从C获取* ABS *符号的值 [英] Getting the value of *ABS* symbols from C

问题描述

从C中访问*ABS*(绝对值，不可重定位)符号的实际值的正确方法是什么?

What is the proper way of accessing the actual value of an *ABS* (absolute, not-to-be-relocated) symbol from C?

我正在使用ld将数据(... HTML模板)嵌入可执行文件中(...按照

I'm using ld to embed data (... a HTML template) in an executable (... following the method described in https://csl.name/post/embedding-binary-data/). As in:

ld -r -b binary [the_file_to_be_embedded] -o [some_object.o]

然后，我正在使用

const char _some_object_start;
const char _some_object_end;
const int _some_object_size;

实际代码中的

.通过使用&_some_object_start获得char指针，实际的链接文本可以很好地工作.读取大小总是以有趣的，与平台相关的方式中断:在ARM64上，(long) &_some_object_size实际上最终成为大小，但是在x64上，编译器试图使其相对于PC，而最终没有得到正确的结果

in the actual code. The actual linked text works nicely by getting char pointers with &_some_object_start; reading the size keeps breaking in interesting, platform-dependent ways though: on ARM64, (long) &_some_object_size actually ends up being the size, but on x64 the compiler is trying to make it PC-relative and doesn't end up with the right thing.

作为参考，这是实际符号表的外观:

For reference, this is how the actual symbol table looks like:

SYMBOL TABLE:
00000000 l    d  .data  00000000 .data
0000000d g       .data  00000000 _binary_hello_world_txt_end
0000000d g       *ABS*  00000000 _binary_hello_world_txt_size
00000000 g       .data  00000000 _binary_hello_world_txt_start

(由objdump -x获取.)

(obtained by objdump -x.)

我假设ld假定存在某种合理的方式来链接代码以读取它，因此添加了大小数据，因此是问题.

I presume ld adds size data assuming that there is some reasonable way for code linked with it to read it, therefore the question.

(...更多是从理论上讲的，因为&_obj_end - &obj_start仍会解析为实际数据长度；不过，我对_obj_size特别好奇.)

(... which is more on the theoretical side, as &_obj_end - &obj_start does still resolve to the actual data length; I'm curious specifically about _obj_size though.)

推荐答案

符号的值是其地址，因此&symbol_name应该对其进行检索.

The value of a symbol is its address, so &symbol_name should retrieve it.

example.c

example.c

#include <stdio.h>
extern char abs_sym;
int main() { printf("%p\n", (void*)&abs_sym); }

运行示例:

$ gcc ldabs.c  -no-pie -Wl,--defsym=abs_sym=0xab && ./a.out
  0xab

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