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(Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> + <i class="fa fa-search"></i> + </button> + + </div> + + <h1 class="menu-title">Notes</h1> + + <div class="right-buttons"> + <a href="print.html" title="Print this book" aria-label="Print this book"> + <i id="print-button" class="fa fa-print"></i> + </a> + + </div> + </div> + </div> + + + <div id="search-wrapper" class="hidden"> + <form id="searchbar-outer" class="searchbar-outer"> + <input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> + </form> + <div id="searchresults-outer" class="searchresults-outer hidden"> + <div id="searchresults-header" class="searchresults-header"></div> + <ul id="searchresults"> + </ul> + </div> + </div> + + + <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> + <script type="text/javascript"> + document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); + document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); + Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { + link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); + }); + </script> + + <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> +<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 + =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> +<pre><code class="language-markdown"> > ldd ./main + >> libc.so.6 => /usr/lib/libc.so.6 + + > LD_PRELOAD=liba.so:libb.so ./main + --> + 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 +</code></pre> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + + + + <a rel="next" href="git.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + + + <div style="clear: both"></div> + </nav> + </div> + </div> + + <nav class="nav-wide-wrapper" aria-label="Page navigation"> + + + + <a href="git.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></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> |