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| author | johannst <johannst@users.noreply.github.com> | 2020-03-17 22:48:49 +0000 |
|---|---|---|
| committer | johannst <johannst@users.noreply.github.com> | 2020-03-17 22:48:49 +0000 |
| commit | 2a064a9d1bbb8de6ce489b685cce026eee167cd2 (patch) | |
| tree | 123cdb825636f0bfe98ef152ace16eb8b3ae2647 /index.html | |
| parent | 21e8db012f8c46f75e43a40da3f3e2676363c291 (diff) | |
| download | notes-2a064a9d1bbb8de6ce489b685cce026eee167cd2.tar.gz notes-2a064a9d1bbb8de6ce489b685cce026eee167cd2.zip | |
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Diffstat (limited to 'index.html')
| -rw-r--r-- | index.html | 177 |
1 files changed, 94 insertions, 83 deletions
@@ -81,7 +81,7 @@ <nav id="sidebar" class="sidebar" aria-label="Table of contents"> <div id="sidebar-scrollbox" class="sidebar-scrollbox"> - <ol class="chapter"><li class="expanded "><a href="ld.so.html"><strong aria-hidden="true">1.</strong> ld.so</a></li><li class="expanded "><a href="git.html"><strong aria-hidden="true">2.</strong> git</a></li><li class="expanded "><a href="gdb.html"><strong aria-hidden="true">3.</strong> gdb</a></li><li class="expanded "><a href="radare2.html"><strong aria-hidden="true">4.</strong> radare2</a></li><li class="expanded "><a href="emacs.html"><strong aria-hidden="true">5.</strong> emacs</a></li><li class="expanded "><a href="fish.html"><strong aria-hidden="true">6.</strong> fish</a></li><li class="expanded "><a href="strace.html"><strong aria-hidden="true">7.</strong> strace</a></li><li class="expanded "><a href="lsof.html"><strong aria-hidden="true">8.</strong> lsof</a></li><li class="expanded "><a href="pidstat.html"><strong aria-hidden="true">9.</strong> pidstat</a></li><li class="expanded "><a href="time.html"><strong aria-hidden="true">10.</strong> time</a></li><li class="expanded "><a href="pmap.html"><strong aria-hidden="true">11.</strong> pmap</a></li><li class="expanded "><a href="pstack.html"><strong aria-hidden="true">12.</strong> pstack</a></li><li class="expanded "><a href="perf.html"><strong aria-hidden="true">13.</strong> perf</a></li><li class="expanded "><a href="oprofile.html"><strong aria-hidden="true">14.</strong> OProfile</a></li><li class="expanded "><a href="od.html"><strong aria-hidden="true">15.</strong> od</a></li><li class="expanded "><a href="xxd.html"><strong aria-hidden="true">16.</strong> xxd</a></li><li class="expanded "><a href="readelf.html"><strong aria-hidden="true">17.</strong> readelf</a></li><li class="expanded "><a href="objdump.html"><strong aria-hidden="true">18.</strong> objdump</a></li><li class="expanded "><a href="nm.html"><strong aria-hidden="true">19.</strong> nm</a></li><li class="expanded "><a href="c++filt.html"><strong aria-hidden="true">20.</strong> c++filt</a></li></ol> + <ol class="chapter"><li class="expanded "><a href="ld.so.html"><strong aria-hidden="true">1.</strong> ld.so</a></li><li class="expanded "><a href="git.html"><strong aria-hidden="true">2.</strong> git</a></li><li class="expanded "><a href="awk.html"><strong aria-hidden="true">3.</strong> awk</a></li><li class="expanded "><a href="gdb.html"><strong aria-hidden="true">4.</strong> gdb</a></li><li class="expanded "><a href="radare2.html"><strong aria-hidden="true">5.</strong> radare2</a></li><li class="expanded "><a href="emacs.html"><strong aria-hidden="true">6.</strong> emacs</a></li><li class="expanded "><a href="fish.html"><strong aria-hidden="true">7.</strong> fish</a></li><li class="expanded "><a href="strace.html"><strong aria-hidden="true">8.</strong> strace</a></li><li class="expanded "><a href="lsof.html"><strong aria-hidden="true">9.</strong> lsof</a></li><li class="expanded "><a href="pidstat.html"><strong aria-hidden="true">10.</strong> pidstat</a></li><li class="expanded "><a href="time.html"><strong aria-hidden="true">11.</strong> time</a></li><li class="expanded "><a href="pgrep.html"><strong aria-hidden="true">12.</strong> pgrep</a></li><li class="expanded "><a href="pstack.html"><strong aria-hidden="true">13.</strong> pstack</a></li><li class="expanded "><a href="pstack.html"><strong aria-hidden="true">14.</strong> pstack</a></li><li class="expanded "><a href="perf.html"><strong aria-hidden="true">15.</strong> perf</a></li><li class="expanded "><a href="oprofile.html"><strong aria-hidden="true">16.</strong> OProfile</a></li><li class="expanded "><a href="od.html"><strong aria-hidden="true">17.</strong> od</a></li><li class="expanded "><a href="xxd.html"><strong aria-hidden="true">18.</strong> xxd</a></li><li class="expanded "><a href="readelf.html"><strong aria-hidden="true">19.</strong> readelf</a></li><li class="expanded "><a href="objdump.html"><strong aria-hidden="true">20.</strong> objdump</a></li><li class="expanded "><a href="nm.html"><strong aria-hidden="true">21.</strong> nm</a></li><li class="expanded "><a href="c++filt.html"><strong aria-hidden="true">22.