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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> + + <a href="https://github.com/johannst/notes" title="Git repository" aria-label="Git repository"> + <i id="git-repository-button" class="fa fa-github"></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="#elf-symbol-versioning" id="elf-symbol-versioning">ELF Symbol Versioning</a></h1> +<p>The <a href="https://refspecs.linuxbase.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/symversion.html">ELF symbol versioning</a> mechanism allows to attach version +information to symbols. +This can be used to express symbol version requirements or to provide certain +symbols multiple times in the same ELF file with different versions (eg for +backwards compatibility).</p> +<p>The <code>libpthread.so</code> library is an example which provides the +<code>pthread_cond_wait</code> symbol multiple times but in different versions. +With readelf the version of the symbol can be seen after the <code>@</code>.</p> +<pre><code class="language-bash">> readelf -W --dyn-syms /lib/libpthread.so + +Symbol table '.dynsym' contains 342 entries: + Num: Value Size Type Bind Vis Ndx Name + ... + 141: 0000f080 696 FUNC GLOBAL DEFAULT 16 pthread_cond_wait@@GLIBC_2.3.2 + 142: 00010000 111 FUNC GLOBAL DEFAULT 16 pthread_cond_wait@GLIBC_2.2.5 +</code></pre> +<p>The <code>@@</code> denotes the <strong>default symbol version</strong> which will be used during +static linking against the library. +The following dump shows that the <code>tmp</code> program linked against <code>lpthread</code> will +depend on the symbol version <code>GLIBC_2.3.2</code>, which is the default version.</p> +<pre><code class="language-bash">> echo "#include <pthread.h> + int main() { + return pthread_cond_wait(0,0); + }" | gcc -o tmp -xc - -lpthread; + readelf -W --dyn-syms tmp | grep pthread_cond_wait; + +Symbol table '.dynsym' contains 7 entries: + Num: Value Size Type Bind Vis Ndx Name + ... + 2: 00000000 0 FUNC GLOBAL DEFAULT UND pthread_cond_wait@GLIBC_2.3.2 (2) +</code></pre> +<blockquote> +<p>Only <strong>one</strong> symbol can be annotated as the <code>@@</code> default version.</p> +</blockquote> +<p>Using the <code>--version-info</code> flag with readelf, more details on the symbol +version info compiled into the <code>tmp</code> ELF file can be obtained.</p> +<ul> +<li>The <code>.gnu.version</code> section contains the version definition for each symbol in +the <code>.dynsym</code> section. <code>pthread_cond_wait</code> is at index <code>2</code> in the <code>.dynsym</code> +section, the corresponding symbol version is at index <code>2</code> in the +<code>.gnu.version</code> section.</li> +<li>The <code>.gnu.version_r</code> section contains symbol version requirements per shared +library dependency (<code>DT_NEEDED</code> dynamic entry).</li> +</ul> +<pre><code class="language-bash">> readelf -W --version-info --dyn-syms tmp + +Symbol table '.dynsym' contains 7 entries: + Num: Value Size Type Bind Vis Ndx Name + 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND + 1: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTable + 2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND pthread_cond_wait@GLIBC_2.3.2 (2) + 3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (3) + 4: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__ + 5: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable + 6: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@GLIBC_2.2.5 (3) + +Version symbols section '.gnu.version' contains 7 entries: + Addr: 0x0000000000000534 Offset: 0x000534 Link: 6 (.dynsym) + 000: 0 (*local*) 0 (*local*) 2 (GLIBC_2.3.2) 3 (GLIBC_2.2.5) + 004: 0 (*local*) 0 (*local*) 3 (GLIBC_2.2.5) + +Version needs section '.gnu.version_r' contains 2 entries: + Addr: 0x0000000000000548 Offset: 0x000548 Link: 7 (.dynstr) + 000000: Version: 1 File: libc.so.6 Cnt: 1 + 0x0010: Name: GLIBC_2.2.5 Flags: none Version: 3 + 0x0020: Version: 1 File: libpthread.so.0 Cnt: 1 + 0x0030: Name: GLIBC_2.3.2 Flags: none Version: 2 +</code></pre> +<p>The gnu dynamic linker allows to inspect the version processing during runtime +by setting the <code>LD_DEBUG</code> environment variable accordingly.