);
    _Unwind_SetGR(unwind_context, REG_R14, signal_mcontext->arm_lr);
    _Unwind_SetGR(unwind_context, REG_R15, signal_mcontext->arm_pc);

    // Program Counter register aka Instruction Pointer will contain
    // the address of the instruction where the crash happened.
    // UnwindBacktraceCallback() will not supply us with it.
    state->AddAddress(signal_mcontext->arm_pc);

    // UnwindBacktraceCallback() does not always supply us with
    // the return address of the frame where the crash happened.
    // Sometimes Link Register will contain this address
    // (noticed when compiling with Clang without optimization),
    // but LR may also contain address of some previously visitied frame
    // (noticed when compiling with GCC without optimization),
    // or LR may contain null address
    // (noticed when compiling with Clang with optimization).
    // These heuristics are unreliable.
#if __clang__
    state->AddAddress(signal_mcontext->arm_lr);
#endif
}

_Unwind_Reason_Code UnwindBacktraceCallback(
        struct _Unwind_Context* unwind_context, void* state_voidp) {
    assert(unwind_context);
    assert(state_voidp);

    BacktraceState* state = (BacktraceState*)state_voidp;
    assert(state);

    // On the first UnwindBacktraceCallback() call,
    // set registers to _Unwind_Context and BacktraceState.
    if (state->address_count == 0) {
        ProcessRegisters(unwind_context, state);
        return _URC_NO_REASON;
    }

    uintptr_t ip = _Unwind_GetIP(unwind_context);
    bool ok = state->AddAddress(ip);
    if (!ok)
        return _URC_END_OF_STACK;

    return _URC_NO_REASON;
}

void CaptureBacktrace(BacktraceState* state) {
    assert(state);
    _Unwind_Backtrace(UnwindBacktraceCallback, state);
}

void SigActionHandler(int sig, siginfo_t* info, void* ucontext) {
    const ucontext_t* signal_ucontext = (const ucontext_t*)ucontext;
    assert(signal_ucontext);

    BacktraceState backtrace_state(signal_ucontext);
    CaptureBacktrace(&backtrace_state);
    // Do something with the backtrace - print, save to file, etc.
}

---

But I am advising you to not use _Unwind_Context(), but instead use precompiled libunwind for 32-bit ARM, bundled with modern Android NDKs (at sources/cxx-stl/llvm-libc++/libs/armeabi-v7a/libunwind.a) and with all LLVM. You will have to use use libc++ (LLVM STL). How to do it, is demonstrated in this my answer, you'd have to combine the examples here.

https://stackoverflow.com/a/50027799/1016580

If you use libstdc++ (GNU STL), you could use the Dar Hoo's solution:

https://stackoverflow.com/a/48593413/1016580

Answer 2

You want to backtrace from the signal triggering function, but you backtrace from the signal handler function. That's two different stacks. (Note, the SA_ONSTACK flag in sigaction is irrelevant to your question.)

To find the stack pointer of the of the triggering function, use the third parameter of the handler, i.e. void *rserved. You can reference to the answer in this question: Getting the saved instruction pointer address from a signal handler