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| |
| <section id="nuttx-flat-binary-format-nxflat"> |
| <h1>NuttX FLAT Binary Format (NXFLAT)<a class="headerlink" href="#nuttx-flat-binary-format-nxflat" title="Permalink to this heading"></a></h1> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>Migrated from: |
| <a class="reference external" href="https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=139630111">https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=139630111</a></p> |
| </div> |
| <section id="overview"> |
| <h2>Overview<a class="headerlink" href="#overview" title="Permalink to this heading"></a></h2> |
| <p>NuttX supports a configurable <a class="reference internal" href="../binfmt.html"><span class="doc">binary loader</span></a> . |
| This binary loader supports loading and executing binary objects from |
| the file system. |
| The NuttX binary loader is capable of supporting multiple binary formats. |
| One of of those binary formats is NXFLAT, the top of this Wiki page.</p> |
| <p>NXFLAT is a customized and simplified version of binary format |
| implemented a few years ago called XFLAT. |
| With the NXFLAT binary format you will be able to do the following:</p> |
| <ul class="simple"> |
| <li><p>Place separately linked programs in a file system, and</p></li> |
| <li><p>Execute those programs by dynamically linking them to the |
| base NuttX code.</p></li> |
| </ul> |
| <p>This allows you to extend the NuttX base code after it has been |
| written into FLASH. |
| One motivation for implementing NXFLAT is support clean CGI under |
| an HTTPD server.</p> |
| <p>This feature is especially attractive when combined with the |
| NuttX ROMFS support: |
| ROMFS allows you to execute programs in place (XIP) in flash |
| without copying anything other than the .data section to RAM. |
| In fact, the initial NXFLAT release only worked on ROMFS. |
| Later extensions also support execution NXFLAT binaries from |
| an SRAM copy as well.</p> |
| <p>This NuttX feature includes:</p> |
| <ul class="simple"> |
| <li><p>A dynamic loader that is built into the NuttX core (See SVN).</p></li> |
| <li><p>Minor changes to RTOS to support position independent code, and</p></li> |
| <li><p>A linker to bind ELF binaries to produce the |
| NXFLAT binary format (See SVN).</p></li> |
| </ul> |
| </section> |
| <section id="toolchain-compatibility-problem"> |
| <h2>Toolchain Compatibility Problem<a class="headerlink" href="#toolchain-compatibility-problem" title="Permalink to this heading"></a></h2> |
| <section id="description"> |
| <h3>Description<a class="headerlink" href="#description" title="Permalink to this heading"></a></h3> |
| <p>NXFLAT flat requires a specific kind of position independence. |
| The ARM family of GCC toolchains has historically supported |
| this method of position independence: All code addresses are |
| accessed relative to the Program Counter (PC) and a special, |
| <cite>PIC register</cite> (usually <code class="docutils literal notranslate"><span class="pre">r10</span></code>) is used to access all data. To |
| load or store a data value, the contents of <code class="docutils literal notranslate"><span class="pre">r10</span></code>, the PIC |
| base, is added to a constant, position-independent offset |
| to produce the absolute address of the data.</p> |
| <img alt="../../_images/nxflat-addressing.png" src="../../_images/nxflat-addressing.png" /> |
| <p>The <cite>Global Offset Table</cite> (GOT) is a special data structure that |
| resides in D-Space. So PIC-base relative addressing may also |
| be specified as GOT-Relative addressing (or <code class="docutils literal notranslate"><span class="pre">GOTOFF</span></code>). The older |
| GCC 4.3.3 GCC compiler, for example, generates <code class="docutils literal notranslate"><span class="pre">GOTOFF</span></code> |
| relocations to the constant strings, like:</p> |
| <div class="highlight-asm notranslate"><div class="highlight"><pre><span></span><span class="nl">.L3:</span> |
| <span class="w"> </span><span class="na">.word</span><span class="w"> </span><span class="no">.LC0</span><span class="p">(</span><span class="no">GOTOFF</span><span class="p">)</span> |
| <span class="w"> </span><span class="na">.word</span><span class="w"> </span><span class="no">.LC1</span><span class="p">(</span><span class="no">GOTOFF</span><span class="p">)</span> |
