为什么回溯不包含Objective-C符号而不管-rdynamic？ [英] Why does backtrace not contain Objective-C symbols regardless of -rdynamic?

问题描述

更新：我正在使用Linux上的GNU运行时。

更新2：我编译了GNU-在MacOS上运行，并用它构建示例。 GNU运行时在MacOS上发生错误 not 。我会说问题是glibc（因为 backtrace 和 backtrace_symbols 是glibc扩展名）。

使用 backtrace 和 backtrace_symbols ，我没有得到任何Objective-C符号。只有文件名，地址和C符号才会出现。

我使用 -g 编译并链接到<$ c


$ p <我的测试应用程序：

  void _printTrace（）
 {
 void * addr [1024]; 
 int aCount = backtrace（addr，1024）; 
 char ** frameStrings = backtrace_symbols（addr，aCount）; 
 for（int i = 0; i< aCount; i ++）{
 printf（％s \ n，frameStrings [i]）; 
} 
 free（frameStrings）; 
} 
 
 @interface TheObject 
 +（void）_printTrace; 
 +（void）printTrace; 
 @end 
 
 @implementation TheObject 
 +（void）_printTrace 
 {
 _printTrace（）; 
} 
 
 +（void）printTrace 
 {
 [self _printTrace]; 
} 
 @end 
 
 void printTrace（）
 {
 [TheObject printTrace]; 
 
 
 int main（int argc，char ** argv）
 {
 printTrace（）; 
返回0; 
} 
  

以及它的输出：

  ./ test.bin（_printTrace + 0x1f）[0x8048e05] 
 ./test.bin（）[0x8048e60] 
 ./test.bin（） [0x8048e8b] 
 ./test.bin(printTrace+0x34）[0x8048ec5] 
 ./test.bin（main+0xf）[0x8048eda] 
 /lib/libc.so.6（ __libc_start_main + 0xe5）[0xb7643bb5] 
 ./test.bin（）[0x8048b51] 
  

有没有一种方法让Objective-C符号出现在这个回溯中？

解决方案

dladdr（） 仅报告全局和弱符号。但是所有的Objective-C函数符号都是本地的：

  $ readelf -s so_backtrace 
 
符号表' .dynsym'包含29个条目：
 ... 
 
符号表'.symtab'包含121个条目：
编号：值大小类型绑定Vis Ndx名称
 ... 
 49：08048a01 13 FUNC LOCAL DEFAULT 14 _c_TheObject___printTrace 
 50：08048a0e 47 FUNC LOCAL DEFAULT 14 _c_TheObject__printTrace 
 ... 
  

您可以通过查看 GNU libc源代码。 backtrace_symbols（）在sysdeps / generic / elf / backtracesyms.c中定义。它依赖于elf / dl-addr.c中定义的 _dl_addr（）来为它提供符号名称。这最终会调用 determine_info（）。如果可以的话，它使用 GNU哈希表，它不包含本地符号设计：

  49 / *我们查看哈希
 50表引用的所有符号表条目。 * / 
 ... 
 60 / *散列表从不引用本地符号，所以
 61我们可以在这里省略这个测试。 * / 
  

如果GNU哈希表不存在，它将回退到标准哈希表。这包括所有符号，但 determine_info（）代码过滤掉全部符号和弱符号：

<$ （（ELFW（ST_BIND）（symtab-> st_info）== STB_GLOBAL
91 || ELFW（ST_BIND）（symtab-> st_info）== STB_WEAK）（p $ p>


为了表示Objective-C函数地址，您必须自己执行查找，而不是过滤出本地函数符号，并且必须将Objective-C函数符号去映射以将 _c_TheObject ___ printTrace 还原为 + [TheObject _printTrace] 。

Update: I'm working with the GNU-runtime on Linux. The problem does not occur on MacOS with the Apple-runtime.

Update 2: I compiled the GNU-runtime on MacOS and build the example with it. The error does not occur on MacOS with the GNU-runtime. I would say the problem is the glibc (since backtrace and backtrace_symbols are glibc extensions).

When printing a backtrace in a GCC compiled Objective-C app using backtraceand backtrace_symbols, I don't get any Objective-C symbols. Only the filenames, addresses and C-symbols appear.

I compiled with -g and linked with -rdynamic.

My test app:

void _printTrace()
{
    void *addr[1024];
    int aCount = backtrace(addr, 1024);
    char **frameStrings = backtrace_symbols(addr, aCount);
    for (int i = 0; i < aCount; i++) {
        printf("%s\n", frameStrings[i]);
    }
    free(frameStrings);
}

@interface TheObject
+ (void)_printTrace;
+ (void)printTrace;
@end

@implementation TheObject
+ (void)_printTrace
{
    _printTrace();
}

+ (void)printTrace
{
    [self _printTrace];
}
@end

void printTrace()
{
    [TheObject printTrace];
}

int main(int argc, char **argv)
{
    printTrace();
    return 0;
}

and it's output:

./test.bin(_printTrace+0x1f) [0x8048e05]
./test.bin() [0x8048e60]
./test.bin() [0x8048e8b]
./test.bin(printTrace+0x34) [0x8048ec5]
./test.bin(main+0xf) [0x8048eda]
/lib/libc.so.6(__libc_start_main+0xe5) [0xb7643bb5]
./test.bin() [0x8048b51]

Is there a way to let the Objective-C symbols appear in this backtrace?

解决方案

dladdr() only reports global and weak symbols. But all Objective-C function symbols are local:

$ readelf -s so_backtrace

Symbol table '.dynsym' contains 29 entries:
…

Symbol table '.symtab' contains 121 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
…
    49: 08048a01    13 FUNC    LOCAL  DEFAULT   14 _c_TheObject___printTrace
    50: 08048a0e    47 FUNC    LOCAL  DEFAULT   14 _c_TheObject__printTrace
…

You can verify that local symbols are never returned by looking at the GNU libc source code yourself. backtrace_symbols() is defined in sysdeps/generic/elf/backtracesyms.c. It relies on _dl_addr(), which is defined in elf/dl-addr.c, to provide it with the symbol names. That ultimately calls determine_info(). If it can, it uses the the GNU hash table, which does not include local symbols by design:

49       /* We look at all symbol table entries referenced by the hash
50          table.  */
…
60                   /* The hash table never references local symbols so
61                      we can omit that test here.  */

If the GNU hash table isn't present, it falls back to standard hash table. This includes all the symbols, but the determine_info() code filters out all but the global symbols and weak symbols:

90         if ((ELFW(ST_BIND) (symtab->st_info) == STB_GLOBAL
91              || ELFW(ST_BIND) (symtab->st_info) == STB_WEAK)

To symbolicate the Objective-C function addresses, you would have to perform the look-up yourself and not filter out the local function symbols. Further, you would have to demangle the Objective-C function symbols to restore _c_TheObject___printTrace to +[TheObject _printTrace].

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