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It uses a combination of +function trampolines (<code>.plt</code> section) and a function pointer table (<code>.got.plt</code> +section). +On the first call the trampoline sets up some metadata and then jumps to the +<code>ld.so</code> runtime resolve function, which in turn patches the table with the +correct function pointer.</p> +<pre><code class="language-makrdown"> .plt ....... procedure linkage table, contains function trampolines, usually + located in code segment (rx permission) + .got.plt ... global offset table for .plt, holds the function pointer table +</code></pre> +<p>Using <code>radare2</code> we can analyze this in more detail:</p> +<pre><code class="language-makrdown"> [0x00401040]> pd 4 @ section..got.plt + ;-- section..got.plt: + ;-- .got.plt: ; [22] -rw- section size 32 named .got.plt + ;-- _GLOBAL_OFFSET_TABLE_: + [0] 0x00404000 .qword 0x0000000000403e10 ; section..dynamic + [1] 0x00404008 .qword 0x0000000000000000 + ; CODE XREF from section..plt @ +0x6 + [2] 0x00404010 .qword 0x0000000000000000 + ;-- reloc.puts: + ; CODE XREF from sym.imp.puts @ 0x401030 + [3] 0x00404018 .qword 0x0000000000401036 ; RELOC 64 puts + + [0x00401040]> pd 6 @ section..plt + ;-- section..plt: + ;-- .plt: ; [12] -r-x section size 32 named .plt + ┌─> 0x00401020 ff35e22f0000 push qword [0x00404008] + ╎ 0x00401026 ff25e42f0000 jmp qword [0x00404010] + ╎ 0x0040102c 0f1f4000 nop dword [rax] + ┌ 6: int sym.imp.puts (const char *s); + └ ╎ 0x00401030 ff25e22f0000 jmp qword [reloc.puts] + ╎ 0x00401036 6800000000 push 0 + └─< 0x0040103b e9e0ffffff jmp sym..plt +</code></pre> +<ul> +<li>At address <code>0x00401030</code> in the <code>.plt</code> section we see the indirect jump for +<code>puts</code> using the function pointer in <code>_GLOBAL_OFFSET_TABLE_[3] (GOT)</code>.</li> +<li><code>GOT[3]</code> initially points to instruction after the <code>puts</code> trampoline +<code>0x00401036</code>.</li> +<li>This pushes the relocation index <code>0</code> and then jumps to the first trampoline +<code>0x00401020</code>.</li> +<li>The first trampoline jumps to <code>GOT[2]</code> which will be filled at program +startup by the <code>ld.so</code> with its resolve function.</li> +<li>The <code>ld.so</code> resolve function fixes the relocation referenced by the +relocation index pushed by the <code>puts</code> trampoline.</li> +<li>The relocation entry at index <code>0</code> tells the resolve function which symbol to +search for and where to put the function pointer: +<pre><code class="language-makrdown"> > readelf -r <main> + >> Relocation section '.rela.plt' at offset 0x4b8 contains 1 entry: + >> Offset Info Type Sym. Value Sym. Name + Addend + >> 000000404018 000200000007 R_X86_64_JUMP_SLO 0000000000000000 puts@GLIBC_2.2.5 + 0 +</code></pre> +As we can see the offset from relocation at index <code>0</code> points to <code>GOT[3]</code>.</li> +</ul> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + + <a rel="prev" href="../development/gcc.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + + + + <div style="clear: both"></div> + </nav> + </div> + </div> + + <nav class="nav-wide-wrapper" aria-label="Page navigation"> + + <a href="../development/gcc.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + + + </nav> + + </div> + + + + + + + + + <script type="text/javascript"> + window.playpen_copyable = true; + </script> + + + + + + <script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../mark.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../searcher.js" type="text/javascript" charset="utf-8"></script> + + + <script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../highlight.js" type="text/javascript" charset="utf-8"></script> + <script src="../book.js" type="text/javascript" charset="utf-8"></script> + + <!-- Custom JS scripts --> + + + + + </body> +</html> |