差异位置无关code：86 VS X86-64 [英] Difference in position-independent code: x86 vs x86-64

问题描述

我最近针对建设x86-64架构有一定的共享库（ELF），像这样的：

I was recently building a certain shared library (ELF) targeting x86-64 architecture, like this:

g++ -o binary.so -shared --no-undefined ... -lfoo -lbar

这失败，出现以下错误：

This failed with the following error:

创建共享对象时对`局部符号搬迁R_X86_64_32不能使用;与-fPIC编译

relocation R_X86_64_32 against `a local symbol' can not be used when making a shared object; recompile with -fPIC

当然，这意味着我需要重建它作为独立的位置code，所以它适合用于链接到共享库。

Of course, it means I need to rebuild it as position-independent code, so it's suitable for linking into a shared library.

但是，这工作得很好在x86上使用完全相同的构建参数。所以，问题是，如何重新定位是在x86来自不同x86-64的，为什么不让我需要 -fPIC 编译于前者？

But this works perfectly well on x86 with exactly the same build arguments. So the question is, how is relocation on x86 different from x86-64 and why don't I need to compile with -fPIC on the former?

推荐答案

我已经找到一个漂亮和详细的解释，这可以归结为：

I have found a nice and detailed explanation, which boils down to:

1. X86-64使用IP-相对偏移加载全局数据，X86-32不能，所以它解引用全局偏移。


2. IP-相对偏移共享库是不行的，因为全局符号可以被覆盖，所以X86-64发生故障时与PIC不建。


3. 如果X86-64与PIC建成，IP-相对偏移解引用，现在产量的的指针，GOT条目的，然后再解除引用。


4. X86-32，然而，的已的使用全局的解引用抵消，因此它变成直接GOT条目。

1. x86-64 uses IP-relative offset to load global data, x86-32 cannot, so it dereferences a global offset.
2. IP-relative offset does not work for shared libraries, because global symbols can be overridden, so x86-64 breaks down when not built with PIC.
3. If x86-64 built with PIC, the IP-relative offset dereference now yields a pointer to GOT entry, which is then dereferenced.
4. x86-32, however, already uses a dereference of a global offset, so it is turned into GOT entry directly.

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