| <span class="w"> </span><span class="na">.word</span><span class="w"> </span><span class="no">.LC2</span><span class="p">(</span><span class="no">GOTOFF</span><span class="p">)</span> |
| <span class="w"> </span><span class="na">.word</span><span class="w"> </span><span class="no">.LC3</span><span class="p">(</span><span class="no">GOTOFF</span><span class="p">)</span> |
| <span class="w"> </span><span class="na">.word</span><span class="w"> </span><span class="no">.LC4</span><span class="p">(</span><span class="no">GOTOFF</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| <p>Where <code class="docutils literal notranslate"><span class="pre">.LC0</span></code>, <code class="docutils literal notranslate"><span class="pre">.LC1</span></code>, <code class="docutils literal notranslate"><span class="pre">.LC2</span></code>, <code class="docutils literal notranslate"><span class="pre">.LC3</span></code>, and <code class="docutils literal notranslate"><span class="pre">.LC4</span></code> are the labels |
| corresponding to strings in the <code class="docutils literal notranslate"><span class="pre">.rodata.str1.1</span></code> section. One |
| consequence of this is that <code class="docutils literal notranslate"><span class="pre">.rodata</span></code> must reside in D-Space |
| since it will addressed relative to the GOT (see the section |
| entitled “Read-Only Data in RAM” here).</p> |
| <p>The newer 4.6.3 GCC compiler, however, generated PC relative |
| relocations to these same strings:</p> |
| <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>.L2: |
| .word .LC0-(.LPIC0+4) |
| .word .LC1-(.LPIC1+4) |
| .word .LC2-(.LPIC2+4) |
| .word .LC3-(.LPIC4+4) |
| .word .LC4-(.LPIC5+4) |
| </pre></div> |
| </div> |
| <p>These are <cite>PC-Relative</cite> relocations. This means that the string |
| data is address not by an offset relative to the PIC register |
| (<code class="docutils literal notranslate"><span class="pre">r10</span></code>) but to the program count (PC). This is good and bad. This |
| is good because it means that <code class="docutils literal notranslate"><span class="pre">.rodata.str1.1</span></code> must now can now |
| reside in FLASH with <code class="docutils literal notranslate"><span class="pre">.text</span></code> and can be accessed using PC-relative |
| addressing. That can be accomplished by simply moving the <code class="docutils literal notranslate"><span class="pre">.rodata</span></code> |
| from the <code class="docutils literal notranslate"><span class="pre">.data</span></code> section to the <code class="docutils literal notranslate"><span class="pre">.text</span></code> section in the linker script. |
| The NXFLAT linker script is located at |
| <code class="docutils literal notranslate"><span class="pre">nuttx/binfmt/libnxflat/gnu-nxflat-?.ld</span></code>. <strong>NOTE</strong>: There are two |
| linker scripts located at <code class="docutils literal notranslate"><span class="pre">nuttx/binfmt/libnxflat/</span></code>:</p> |
| <ol class="arabic simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">binfmt/libnxflat/gnu-nxflat-gotoff.ld.</span></code> Older versions |
| of GCC (at least up to GCC 4.3.3), use GOT-relative |
| addressing to access RO data. In that case, read-only |
| data (<code class="docutils literal notranslate"><span class="pre">.rodata</span></code>) must reside in D-Space and this linker |
| script should be used.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">binfmt/libnxflat/gnu-nxflat-pcrel.ld.</span></code> Newer versions |
| of GCC (at least as of GCC 4.6.3), use PC-relative |
| addressing to access RO data. In that case, read-only |
| data (<code class="docutils literal notranslate"><span class="pre">.rodata</span></code>) must reside in I-Space and this linker |
| script should be used.</p></li> |
| </ol> |
| <p>But this is very bad because a lot of NXFLAT is now broken. |
| For it appears that not just constant strings, but that all |
| data may now be referenced using PC-relative addressing to |
| .bss and .data values. I do not yet know the extent of this |
| problem or the direction that GCC is going but certainly |
| version 4.6.3 cannot be used with NXFLAT.</p> |
