ARMv7-A 那些事 - 7.栈回溯浅析
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}</style><title>ARMv7-A 那些事 - 7.栈回溯浅析</title><p>在嵌入式开发过程中，经常需要对代码进行调试来解决各种各样的问题，常用的调试手段有：</p><p>(1)、开发环境搭配硬件仿真器进行在线调试。优点：调试过程中能够清楚的知道各个寄存器的值以及各个变量的值，程序的执行流程也能够一目了然。缺点：板卡需要引出硬件仿真器的连接口，并且需要购买硬件仿真器。</p><p>(2)、通过调试串口打印信息梳理程序的执行流程，结合代码分析问题产生的原因。优点：足够简单，通过增加较多的打印信息来分析问题出现的位置，再结合代码分析问题产生的原因。缺点：没法准确的定位问题产生的位置和原因。</p><p>(3)、在应用或者操作系统死机的时候，根据操作系统输出的异常栈信息进行分析，再结合镜像或者应用的反汇编代码进行定位。通常这种方法和方法(2)结合使用。</p><p>本文主要简单的讲讲栈回溯，对于以后去理解操作系统的异常栈处理打个基础吧。</p><p>ARM处理器的栈回溯主要有两种方式：一种是基于栈帧寄存器（FP）的栈回溯，另一种是unwind形式的栈回溯。本文主要讲讲基于栈帧寄存器（FP）的栈回溯。</p><h2 id="栈回溯相关寄存器">栈回溯相关寄存器</h2><p>在栈回溯过程中，主要涉及如下寄存器：</p><p>R15：又叫程序计数器（Program Counter）PC，PC主要用于存放CPU取指的地址。</p><p>R14：又叫链接寄存器（Link register）LR，LR主要用于存放函数的返回地址，即当函数返回时，知道自己该回到哪儿去继续运行。</p><p>R13：又叫堆栈指针寄存器（Stack pointer）SP，SP通常用于保存堆栈地址，在使用入栈和出栈指令时，SP中的堆栈地址会自动的更新。</p><p>R12：又叫内部过程调用暂存寄存器（Intra-Procedure-call scratch register）IP，主要用于暂存SP。</p><p>R11：又叫帧指针寄存器（Frame pointer）FP，通常指向一个函数的栈帧底部，表示一个函数栈的开始位置。</p><h2 id="arm栈帧结构">ARM栈帧结构</h2><p>依据AAPCS (ARM Archtecture Procedure Call Standard)规范，当调用子函数时，子函数一开始的代码总是会执行压栈操作来保留父函数的相关信息，压栈步骤示例如下所示：</p><pre class="md-fences md-end-block ty-contain-cm modeLoaded md-focus" spellcheck="false" lang="c">mov ip, sp
push {fp, ip, lr, pc}
sub fp, ip, #4
sub sp, sp, #16
...</pre><p>每个函数都有自己的栈空间，这一部分称为栈帧。栈帧在函数被调用的时候创建，在函数返回后销毁。每个函数的栈帧是由SP寄存器和FP寄存器来界定的，ARM栈帧结构典型示意图如下所示：</p><p style="text-align:center"><img src="/uploads/AilsonJack/2023.10.14/112414860569798.png" onclick="preview_image('/uploads/AilsonJack/2023.10.14/112414860569798.png')"/></p><p>上图描述的栈帧，main函数和func1函数的示意代码如下：</p><pre class="brush:cpp;toolbar:false;" spellcheck="false" lang="c" style="break-inside: unset;">int func1(int p1, int p2, int p3, int p4, int p5)
{
int i;
int j;
i = 0xf3;
j = 0xf6;
return 0;
}
int main(int argc, char *argv[])
{
int i;
int j;
i = 0x33;
j = 0x66;
func1(0xa1, 0xa2, 0xa3, 0xa4, 0xa5);
return 0;
}</pre><p>每个函数的栈帧中都会保存调用该函数之前的PC、LR、SP、FP寄存器的值；如果函数具有参数并且函数内部使用了局部变量，那么函数栈帧中也会保存函数的参数和局部变量；如果被调用的子函数参数过多，那么多余的参数会通过父函数的栈进行传递。比如func1函数的参数p5通过main函数的栈帧进行传递的。（注：编译器的版本不同，函数栈帧中参数和局部变量的压栈顺序可能不同，PC，LR，SP和FP这4个寄存器的压栈顺序一般是固定的）</p><p>函数栈帧中的PC和LR均指向代码段，PC表示执行入栈指令时CPU正在取指的地址，LR表示当前函数返回后继续执行的地址。</p><h2 id="栈回溯原理">栈回溯原理</h2><p>在栈回溯的过程中，我们主要利用FP寄存器进行栈回溯。通过FP就可以知道当前函数的栈底，从而可以找到存储在栈帧中的LR寄存器的数据，这个数据就是函数的返回地址。同时也可以找到保存在函数栈帧中的上一级函数FP的数据，这个数据指向了上一级函数的栈底，按照同样的方法可以找出上一级函数栈帧中存储的LR和FP数据，就知道哪个函数调用了上一级函数以及这个函数的栈底地址。这就是栈回溯的流程，整个流程以FP为核心，依次找出每个函数栈帧中存储的LR和FP数据，计算出函数返回地址和上一级函数栈底地址，从而找出每一级函数调用关系。</p><h2 id="栈回溯编译选项">栈回溯编译选项</h2><p>当gcc的编译选项带有<code>-mapcs-frame</code>时，编译出来的代码能够将PC，LR，SP和FP寄存器的值压入函数的栈帧中。默认情况下gcc的编译选项为<code>-mno-apcs-frame</code>，此时编译出来的代码不一定会将PC，LR，SP和FP这四个寄存器的值压入函数的栈帧中，可能只会将LR和FP寄存器的值压入函数的栈帧中。关于<code>-mapcs-frame</code>选项，gcc的手册描述如下：</p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="">Generate a stack frame that is compliant with the ARM Procedure Call Standard for all
