简介

应用代码不太健壮时, 会遇到程序 crash 情况, 通常可通过 PC 寄存器, A0 寄存器, EXCCAUSE 寄存器和 backtrace 来初步定位问题.
例如截图中 crash 信息:

通过如下方式定位分析问题时, 旨在让读者能够初步感知 crash 时的现场. 不代表程序 100% crash 在分析出的位置上.

通过 backtrace 定位

如上截图所示, 黄色打印是 backtrace 信息, 即程序执行时的调用栈.
从 backtrace 中初步定位到问题出现在 user_main.c 的 21 行附近, 即如图的 printf 可能会有问题.

通过 PC/A0 寄存器定位

通过 PC/A0 寄存器中信息通常也可以初步看到 crash 附近代码位置.
PC 判断 crash 原因参考:

1.PC 寄存器值若为 0x00000000, 可能是代码中执行某个空 callback, 或执行调用某个为空的函数指针.
2.PC 寄存器值若为野指针, 例如 0x80001210, 可能是内存踩踏. 一旦踩踏到栈底的 PC, 形如 retw.n 就会弹出野指针.

PC 野指针: 笔者认为指不在正常 IRAM 范围内的指针.
ESP8266 正常 IRAM 地址为 0x40100000-0x40201010+appsize(可在 esp8266.ld 中查看);
ESP32 正常 IRAM 地址为 0x40080000-0x40400000(可在 esp32.ld 中查看).

内存踩踏原因很多, 例如:

• 使用不安全的 APIs, strcat(), strcpy(), sprintf() 等
• 内存操作越界, memset, memcpy, memmove
• 边界问题: 数组操作越界, strlen 写错为 sizeof
• task stack 过小, 导致程序执行时越界
• 函数返回局部变量引起的惨案
• 操作已经 free 过的指针

3.PC 寄存器值看起来正常, 例如 0x40201234
第一种是 PC 寄存器值真的正常, 这种情况需看下 PC 寄存器值对应的函数.
第二种是 PC 寄存器值看起来正常, 实际 PC 寄存器值无效.
例如 PC 指向一個 flash 上函数, 而刚好在执行此函数前, cache 被禁用(通常是 flash 相关操作), 则也会出现 因无法访问此函数而 crash.

通过 PC/A0 寄存器定位具体方法如下:
进入应用编译目录, 通过下面命令进行反汇编.
ESP8266:

cd build
xtensa-lx106-elf-objdump -S build/xxx.elf > a.S

ESP32:

cd build
xtensa-esp32-elf-objdump -S build/xxx.elf > a.S

• -S : 展示源代码和反汇编代码

1. 对 PC/A0 寄存器作用了解不深读者, 可以看下计算机组成原理或相关书籍.
2. xtensa-lx106-elf-objdump/xtensa-esp32-elf-objdump 和 linux 上 objdump 是几乎一致的.
3. 可通过 xtensa-lx106-elf-objdump --help/xtensa-esp32-elf-objdump --help 查看详细说明.

通过查看 a.S 文件中的 PC 寄存器和 A0 寄存器信息, 这里以 PC: 0x40228115 为例:

如图所示:
PC: 0x40228115 指向 printf 的反汇编代码, 前两条汇编指令对应: 从内存地址 0 读取值到寄存器 a3.
按照 C 入参规则, 即对应 *p 入参错误, 由于 p 为 NULL, 而读取 0x0 地址内容是属于非法读取, 因此程序 crash.

通过 EXCCAUSE 寄存器定位

EXCCAUSE寄存器: 即 exception causes, 程序异常时, 该寄存器会保存 crash 原因, 供读者参考.

如本文简介中截图所示:
EXCCAUSE: 0x0000001c 对应的是 LoadProhibitedCause, 即一个加载引用了一个映射了不允许加载的属性的页.
其他 EXCCAUSE Code 可通过查看如下 Exception Causes.

Exception Causes

EXCCAUSE Code	Cause Name	Cause Description [Required Option]	EXCVADDR Loaded
0	IllegalInstructionCause	Illegal instruction [Exception Option]	No
1	SyscallCause	SYSCALL instruction [Exception Option]	No
2	InstructionFetchErrorCause	Processor internal physical address or data error during instruction fetch [Exception Option]	Yes
3	LoadStoreErrorCause	Processor internal physical address or data error during load or store [Exception Option]	Yes
4	Level1InterruptCause	Level-1 interrupt as indicated by set level-1 bits in the INTERRUPT register [ Interrupt Option ]	No
5	AllocaCause	MOVSP instruction, if caller’s registers are not in the register file [ Windowed Register Option]	No
6	IntegerDivideByZeroCause	QUOS, QUOU, REMS, or REMU divisor operand is zero [ 32-bit Integer Divide Option]	No
7	/	Reserved for Tensilica	/
8	PrivilegedCause	Attempt to execute a privileged operation when CRING ≠ 0 [MMU Option]	No
9	LoadStoreAlignmentCause	Load or store to an unaligned address [Unaligned Exception Option]	Yes
10…11	/	Reserved for Tensilica	/
12	InstrPIFDataErrorCause	PIF data error during instruction fetch [Processor Interface Option]	Yes
13	LoadStorePIFDataErrorCause	Synchronous PIF data error during LoadStore access [Processor Interface Option]	Yes
14	InstrPIFAddrErrorCause	PIF address error during instruction fetch [Processor Interface Option]	Yes
15	LoadStorePIFAddrErrorCause	Synchronous PIF address error during LoadStore access [Processor Interface Option]	Yes
16	InstTLBMissCause	Error during Instruction TLB refill [MMU Option]	Yes
17	InstTLBMultiHitCause	Multiple instruction TLB entries matched [MMU Option]	Yes
18	InstFetchPrivilegeCause	An instruction fetch referenced a virtual address at a ring level less than CRING [MMU Option]	Yes
19	/	Reserved for Tensilica	/
20	InstFetchProhibitedCause	An instruction fetch referenced a page mapped with an attribute that does not permit instruction fetch [Region Protection Option or MMU Option]	Yes
21…23	/	Reserved for Tensilica	/
24	LoadStoreTLBMissCause	Error during TLB refill for a load or store [MMU Option]	Yes
25	LoadStoreTLBMultiHitCause	Multiple TLB entries matched for a load or store [MMU Option]	Yes
26	LoadStorePrivilegeCause	A load or store referenced a virtual address at a ring level less than CRING [MMU Option]	Yes
27	/	Reserved for Tensilica	/
28	LoadProhibitedCause	A load referenced a page mapped with an attribute that does not permit loads [Region Protection Option or MMU Option]	Yes
29	StoreProhibitedCause	A store referenced a page mapped with an attribute that does not permit stores [Region Protection Option or MMU Option]	Yes
30…31	/	Reserved for Tensilica	/
32…39	CoprocessornDisabled	Coprocessor n instruction when cpn disabled. n varies 0…7 as the cause varies 32…39 [Coprocessor Option]	No
40…63	/	Reserved for Tensilica	/