ARM 架構 dump_stack 實現分析(3.0 printk %pS選項實現)

上篇提到了函數：

void dump_backtrace_entry(unsigned long where, unsigned long from, unsigned long frame)
{
#ifdef CONFIG_KALLSYMS
        //記住%pS是關鍵  printk 與 普通printf最大的不同
        //慚愧啊，現在才知道此選項
	printk("symbol<%08lx>] (%pS) from [<%08lx>] (%pS)\n", where, (void *)where, from, (void *)from);
#else
	printk("Function entered at [<%08lx>] from [<%08lx>]\n", where, from);
#endif
	if (in_exception_text(where))
		dump_mem("", "Exception stack", frame + 4, frame + 4 + sizeof(struct pt_regs));
}

實際打印的卻類似下面的語句：

symbol<bf00c0c4>] (handler_pre+0x0/0x19c [kk]) from [<c063d174>] (kprobe_handler+0x194/0x234)

明明只是傳入了一個PC指針而已，卻可以打印出函數名字及偏移量。

查看了源碼，發現是printk的功勞。

參考： kernel/lib/vsprintf
              kernel/kernel/printk

回顧下printk實現：
printk
   -->vprintk
     -->vsnprintf (格式話，及做更多功能)

* This function follows C99 vsnprintf, but has some extensions:
 * %pS output the name of a text symbol with offset    關鍵在這兩個格式化選項
 * %ps output the name of a text symbol without offset
 * %pF output the name of a function pointer with its offset
 * %pf output the name of a function pointer without its offset
 * %pB output the name of a backtrace symbol with its offset
 * %pR output the address range in a struct resource with decoded flags
 * %pr output the address range in a struct resource with raw flags
 * %pM output a 6-byte MAC address with colons
 * %pm output a 6-byte MAC address without colons
 * %pI4 print an IPv4 address without leading zeros
 * %pi4 print an IPv4 address with leading zeros
 * %pI6 print an IPv6 address with colons
 * %pi6 print an IPv6 address without colons
 * %pI6c print an IPv6 address as specified by RFC 5952
 * %pU[bBlL] print a UUID/GUID in big or little endian using lower or upper
 *   case.
 * %n is ignored

int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
{
  ...
    case FORMAT_TYPE_PTR:
                        //格式化爲函數名+偏移量
   str = pointer(fmt+1, str, end, va_arg(args, void *),
          spec);
                        //傳入的必須是函數指針或地址 （text段）
   while (isalnum(*fmt))
    fmt++;
   break;
}

char *pointer(const char *fmt, char *buf, char *end, void *ptr,
	      struct printf_spec spec)
{
        ......
	switch (*fmt) {
	case 'S':
	case 's':
	case 'B':
		return symbol_string(buf, end, ptr, spec, *fmt);
}

char *symbol_string(char *buf, char *end, void *ptr,
		    struct printf_spec spec, char ext)
{
	unsigned long value = (unsigned long) ptr;
        // 函數名不能超過此長度，否則可能數組越界，導致奇怪問題的產生
	char sym[KSYM_SYMBOL_LEN];
        //真正去查找函數名的實現
	kallsyms_lookup(value, NULL, NULL, NULL, sym);
	return string(buf, end, sym, spec);
}

參考： kernel/kernel/kallsyms.c
呵呵，有時間好好看看kallsyms_lookup的實現