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| <section id="armv7-m-run-time-stack-checking"> |
| <h1>ARMv7-M Run Time Stack Checking<a class="headerlink" href="#armv7-m-run-time-stack-checking" 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/display/NUTTX/ARMv7-M+Run+Time+Stack+Checking">https://cwiki.apache.org/confluence/display/NUTTX/ARMv7-M+Run+Time+Stack+Checking</a></p> |
| </div> |
| <section id="overview"> |
| <h2>Overview<a class="headerlink" href="#overview" title="Permalink to this heading"></a></h2> |
| <p>Nuttx supports facilities to verify the dynamically allocated stacks and fixed |
| stacks used by the tasks and interrupt context running under Nuttx. There are 2 |
| types of stack checking that can be used together or separately.</p> |
| <ol class="arabic simple"> |
| <li><p>The Stack Monitor</p></li> |
| <li><p>Per function Call (ARMV7 Only)</p></li> |
| </ol> |
| <section id="the-stack-monitor"> |
| <h3>The Stack Monitor<a class="headerlink" href="#the-stack-monitor" title="Permalink to this heading"></a></h3> |
| <p>The use of the Stack Monitor application requires that |
| <code class="docutils literal notranslate"><span class="pre">CONFIG_STACK_COLORATION</span></code> be enabled. This compile time option enables the |
| writing of a know pattern <code class="docutils literal notranslate"><span class="pre">STACK_COLOR</span></code> to the stack memory at creation time. |
| In the case of the idle task and interrupt, this is done in the code that runs |
| just after reset at startup. This is known as stack coloring.</p> |
| <p>Once the pattern has been established, the functions <code class="docutils literal notranslate"><span class="pre">up_check_stack</span></code> and |
| friends are used to perform the stack checking by finding the lowest word of |
| the pattern in the allocated stack.</p> |
| <p>The Stack Monitor is enabled with <code class="docutils literal notranslate"><span class="pre">CONFIG_SYSTEM_STACKMONITOR</span></code> which will |
| enable a daemon that will periodically run and check the stack penetration of |
| the tasks running on the system.</p> |
| <p>The stack monitor is good to help size and check the usage of tasks. However it |
| is not really useful to detect, certain kinds, nor the cause of a stack overrun.</p> |
| <p>The reason for this is because a corrupted stack may not be eaten away at. It |
| may have overruns where the stack pointer is set way below the stack bottom in |
| a function call as it allocates local variables on the stack. The code in the |
| function call can then corrupt the memory below the stack bottom, restores the |
| stack pointer and returns to the caller without actually overwriting the |
| coloring at the base of the stack.</p> |
| <p>This brings us the the next method of stack checking.</p> |
| </section> |
| <section id="per-function-call"> |
| <h3>Per function Call<a class="headerlink" href="#per-function-call" title="Permalink to this heading"></a></h3> |
| <p>This method of stack checking leverages the profiler hook mechanism supported |
| by the compiler. Once enabled using <code class="docutils literal notranslate"><span class="pre">CONFIG_ARMV7M_STACKCHECK</span></code>, one register |
| is set aside (<code class="docutils literal notranslate"><span class="pre">R10</span></code> is the default) and the value of the base of stack is |
| saved there (<code class="docutils literal notranslate"><span class="pre">rBS</span></code>). Then every function call will have a preamble and a |
| postamble code added to it. The preamble (<code class="docutils literal notranslate"><span class="pre">__cyg_profile_func_enter</span></code>) checks |
| the current stack pointer, minus a margin of 64 bytes (with an additional 136 |
| bytes for the <code class="docutils literal notranslate"><span class="pre">FP</span></code> registers) against the value in the reserved register |
| <code class="docutils literal notranslate"><span class="pre">rBS</span></code>. If the computed value lies below the value in <code class="docutils literal notranslate"><span class="pre">rBS</span></code> a hard fault is |
| generated. The postamble code (<code class="docutils literal notranslate"><span class="pre">__cyg_profile_func_exit</span></code>) just returns to |
| the caller.</p> |
| <p>The rationale for subtracting the margin can be viewed two ways. If the |
