mips - 在SIGSEGV上显示_Unwind_Backtrace

Question

在mips平台上，我正在尝试让Unwind工作。当前，如果我手动发出print_trace，则堆栈跟踪正确显示如下：


backtrace_helper 0x4b6958

backtrace_helper 0x4b6ab4

backtrace_helper 0x2ac2f628

获得3个堆叠帧。

./v(print_trace+0x38）[0x4b6958]

./v(main+0x90）[0x4b6ab4]

/lib/libc.so.0(__uClibc_main+0x24c）[0x2ac2f628]


但是，当发生SIGSEGV时，堆栈跟踪不会显示正确的函数调用顺序。


backtrace_helper 0x4b7a74

backtrace_helper 0x2ab9b84c

获得2个堆叠帧。

./v(getLineIDByPhyIdx+0x3d8）[0x4b7a74]

/lib/libpthread.so.0(__new_sem_post+0x2c8）[0x2ab9b84c]


我正在使用-g -fexceptions -rdynamic进行编译。我也看到了How to generate a stacktrace when my gcc C++ app crashes，其中第二个答案提到了错误的地址，但是当我像他一样设置时，却只改变了第二个帧，其余的都一样，代码片段如下：

caller_address = (void *) uc->uc_mcontext.gregs[30]; // Frame pointer (from wikipedia here)  

 fprintf(stderr, "signal %d (%s), address is %p from %p\n", 
  sig_num, strsignal(sig_num), info->si_addr, 
  (void *)caller_address);

 size = backtrace(array, 50);

 /* overwrite sigaction with caller's address */
 array[1] = caller_address;

 messages = backtrace_symbols(array, size);

码：

int main(int argc, char *argv[]) {
    registerSignalHandler(signalHandler);

    print_trace();

    {
        // Seg Fault
        int *p = NULL;
        *p = 54;
    }
}

void print_trace(void) {
    void *array[10];
    size_t size;
    char **strings;
    size_t i;

    /* Get the address at the time the signal was raised from the EIP (x86) */
    size = backtrace(array, 10);
    strings = backtrace_symbols(array, size);

    printf("Obtained %zd stack frames.\n", size);

    for (i = 0; i < size; i++)
        printf("%s\n", strings[i]);

    free(strings);
}



static _Unwind_Reason_Code
backtrace_helper (struct _Unwind_Context *ctx, void *a)
{
    struct trace_arg *arg = a;

    assert (unwind_getip != NULL);

    /* We are first called with address in the __backtrace function. Skip it. */
    if (arg->cnt != -1) {
        arg->array[arg->cnt] = (void *) unwind_getip (ctx);
        printf("backtrace_helper %p \n", arg->array[arg->cnt]);
    }
    if (++arg->cnt == arg->size)
        return _URC_END_OF_STACK;
    return _URC_NO_REASON;
}

/*
 * Perform stack unwinding by using the _Unwind_Backtrace.
 *
 * User application that wants to use backtrace needs to be
 * compiled with -fexceptions option and -rdynamic to get full
 * symbols printed.
 */
int backtrace (void **array, int size)
{
    struct trace_arg arg = { .array = array, .size = size, .cnt = -1 };

    if (unwind_backtrace == NULL)
        backtrace_init();

    if (size >= 1)
        unwind_backtrace (backtrace_helper, &arg);

    return arg.cnt != -1 ? arg.cnt : 0;
}


void signalHandler( int sig, siginfo_t* siginfo, void* notused)
{
    /* Print out the signal info */
    signalInfo(sig, siginfo);

    switch (sig) {
        case SIGSEGV:
        {
            print_trace();
            abort();
        }
    }
}

Answer 1

帧指针实际上从未在MIPS上使用，并且在不深入了解符号的情况下获取回溯就需要一些启发。

典型的方法是分析当前指令指针之前的代码，并尝试查找调整SP的函数序言。使用该信息，可以知道前一帧的位置，等等。

有关这些血腥细节，请参见以下幻灯片：
http://elinux.org/images/0/07/Intricacies_of_a_MIPS_Stack_Backtrace_Implementation.pdf