musl+libunwind的堆栈获取方案

发表于 更新于 Changyan：

背景

musl静态链接后，通过_Unwind_Backtrace某些架构无法获取到堆栈信息了，如arm32的SIGSEGV，libbacktrace是对其进行封装，所以也是如此。

使用http://musl.cc/下载的静态编译工具+libunwind同样无法获取到堆栈信息。

需要加入libunwind库的方法解决，具体如下。

参考：
https://cloud.tencent.com/developer/article/1173442
https://github.com/boostorg/stacktrace
https://www.boost.org/doc/libs/develop/doc/html/stacktrace.html
https://www.jianshu.com/p/58d32fbd8dfa
https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html
https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/main/libunwind/musl-mips-fix.patch

交叉编译libunwind

x86的需要链接libucontext才可以正常编译，mipsel的需要自己打patch，文件如musl-mips-fix.patch

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$ autoreconf -i # Needed only for building from git. Depends on libtool.
# HOST=x86_64-linux-gnu
# HOST=x86-linux-gnu
# HOST=arm-linux-gnueabihf
# HOST=aarch64-linux-gnu
# HOST=mipsel-linux-gnu
$ ./configure --host arm-linux CC=arm-linux-musleabi-gcc CFLAGS=-std=c11
# x86的需要链接libucontext才可以正常编译
# libcontext编译
$ make ARCH=x86 CC=/root/compile/old/i486-linux-musl-cross/bin/i486-linux-musl-gcc DESTDIR=x86-out install
$ ./configure --host x86-linux CC=i486-linux-musleabi-gcc CFLAGS="-std=c11 -m32" LDFLAGS="-L libucontext/lib/path -lucontext"
$ make
# 编译出来的库在src/.libs下
$ ls src/.libs/

具体应用

链接unwind库，该库需要使用arm-linux-musleabi-gcc工具进行编译。

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# 编译方法
$ arm-linux-musleabi-gcc backtrace.c -o backtrace -lunwind -static

backtrace.c文件内容如下：

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#define __STDC_FORMAT_MACROS
#include <inttypes.h>

#define UNW_LOCAL_ONLY
#include <libunwind.h>

#include <cxxabi.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
                       int flags)
{
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    sa.sa_sigaction = handler;
    sa.sa_flags = SA_SIGINFO | flags;
    sigemptyset(&sa.sa_mask);
    if (sigaction(sig, &sa, 0))
        err(1, "sigaction");
}

void backtrace()
{
  unw_cursor_t cursor;
  unw_context_t context;

  unw_getcontext(&context);
  unw_init_local(&cursor, &context);

  int n=0;
  while ( unw_step(&cursor) ) {
    unw_word_t ip, sp, off;

    unw_get_reg(&cursor, UNW_REG_IP, &ip);
    unw_get_reg(&cursor, UNW_REG_SP, &sp);

    char symbol[256] = {"<unknown>"};
    char *name = symbol;

    if ( !unw_get_proc_name(&cursor, symbol, sizeof(symbol), &off) ) {
      int status;
      if ( (name = abi::__cxa_demangle(symbol, NULL, NULL, &status)) == 0 )
        name = symbol;
    }

    printf("#%-2d 0x%016" PRIxPTR " sp=0x%016" PRIxPTR " %s + 0x%" PRIxPTR "\n",
        ++n,
        static_cast<uintptr_t>(ip),
        static_cast<uintptr_t>(sp),
        name,
        static_cast<uintptr_t>(off));

    if ( name != symbol )
      free(name);
  }
}

static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
{
    long ip = 0;
    ucontext_t *uc = (ucontext_t *)ctx_void;
    ip = uc->uc_mcontext.arm_pc;
    printf("In signal handler.  Trying to unwind.\n");
    dprintf(2, "signal:%d address:0x%lx ip:0x%lx\n",
        sig,
        /* this is void*, but using %p would print "(null)"
            * even for ptrs which are not exactly 0, but, say, 0x123:
            */
        (long)info->si_addr,
        ip);
    backtrace();
    exit(0);
}

int fac(int n)
{
  if ( n == 0 ) {
    printf("Unwind from signal handler\n");
    sethandler(SIGSEGV, sigusr1, 0);
    char *p = NULL;
    *p = 11;
    return 1;
  } else {
    return n*fac(n-1);
  }
}

int main()
{
  fac(10);
  return 0;
}