| configuration is not using a separate ISR stack, then the space reserved will |
| accommodate the context save of the CPU and optionally the FPU registers to |
| service an interrupt on the users stack. If the configuration is using a |
| separate ISR stack, some the 64 bytes will accommodate the transition to the |
| interrupt stack and the remaining 60-200 bytes are just margin. Either way |
| stacks should always be allocated with at least 200 bytes of margin.</p> |
| <p>Because of the reserved register rBS contains the current context’s stack base, |
| and rBS is not updated on the entry to an ISR, it is not possible to check the |
| stack penetration for an interrupt with Per function Call stack checking.</p> |
| <p>One thing to consider is the impact on code size and speed this method of stack |
| checking will have. Each function will have two additional call and return |
| instructions added to it. In the execution path of each function, there will be |
| an added set of instructions to perform the preamble and postamble |
| functionality. In a call tree that is nested several layers deep, this can add |
| up. In one particular use, we saw an increase of 30% to 35% additional CPU |
| utilization required to support per function call stack checking.</p> |
| <p>Is this just a debugging tool? One could imagine that in a mission critical |
| application, this might be part of a release build if the code size and speed |
| impact can be tolerated.</p> |
| <p>As of commit 4942867 Register R11 will contain the value that the stack pointer |
| would hit that caused the fault. This can be used to calculate the stack size |
| needed for that task that faulted. To do so, take the difference of R10-R11 and |
| round it up by 12 bytes (The round up is to make up for the 8 byte stack |
| alignment and 4 byte decrease that may happen in the stack allocation) Then add |
| that amount to the failing tasks stack size.</p> |
| </section> |
| <section id="details-for-support-of-per-call-stack-checking"> |
| <h3>Details for Support of Per Call Stack Checking<a class="headerlink" href="#details-for-support-of-per-call-stack-checking" title="Permalink to this heading"></a></h3> |
| <p>Currently only ARMV7 derivatives support Per Call Stack Checking. Support |
| requires the following components:</p> |
| <p>The start function must establish the value in <code class="docutils literal notranslate"><span class="pre">rBS</span></code> (<code class="docutils literal notranslate"><span class="pre">R10</span></code> by default see |
| below). Yet to do this the start function must NOT have the preamble and |
| postamble code added to it. This is accomplished with the use the following |
| gcc attribute:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#ifdef CONFIG_ARMV7M_STACKCHECK</span> |
| <span class="cm">/* we need to get r10 set before we can allow instrumentation calls */</span> |
| |
| <span class="kt">void</span><span class="w"> </span><span class="nf">__start</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span><span class="w"> </span><span class="n">__attribute__</span><span class="w"> </span><span class="p">((</span><span class="n">no_instrument_function</span><span class="p">));</span> |
| <span class="cp">#endif</span> |
| </pre></div> |
| </div> |
| <p>…</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span><span class="w"> </span><span class="nf">__start</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> |
| <span class="p">{</span> |
| <span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="o">*</span><span class="n">src</span><span class="p">;</span> |
| <span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="o">*</span><span class="n">dest</span><span class="p">;</span> |
| |
| <span class="cp">#ifdef CONFIG_ARMV7M_STACKCHECK</span> |
| |
| <span class="w"> </span><span class="cm">/* Set the stack limit before we attempt to call any functions */</span> |
| |
| <span class="w"> </span><span class="n">__asm__</span><span class="w"> </span><span class="k">volatile</span><span class="w"> </span><span class="p">(</span><span class="s">"sub r10, sp, %0"</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="s">"r"</span><span class="w"> </span><span class="p">(</span><span class="n">CONFIG_IDLETHREAD_STACKSIZE</span><span class="w"> </span><span class="mi">-64</span><span class="p">)</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="p">);</span> |
