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                    <main>
                        <h1 id="ldso8"><a class="header" href="#ldso8">ld.so(8)</a></h1>
<h2 id="environment-variables"><a class="header" href="#environment-variables">Environment Variables</a></h2>
<pre><code class="language-console">  LD_PRELOAD=&lt;l_so&gt;       colon separated list of libso's to be pre loaded
  LD_DEBUG=&lt;opts&gt;         comma separated list of debug options
          =help           list available options
          =libs           show library search path
          =files          processing of input files
          =symbols        show search path for symbol lookup
          =bindings       show against which definition a symbol is bound
</code></pre>
<h2 id="ld_preload-initialization-order-and-link-map"><a class="header" href="#ld_preload-initialization-order-and-link-map">LD_PRELOAD: Initialization Order and Link Map</a></h2>
<p>Libraries specified in <code>LD_PRELOAD</code> are loaded from <code>left-to-right</code> but
initialized from <code>right-to-left</code>.</p>
<pre><code class="language-markdown">  &gt; ldd ./main
    &gt;&gt; libc.so.6 =&gt; /usr/lib/libc.so.6

  &gt; LD_PRELOAD=liba.so:libb.so ./main
             --&gt;
      preloaded in this order
             &lt;--
      initialized in this order
</code></pre>
<p>The preload order determines:</p>
<ul>
<li>the order libraries are inserted into the <code>link map</code></li>
<li>the initialization order for libraries</li>
</ul>
<p>For the example listed above the resulting <code>link map</code> will look like the
following:</p>
<pre><code class="language-makrdown">  +------+    +------+    +------+    +------+
  | main | -&gt; | liba | -&gt; | libb | -&gt; | libc |
  +------+    +------+    +------+    +------+
</code></pre>
<p>This can be seen when running with <code>LD_DEBUG=files</code>:</p>
<pre><code class="language-makrdown">  &gt; LD_DEBUG=files LD_PRELOAD=liba.so:libb.so ./main
    # load order (-&gt; determines link map)
    &gt;&gt; file=liba.so [0];  generating link map
    &gt;&gt; file=libb.so [0];  generating link map
    &gt;&gt; file=libc.so.6 [0];  generating link map

    # init order
    &gt;&gt; calling init: /usr/lib/libc.so.6
    &gt;&gt; calling init: &lt;path&gt;/libb.so
    &gt;&gt; calling init: &lt;path&gt;/liba.so
    &gt;&gt; initialize program: ./main
</code></pre>
<p>To verify the <code>link map</code> order we let <code>ld.so</code> resolve the <code>memcpy(3)</code> libc
symbol (used in <em>main</em>) dynamically, while enabling <code>LD_DEBUG=symbols,bindings</code>
to see the resolving in action.</p>
<pre><code class="language-makrdown">  &gt; LD_DEBUG=symbols,bindings LD_PRELOAD=liba.so:libb.so ./main
    &gt;&gt; symbol=memcpy;  lookup in file=./main [0]
    &gt;&gt; symbol=memcpy;  lookup in file=&lt;path&gt;/liba.so [0]
    &gt;&gt; symbol=memcpy;  lookup in file=&lt;path&gt;/libb.so [0]
    &gt;&gt; symbol=memcpy;  lookup in file=/usr/lib/libc.so.6 [0]
    &gt;&gt; binding file ./main [0] to /usr/lib/libc.so.6 [0]: normal symbol `memcpy' [GLIBC_2.14]
</code></pre>
<h2 id="dynamic-linking-x86_64"><a class="header" href="#dynamic-linking-x86_64">Dynamic Linking (x86_64)</a></h2>
<p>Dynamic linking basically works via one indirect jump. 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]&gt; 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]&gt; pd 6 @ section..plt
              ;-- section..plt:
              ;-- .plt:       ; [12] -r-x section size 32 named .plt
          ┌─&gt; 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
          └─&lt; 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">  &gt; readelf -r &lt;main&gt;
    &gt;&gt; Relocation section '.rela.plt' at offset 0x4b8 contains 1 entry:
    &gt;&gt;   Offset          Info           Type           Sym. Value    Sym. Name + Addend
    &gt;&gt; 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>

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