functions, even if this is not strictly necessary for correct execution of the code.
Specifying ‘-fomit-frame-pointer’ with this option causes the stack frames not to be
generated for leaf functions. The default is ‘-mno-apcs-frame’.
This option is deprecated.</pre><p>我这里使用的gcc信息如下：</p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="">$ arm-none-eabi-gcc -v
...
gcc version 10.3.1 20210824 (release) (GNU Arm Embedded Toolchain 10.3-2021.10)</pre><p>虽然gcc手册上说<code>-mapcs-frame</code>选项被废弃了，但是只有添加了该选项，编译出来的代码才会将PC，LR，SP和FP寄存器的值压入函数的栈帧中。</p><p>我这里编译代码仍然使用<code>-mapcs-frame</code>选项，有知道该选项对应的新的栈帧配置选项的兄弟可以告知我一下。</p><h2 id="栈回溯示例">栈回溯示例</h2><p>根据前面的内容，这里简单的写了一个栈回溯的示例，函数调用流程为：<code>main -> test_a -> test_b -> test_c</code>。</p><p>函数的源代码如下：</p><pre class="brush:cpp;toolbar:false;" spellcheck="false" lang="c" style="break-inside: unset;">int test_a(int arg0, int arg1, int arg2, int arg3, int arg4)
{
int a;
a = 0xff11;
test_b(0xbb00);
return a;
}
int test_b(int arg0)
{
int b;
b = 0xff22;
test_c(0xcc00);
return b;
}
int test_c(int arg0)
{
int c;
c = 0xff33;
return c;
}
int main(void)
{
int val;
val = 0xff00;
test_a(0xaa00, 0xaa11, 0xaa22, 0xaa33, 0xaa44);
return 0;
}</pre><p>上述函数的反汇编内容如下：</p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c" style="break-inside: unset;">80002164 <test_a>:
int test_a(int arg0, int arg1, int arg2, int arg3, int arg4)
{
80002164: e1a0c00d mov ip, sp
80002168: e92dd800 push {fp, ip, lr, pc}
8000216c: e24cb004 sub fp, ip, #4
80002170: e24dd018 sub sp, sp, #24
80002174: e50b0018 str r0, [fp, #-24] ; 0xffffffe8
80002178: e50b101c str r1, [fp, #-28] ; 0xffffffe4
8000217c: e50b2020 str r2, [fp, #-32] ; 0xffffffe0
80002180: e50b3024 str r3, [fp, #-36] ; 0xffffffdc
int a;
a = 0xff11;
80002184: e30f3f11 movw r3, #65297 ; 0xff11
80002188: e50b3010 str r3, [fp, #-16]
test_b(0xbb00);
8000218c: e3a00cbb mov r0, #47872 ; 0xbb00
80002190: eb000003 bl 800021a4 <test_b>
return a;
80002194: e51b3010 ldr r3, [fp, #-16]
}
80002198: e1a00003 mov r0, r3
8000219c: e24bd00c sub sp, fp, #12
800021a0: e89da800 ldm sp, {fp, sp, pc}
800021a4 <test_b>:
int test_b(int arg0)
{
800021a4: e1a0c00d mov ip, sp
800021a8: e92dd800 push {fp, ip, lr, pc}
800021ac: e24cb004 sub fp, ip, #4
800021b0: e24dd010 sub sp, sp, #16
800021b4: e50b0018 str r0, [fp, #-24] ; 0xffffffe8
int b;
b = 0xff22;