</strong> c++filt</a></li></ol> </div> <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div> </nav> @@ -149,16 +149,18 @@ <div id="content" class="content"> <main> <h1><a class="header" href="#ldso8" id="ldso8">ld.so(8)</a></h1> -<h2><a class="header" href="#environment-variables" id="environment-variables">Environment variables</a></h2> +<h2><a class="header" href="#environment-variables" id="environment-variables">Environment Variables</a></h2> <pre><code class="language-console"> LD_PRELOAD=<l_so> colon separated list of libso's to be pre loaded - LD_DEBUG=<opts> comman separated list of debug options + LD_DEBUG=<opts> 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><a class="header" href="#ld_preload-load--init-order" id="ld_preload-load--init-order">LD_PRELOAD load & init order</a></h2> +<h2><a class="header" href="#ld_preload-initialization-order-and-link-map" id="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"> > ldd ./main >> libc.so.6 => /usr/lib/libc.so.6 @@ -167,87 +169,96 @@ preloaded in this order <-- initialized in this order - - - preload order determines the order libs are inserted into the link map - - - resulting link map: - +------+ +------+ +------+ +------+ - | main | -> | liba | -> | libb | -> | libc | - +------+ +------+ +------+ +------+ - - - see preload and init order in action - > LD_DEBUG=files LD_PRELOAD=liba.so:libb.so ./main - # load order (-> determines link map) - >> file=liba.so [0]; generating link map - >> file=libb.so [0]; generating link map - >> file=libc.so.6 [0]; generating link map - - # init order - >> calling init: /usr/lib/libc.so.6 - >> calling init: <path>/libb.so - >> calling init: <path>/liba.so - >> initialize program: ./main - - - see the symbol lookup in action and therefore the link map order - > LD_DEBUG=symbols,bindings LD_PRELOAD=liba.so:libb.so ./main - >> symbol=memcpy; lookup in file=./main [0] - >> symbol=memcpy; lookup in file=<path>/liba.so [0] - >> symbol=memcpy; lookup in file=<path>/libb.so [0] - >> symbol=memcpy; lookup in file=/usr/lib/libc.so.6 [0] - >> binding file ./main [0] to /usr/lib/libc.so.6 [0]: normal symbol - `memcpy' [GLIBC_2.14] </code></pre> -<h2><a class="header" href="#dynamic-linking-x86_64" id="dynamic-linking-x86_64">dynamic linking (x86_64)</a></h2> -<pre><code class="language-makrdown"> - dynamic linking basically works via one indirect jump. It uses a - combination of function trampolines (.plt) and a function pointer table - (.got.plt). On the first call the trampoline sets up some metadata and - then jumps to the ld.so runtime resolve function, which in turn patches - the table with the correct function pointer. - .plt ....... contains function trampolines, usually located in code - segment (rx permission) - .got.plt ... hold the function pointer table - - - following r2 dump shows this - - [0x00401030] indirect jump for 'puts' using function pointer in - _GLOBAL_OFFSET_TABLE_[3] - - initially points to instruction behind 'puts' trampoline [0x00401036] - - this pushes relocation index and then jumps to the first trampoline - [0x00401020] - - the first trampoline jumps to _GLOBAL_OFFSET_TABLE_[2] which will be - filled at program startup by the ld.so with its resolve function - - the resolve function fixes the relocation referenced by the - relocation index pushed by the 'puts' trampoline - - the relocation entry tells the resolve function which symbol to - search for and where to put the function pointer - > 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 - - offset points to _GLOBAL_OFFSET_TABLE_[3] - - [0x00401040]> pd 4 @ section..got.plt - ;-- section..got.plt: - ;-- .got.plt: ; [22] -rw- section size 32 named .got.plt - ;-- _GLOBAL_OFFSET_TABLE_: - 0x00404000 .qword 0x0000000000403e10 ; section..dynamic ; obj._DYNAMIC - 0x00404008 .qword 0x0000000000000000 - ; CODE XREF from section..plt @ +0x6 - 0x00404010 .qword 0x0000000000000000 - ;-- reloc.puts: - ; CODE XREF from sym.imp.puts @ 0x401030 - 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 +<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 | -> | liba | -> | libb | -> | libc | + +------+ +------+ +------+ +------+ +</code></pre> +<p>This can be seen when running with <code>LD_DEBUG=files</code>:</p> +<pre><code class="language-makrdown"> > LD_DEBUG=files LD_PRELOAD=liba.so:libb.so ./main + # load order (-> determines link map) + >> file=liba.so [0]; generating link map + >> file=libb.so [0]; generating link map + >> file=libc.so.6 [0]; generating link map + + # init order + >> calling init: /usr/lib/libc.so.6 + >> calling init: <path>/libb.so + >> calling init: <path>/liba.so + >> 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"> > LD_DEBUG=symbols,bindings LD_PRELOAD=liba.so:libb.so ./main + >> symbol=memcpy; lookup in file=./main [0] + >> symbol=memcpy; lookup in file=<path>/liba.so [0] + >> symbol=memcpy; lookup in file=<path>/libb.so [0] + >> symbol=memcpy; lookup in file=/usr/lib/libc.so.6 [0] + >> binding file ./main [0] to /usr/lib/libc.so.6 [0]: normal symbol `memcpy' [GLIBC_2.14] +</code></pre> +<h2><a class="header" href="#dynamic-linking-x86_64" id="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]> 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> |