| <span class="cp">#endif</span> |
| </pre></div> |
| </div> |
| <p>The minus 64 is setting the limit 64 bytes above the bottom of the stack. Note: |
| This may be adding another 64 bytes of margin</p> |
| <p>For the creation of a task’s context the following code is needed to set up |
| <code class="docutils literal notranslate"><span class="pre">rBS</span></code></p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span><span class="w"> </span><span class="nf">up_initial_state</span><span class="p">(</span><span class="k">struct</span><span class="w"> </span><span class="nc">tcb_s</span><span class="w"> </span><span class="o">*</span><span class="n">tcb</span><span class="p">)</span> |
| <span class="p">{</span> |
| <span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">xcptcontext</span><span class="w"> </span><span class="o">*</span><span class="n">xcp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">&</span><span class="n">tcb</span><span class="o">-></span><span class="n">xcp</span><span class="p">;</span> |
| |
| <span class="w"> </span><span class="cm">/* Initialize the initial exception register context structure */</span> |
| |
| <span class="w"> </span><span class="n">memset</span><span class="p">(</span><span class="n">xcp</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="k">struct</span><span class="w"> </span><span class="nc">xcptcontext</span><span class="p">));</span> |
| |
| <span class="w"> </span><span class="cm">/* Save the initial stack pointer */</span> |
| |
| <span class="w"> </span><span class="n">xcp</span><span class="o">-></span><span class="n">regs</span><span class="p">[</span><span class="n">REG_SP</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="kt">uint32_t</span><span class="p">)</span><span class="n">tcb</span><span class="o">-></span><span class="n">adj_stack_ptr</span><span class="p">;</span> |
| |
| <span class="cp">#ifdef CONFIG_ARMV7M_STACKCHECK</span> |
| <span class="w"> </span><span class="cm">/* Set the stack limit value */</span> |
| |
| <span class="w"> </span><span class="n">xcp</span><span class="o">-></span><span class="n">regs</span><span class="p">[</span><span class="n">REG_R10</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="kt">uint32_t</span><span class="p">)</span><span class="n">tcb</span><span class="o">-></span><span class="n">stack_alloc_ptr</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">64</span><span class="p">;</span> |
| <span class="cp">#endif</span> |
| </pre></div> |
| </div> |
| <p>And finally up_stackcheck.c needs to be included in the build and the compiler |
| flags set to reserve <code class="docutils literal notranslate"><span class="pre">R10</span></code> and enable the instrumentation.</p> |
| <p>This is done for a given architecture in nuttx/arch/arm/src/<arch>/Make.defs:</p> |
| <div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="cp">ifeq ($(CONFIG_ARMV7M_STACKCHECK),y)</span> |
| <span class="nv">CMN_CSRCS</span><span class="w"> </span><span class="o">+=</span><span class="w"> </span>up_stackcheck.c |
| <span class="cp">endif</span> |
| </pre></div> |
| </div> |
| <p>The compiler flags are added in the nuttx/arch/arm/src/armv7-m/Toolchain.defs</p> |
| <div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="c"># enable precise stack overflow tracking</span> |
| <span class="cp">ifeq ($(CONFIG_ARMV7M_STACKCHECK),y)</span> |
| <span class="nv">INSTRUMENTATIONDEFINES</span><span class="w"> </span><span class="o">=</span><span class="w"> </span>-finstrument-functions<span class="w"> </span>-ffixed-r10 |
| <span class="cp">endif</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="other-considerations"> |
| <h3>Other Considerations<a class="headerlink" href="#other-considerations" title="Permalink to this heading"></a></h3> |
| <p>If using the export build feature of Nuttx: For the runtime stack checking both |
| the Application and Nuttx need to be built with the |
| <code class="docutils literal notranslate"><span class="pre">CONFIG_ARMV7M_STACKCHECK</span></code> option set the same state, enabled or disabled. |
| Any mismatch will created either compile time or runtime issues.</p> |
| </section> |
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