| <p>The workaround for now is to use the older, 4.3.3 OABI |
| compiler. In the long run, this might spell the end |
| to NXFLAT.</p> |
| </section> |
| <section id="update-restored-gcc-support"> |
| <h3>Update: Restored GCC Support<a class="headerlink" href="#update-restored-gcc-support" title="Permalink to this heading"></a></h3> |
| <p>This post was pointed out by Michael Jung:</p> |
| <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>MCU:<span class="w"> </span>STM32F4<span class="w"> </span><span class="o">(</span>ARM<span class="w"> </span>Cortex<span class="w"> </span>M4<span class="o">)</span> |
| Build<span class="w"> </span>environment:<span class="w"> </span>arm-none-eabi-gcc<span class="w"> </span><span class="m">4</span>.8.4<span class="w"> </span><span class="m">20140725</span> |
| |
| My<span class="w"> </span>goal<span class="w"> </span>is<span class="w"> </span>to<span class="w"> </span>build<span class="w"> </span>an<span class="w"> </span>image<span class="w"> </span>that<span class="w"> </span>can<span class="w"> </span>be<span class="w"> </span>run<span class="w"> </span>from<span class="w"> </span>any<span class="w"> </span>properly-aligned |
| offset<span class="w"> </span><span class="k">in</span><span class="w"> </span>internal<span class="w"> </span>flash<span class="w"> </span><span class="o">(</span>i.e.,<span class="w"> </span>position-independent<span class="o">)</span>.<span class="w"> </span>I<span class="w"> </span>found<span class="w"> </span>the |
| following<span class="w"> </span><span class="nb">set</span><span class="w"> </span>of<span class="w"> </span>gcc<span class="w"> </span>flags<span class="w"> </span>that<span class="w"> </span>achieves<span class="w"> </span>this<span class="w"> </span>goal: |
| |
| <span class="w"> </span><span class="c1"># Generate position independent code.</span> |
| <span class="w"> </span>-fPIC |
| |
| <span class="w"> </span><span class="c1"># Access bss via the GOT.</span> |
| <span class="w"> </span>-mno-pic-data-is-text-relative |
| |
| <span class="w"> </span><span class="c1"># GOT is not PC-relative; store GOT location in a register.</span> |
| <span class="w"> </span>-msingle-pic-base |
| |
| <span class="w"> </span><span class="c1"># Store GOT location in r9.</span> |
| <span class="w"> </span>-mpic-register<span class="o">=</span>r9 |
| </pre></div> |
| </div> |
| <p>Reference: <a class="reference external" href="https://gcc.gnu.org/ml/gcc-help/2015-07/msg00027.html">https://gcc.gnu.org/ml/gcc-help/2015-07/msg00027.html</a></p> |
| <p>Michael has verified that <code class="docutils literal notranslate"><span class="pre">-mno-pic-data-is-text-relative</span></code> is, |
| indeed, a solution to the above NXFLAT problem in newer |
| compilers. You simply need to modify the board Make.defs |
| file like:</p> |
| <ol class="arabic simple"> |
| <li><p>ARCHPICFLAGS = -fpic -msingle-pic-base -mpic-register=r10</p></li> |
| </ol> |
| <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>+ARCHPICFLAGS<span class="w"> </span><span class="o">=</span><span class="w"> </span>-fpic<span class="w"> </span>-msingle-pic-base<span class="w"> </span>-mpic-register<span class="o">=</span>r10<span class="w"> </span>-mno-pic-data-is-text-relative |
| </pre></div> |
| </div> |
| <p>NOTE the minor difference from the post: NuttX uses <code class="docutils literal notranslate"><span class="pre">r10</span></code> as |
| the PIC base register by default in all configurations.</p> |
| <p>See this <a class="reference external" href="https://groups.google.com/forum/">thread</a> for additional information.</p> |
| </section> |
| <section id="references"> |
| <h3>References<a class="headerlink" href="#references" title="Permalink to this heading"></a></h3> |
| <ul class="simple"> |
| <li><p><a class="reference internal" href="../nxflat.html"><span class="doc">NXFLAT</span></a></p></li> |
| <li><p><a class="reference external" href="http://xflat.sourceforge.net/">XFLATFLAT</a></p></li> |
| <li><p><a class="reference external" href="http://retired.beyondlogic.org/uClinux/bflt.htm">FLAT</a></p></li> |
| <li><p><a class="reference external" href="http://romfs.sourceforge.net/">ROMFS</a></p></li> |
| </ul> |
| </section> |
| </section> |
| </section> |
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