800021b8: e30f3f22 movw r3, #65314 ; 0xff22
800021bc: e50b3010 str r3, [fp, #-16]
test_c(0xcc00);
800021c0: e3a00b33 mov r0, #52224 ; 0xcc00
800021c4: eb000003 bl 800021d8 <test_c>
return b;
800021c8: e51b3010 ldr r3, [fp, #-16]
}
800021cc: e1a00003 mov r0, r3
800021d0: e24bd00c sub sp, fp, #12
800021d4: e89da800 ldm sp, {fp, sp, pc}
800021d8 <test_c>:
int test_c(int arg0)
{
800021d8: e1a0c00d mov ip, sp
800021dc: e92dd800 push {fp, ip, lr, pc}
800021e0: e24cb004 sub fp, ip, #4
800021e4: e24dd010 sub sp, sp, #16
800021e8: e50b0018 str r0, [fp, #-24] ; 0xffffffe8
int c;
c = 0xff33;
800021ec: e30f3f33 movw r3, #65331 ; 0xff33
800021f0: e50b3010 str r3, [fp, #-16]
return c;
800021f4: e51b3010 ldr r3, [fp, #-16]
}
800021f8: e1a00003 mov r0, r3
800021fc: e24bd00c sub sp, fp, #12
80002200: e89da800 ldm sp, {fp, sp, pc}
80002204 <main>:
int main(void)
{
80002204: e1a0c00d mov ip, sp
80002208: e92dd800 push {fp, ip, lr, pc}
8000220c: e24cb004 sub fp, ip, #4
80002210: e24dd010 sub sp, sp, #16
int val;
val = 0xff00;
80002214: e3a03cff mov r3, #65280 ; 0xff00
80002218: e50b3010 str r3, [fp, #-16]
test_a(0xaa00, 0xaa11, 0xaa22, 0xaa33, 0xaa44);
8000221c: e30a3a44 movw r3, #43588 ; 0xaa44
80002220: e58d3000 str r3, [sp]
80002224: e30a3a33 movw r3, #43571 ; 0xaa33
80002228: e30a2a22 movw r2, #43554 ; 0xaa22
8000222c: e30a1a11 movw r1, #43537 ; 0xaa11
80002230: e3a00caa mov r0, #43520 ; 0xaa00
80002234: ebffffca bl 80002164 <test_a>
return 0;
80002238: e3a03000 mov r3, #0
}
8000223c: e1a00003 mov r0, r3
80002240: e24bd00c sub sp, fp, #12
80002244: e89da800 ldm sp, {fp, sp, pc}</pre><p>当程序运行到<code>test_c()</code>函数的<code>return c;</code>代码处时，FP的值为0x9FDFFF94，此时内存数据如下：</p><p style="text-align:center"><img src="/uploads/AilsonJack/2023.10.14/112414248826393.png" onclick="preview_image('/uploads/AilsonJack/2023.10.14/112414248826393.png')"/></p><p><code>test_c()</code>函数的栈底为0x9FDFFF94，可以得到<code>test_c()</code>函数栈帧中LR为0x800021C8、FP为0x9FDFFFB4，LR是<code>test_c()</code>函数执行完成后的返回地址，与反汇编代码中<code>test_b()</code>函数调用完<code>test_c()</code>之后的下一个执行地址一致：</p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c">800021c0: e3a00b33 mov r0, #52224 ; 0xcc00
800021c4: eb000003 bl 800021d8 <test_c>
return b;
800021c8: e51b3010 ldr r3, [fp, #-16] // test_c() 函数返回后继续执行的地址</pre><p>FP为0x9FDFFFB4表示<code>test_b()</code>函数的栈底为0x9FDFFFB4，有了<code>test_b()</code>函数的栈底就可以得到<code>test_b()</code>函数栈帧中LR为0x80002194、FP为0x9FDFFFDC，从而知道<code>test_b()</code>函数执行完成后的返回地址以及<code>test_a()</code>函数的栈底，依次逐级回溯，就可以知道程序的整个运行流程了。</p><p>在栈回溯的过程中我们可以利用addr2line工具辅助我们对程序执行流程的分析。</p><!--!doctype-->
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