</p> +<pre><code class="language-text"># version: Display version dependencies. +> LD_DEBUG=versions ./tmp + 717904: checking for version `GLIBC_2.2.5' in file /usr/lib/libc.so.6 [0] required by file ./tmp [0] + 717904: checking for version `GLIBC_2.3.2' in file /usr/lib/libpthread.so.0 [0] required by file ./tmp [0] + ... + +# symbols : Display symbol table processing. +# bindings: Display information about symbol binding. +> LD_DEBUG=symbols,bindings ./tmp + ... + 718123: symbol=pthread_cond_wait; lookup in file=./tmp [0] + 718123: symbol=pthread_cond_wait; lookup in file=/usr/lib/libpthread.so.0 [0] + 718123: binding file ./tmp [0] to /usr/lib/libpthread.so.0 [0]: normal symbol `pthread_cond_wait' [GLIBC_2.3.2] +</code></pre> +<h2><a class="header" href="#example-version-script" id="example-version-script">Example: version script</a></h2> +<p>The following shows an example C++ library <code>libfoo</code> which provides the same +symbol multiple times but in different versions.</p> +<pre><code class="language-cpp">// file: libfoo.cc +#include<stdio.h> + +// Bind function symbols to version nodes. +// +// ..@ -> Is the unversioned symbol. +// ..@@.. -> Is the default symbol. + +__asm__(".symver func_v0,func@"); +__asm__(".symver func_v1,func@LIB_V1"); +__asm__(".symver func_v2,func@@LIB_V2"); + +extern "C" { + void func_v0() { puts("func_v0"); } + void func_v1() { puts("func_v1"); } + void func_v2() { puts("func_v2"); } +} + +__asm__(".symver _Z11func_cpp_v1i,_Z8func_cppi@LIB_V1"); +__asm__(".symver _Z11func_cpp_v2i,_Z8func_cppi@@LIB_V2"); + +void func_cpp_v1(int) { puts("func_cpp_v1"); } +void func_cpp_v2(int) { puts("func_cpp_v2"); } + +void func_cpp(int) { puts("func_cpp_v2"); } +</code></pre> +<p>Version script for <code>libfoo</code> which defines which symbols for which versions are +exported from the ELF file.</p> +<pre><code class="language-ld"># file: libfoo.ver +LIB_V1 { + global: + func; + extern "C++" { + "func_cpp(int)"; + }; + local: + *; +}; + +LIB_V2 { + global: + func; + extern "C++" { + "func_cpp(int)"; + }; +} LIB_V1; +</code></pre> +<blockquote> +<p>The <strong>local:</strong> section in <code>LIB_V1</code> is a catch all, that matches any symbol +not explicitly specified, and defines that the symbol is local and therefore +not exported from the ELF file.</p> +</blockquote> +<p>The library <code>libfoo</code> can be linked with the version definitions in <code>libfoo.ver</code> +by passing the version script to the linker with the <code>--version-script</code> flag.</p> +<pre><code class="language-bash">> g++ -shared -fPIC -o libfoo.so libfoo.cc -Wl,--version-script=libfoo.ver +> readelf -W --dyn-syms libfoo.so | c++filt + +Symbol table '.dynsym' contains 14 entries: + Num: Value Size Type Bind Vis Ndx Name + ... + 6: 0000000000000000 0 OBJECT GLOBAL DEFAULT ABS LIB_V1 + 7: 000000000000114b 29 FUNC GLOBAL DEFAULT 13 func_cpp(int)@LIB_V1 + 8: 0000000000001168 29 FUNC GLOBAL DEFAULT 13 func_cpp(int)@@LIB_V2 + 9: 0000000000001185 29 FUNC GLOBAL DEFAULT 13 func_cpp(int)@@LIB_V1 + 10: 0000000000000000 0 OBJECT GLOBAL DEFAULT ABS LIB_V2 + 11: 0000000000001109 22 FUNC GLOBAL DEFAULT 13 func + 12: 000000000000111f 22 FUNC GLOBAL DEFAULT 13 func@LIB_V1 + 13: 0000000000001135 22 FUNC GLOBAL DEFAULT 13 func@@LIB_V2 +</code></pre> +<p>The following program demonstrates how to make use of the different versions:</p> +<pre><code class="language-cpp">// file: main.cc +#include <dlfcn.h> +#include <assert.h> + +// Links against default symbol in the lib.so. +extern "C" void func(); + +int main() { + // Call the default version. + func(); + +#ifdef _GNU_SOURCE + typedef void (*fnptr)(); + + // Unversioned lookup. + fnptr fn_v0 = (fnptr)dlsym(RTLD_DEFAULT, "func"); + // Version lookup. + fnptr fn_v1 = (fnptr)dlvsym(RTLD_DEFAULT, "func", "LIB_V1"); + fnptr fn_v2 = (fnptr)dlvsym(RTLD_DEFAULT, "func", "LIB_V2"); + + assert(fn_v0 != 0); + assert(fn_v1 != 0); + assert(fn_v2 != 0); + + fn_v0(); + fn_v1(); + fn_v2(); +#endif + + return 0; +} +</code></pre> +<p>Compiling and running results in:</p> +<pre><code class="language-bash">> g++ -o main main.cc -ldl ./libfoo.so && ./main +func_v2 +func_v0 +func_v1 +func_v2 +</code></pre> +<h2><a class="header" href="#references" id="references">References</a></h2> +<ul> +<li><a href="https://akkadia.org/drepper/symbol-versioning">ELF Symbol Versioning</a></li> +<li><a href="https://sourceware.org/binutils/docs/ld/VERSION.html">Binutils ld: Symbol Versioning</a></li> +<li><a href="https://refspecs.linuxbase.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/symversion.html">LSB: Symbol Versioning</a></li> +<li><a href="https://akkadia.org/drepper/dsohowto.pdf">How To Write Shared Libraries</a></li> +</ul> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + + <a rel="prev" href="../development/ld.so.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + + + <a rel="next" href="../development/python.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="../development/ld.so.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + + + <a href="../development/python.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; 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