如何展开堆栈获得回溯指定的堆栈指针（SP）？ [英] How to unwind the stack to get backtrace for the specified stack pointer (SP)?
问题描述
我在写这为Android（仅ARM），但我相信原则是通用Linux一样好。
I'm writing this for Android (ARM only), but I believe the principle is the same for generic Linux as well.
我想从信号处理程序中捕获堆栈跟踪，这样我就可以登录它时，我的应用程序崩溃。这是我想出用＆LT; unwind.h方式＆gt; 结果
初始化：
I'm trying to capture the stack trace from within the signal handler, so that I can log it when my app crashes. This is what I've come up with using <unwind.h>.
Initialization:
struct sigaction signalhandlerDescriptor;
memset(&signalhandlerDescriptor, 0, sizeof(signalhandlerDescriptor));
signalhandlerDescriptor.sa_flags = SA_SIGINFO;
signalhandlerDescriptor._u._sa_sigaction = signalHandler;
sigaction(SIGSEGV, &signalhandlerDescriptor, 0);
在code本身：
The code itself:
struct BacktraceState
{
void** current;
void** end;
void* pc;
};
inline _Unwind_Reason_Code unwindCallback(struct _Unwind_Context* context, void* arg)
{
BacktraceState* state = static_cast<BacktraceState*>(arg);
state->pc = (void*)_Unwind_GetIP(context);
if (state->pc)
{
if (state->current == state->end)
return _URC_END_OF_STACK;
else
*state->current++ = reinterpret_cast<void*>(state->pc);
}
return _URC_NO_REASON;
}
inline size_t captureBacktrace(void** addrs, size_t max, unsigned long pc)
{
BacktraceState state = {addrs, addrs + max, (void*)pc};
_Unwind_Backtrace(unwindCallback, &state);
personality_routine();
return state.current - addrs;
}
inline void dumpBacktrace(std::ostream& os, void** addrs, size_t count)
{
for (size_t idx = 0; idx < count; ++idx) {
const void* addr = addrs[idx];
const char* symbol = "";
Dl_info info;
if (dladdr(addr, &info) && info.dli_sname) {
symbol = info.dli_sname;
}
int status = -3;
char * demangledName = abi::__cxa_demangle(symbol, 0, 0, &status);
os << "#" << idx << ": " << addr << " " << (status == 0 ? demangledName : symbol) << "\n";
free(demangledName);
}
}
void signalHandler(int sig, siginfo_t *siginfo, void *uctx)
{
ucontext * context = (ucontext*)uctx;
unsigned long PC = context->uc_mcontext.arm_pc;
unsigned long SP = context->uc_mcontext.arm_sp;
Logger() << __PRETTY_FUNCTION__ << "Fatal signal:" << sig;
const size_t maxNumAddresses = 50;
void* addresses[maxNumAddresses];
std::ostringstream oss;
const size_t actualNumAddresses = captureBacktrace(addresses, maxNumAddresses, PC);
dumpBacktrace(oss, addresses, actualNumAddresses);
Logger() << oss.str();
exit(EXIT_FAILURE);
}
问题：如果我通过调用 _Unwind_GetIP（上下文）在 unwindCallback 获得PC寄存器，我得到了完整跟踪的的信号处理器堆栈的。它是一个独立的堆栈，而这显然不是我想要的。所以，我想提供从 ucontext 在信号处理程序采取的PC，并得到了怪异的结果：我得到一个堆栈条目，这是正确的入口 - 这引起了功能首先发出信号。但它记录两次（即使该地址是相同的，所以它不是一个象征性的名称来查找错误）。显然，这还不够好 - 我需要整个堆栈。我不知道这样的结果仅仅是偶然的（即它不应该在一般的工作。
Problem: if I get the PC register by calling _Unwind_GetIP(context) in unwindCallback, I get the complete trace for the signal handler stack. Which is a separate stack, and that's obviously not what I want. So I tried supplying the PC taken from the ucontext in signal handler, and got a weird result: I get one stack entry, it is the correct entry - the function which caused the signal in the first place. But it's logged twice (even the address is the same, so it's not a symbolic name look up bug). Obviously, that's not good enough - I need the whole stack. And I wonder if this result is merely accidental (i. e. it shouldn't work in general.
现在，我读我需要还供应堆栈指针，这显然我可以从获得ucontext ，同PC。但我不知道该怎么办。我一定要手动，而不是放松使用 _Unwind_Backtrace ？如果是这样，你可以给我的样品code？我一直在寻找了一天的大部分时间，仍然无法找到任何东西，我可以复制并粘贴到我的项目。
Now, I read I need to also supply the stack pointer, which I apparently can get from ucontext, same as PC. But I don't know what to do with it. Do I have to unwind manually instead of using _Unwind_Backtrace? If so, can you give me sample code? I've been searching for the better part of a day, and still couldn't find anything I could copy and paste into my project.
有关它的价值，这里包含 libunwind 源 _Unwind_Backtrace 定义。以为我可以想办法的，如果我看到它的来源，但它比我预想的方式更加复杂。
For what it's worth, here's the libunwind source which contains _Unwind_Backtrace definition. Thought I could figure something out if I see its source, but it's way more complicated than I expected.
推荐答案
首先，你需要阅读部分的异步信号安全功能：
First, you need to read the section on "async signal safe" functions:
http://man7.org/linux/man-pages/ man7 / signal.7.html
这就是是安全的信号处理函数调用函数整个集合。关于你能做的最糟糕的事情就是打电话给任何调用引擎盖下的malloc（）/ free（）的 - 或者自己做。
That's the entire set of functions that are safe to call in a signal handler. About the worst thing you can do is to call anything that calls malloc()/free() under the hood - or do it yourself.
二，搞不定的信号处理程序以外的第一。
Second, get it working outside of a signal handler first.
第三，这些可能是中肯的：
Third, these are probably apropos:
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