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387 lines
14 KiB
Markdown
387 lines
14 KiB
Markdown
# 3.8. Delve调试器
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目前Go语言支持GDB、LLDB和Delve几种调试器。其中GDB是最早支持的调试工具，LLDB是macOS系统推荐的标准调试工具。但是GDB和LLDB对Go语言的专有特性都缺乏很大支持，而只有Delve是专门为Go语言设计开发的调试工具。而且Delve本身也是采用Go语言开发，对Windows平台也提供了一样的支持。本节我们基于Delve简单解释如何调试Go汇编程序。
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## Delve入门
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首先根据官方的文档正确安装Delve调试器。我们会先构造一个简单的Go语言代码，用于熟悉下Delve的简单用法。
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创建main.go文件，main函数先通过循初始化一个切片，然后输出切片的内容：
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```go
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package main
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import (
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"fmt"
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)
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func main() {
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nums := make([]int, 5)
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for i := 0; i < len(nums); i++ {
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nums[i] = i * i
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}
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fmt.Println(nums)
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}
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```
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命令行进入包所在目录，然后输入`dlv debug`命令进入调试：
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```
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$ dlv debug
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Type 'help' for list of commands.
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(dlv)
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```
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输入help命令可以查看到Delve提供的调试命令列表：
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```
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(dlv) help
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The following commands are available:
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args ------------------------ Print function arguments.
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break (alias: b) ------------ Sets a breakpoint.
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breakpoints (alias: bp) ----- Print out info for active breakpoints.
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clear ----------------------- Deletes breakpoint.
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clearall -------------------- Deletes multiple breakpoints.
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condition (alias: cond) ----- Set breakpoint condition.
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config ---------------------- Changes configuration parameters.
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continue (alias: c) --------- Run until breakpoint or program termination.
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disassemble (alias: disass) - Disassembler.
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down ------------------------ Move the current frame down.
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exit (alias: quit | q) ------ Exit the debugger.
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frame ----------------------- Set the current frame, or execute command on a different frame.
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funcs ----------------------- Print list of functions.
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goroutine ------------------- Shows or changes current goroutine
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goroutines ------------------ List program goroutines.
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help (alias: h) ------------- Prints the help message.
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list (alias: ls | l) -------- Show source code.
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locals ---------------------- Print local variables.
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next (alias: n) ------------- Step over to next source line.
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on -------------------------- Executes a command when a breakpoint is hit.
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print (alias: p) ------------ Evaluate an expression.
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regs ------------------------ Print contents of CPU registers.
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restart (alias: r) ---------- Restart process.
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set ------------------------- Changes the value of a variable.
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source ---------------------- Executes a file containing a list of delve commands
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sources --------------------- Print list of source files.
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stack (alias: bt) ----------- Print stack trace.
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step (alias: s) ------------- Single step through program.
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step-instruction (alias: si) Single step a single cpu instruction.
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stepout --------------------- Step out of the current function.
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thread (alias: tr) ---------- Switch to the specified thread.
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threads --------------------- Print out info for every traced thread.
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trace (alias: t) ------------ Set tracepoint.
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types ----------------------- Print list of types
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up -------------------------- Move the current frame up.
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vars ------------------------ Print package variables.
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whatis ---------------------- Prints type of an expression.
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Type help followed by a command for full documentation.
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(dlv)
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```
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每个Go程序的入口是main.main函数，我们可以用break在此设置一个断点：
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```
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(dlv) break main.main
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Breakpoint 1 set at 0x10ae9b8 for main.main() ./main.go:7
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```
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然后通过breakpoints查看已经设置的所有断点：
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```
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(dlv) breakpoints
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Breakpoint unrecovered-panic at 0x102a380 for runtime.startpanic() /usr/local/go/src/runtime/panic.go:588 (0)
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print runtime.curg._panic.arg
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Breakpoint 1 at 0x10ae9b8 for main.main() ./main.go:7 (0)
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```
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我们发现除了我们自己设置的main.main函数断点外，Delve内部已经为panic异常函数设置了一个断点。
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通过vars命令可以查看全部包级的变量。因为最终的目标程序可能含有大量的全局变量，我们可以通过一个正则参数选择想查看的全局变量：
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```
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(dlv) vars main
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main.initdone· = 2
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runtime.main_init_done = chan bool 0/0
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runtime.mainStarted = true
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(dlv)
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```
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然后就可以通过continue命令让程序运行到下一个断点处：
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```
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(dlv) continue
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> main.main() ./main.go:7 (hits goroutine(1):1 total:1) (PC: 0x10ae9b8)
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2:
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3: import (
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4: "fmt"
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5: )
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6:
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=> 7: func main() {
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8: nums := make([]int, 5)
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9: for i := 0; i < len(nums); i++ {
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10: nums[i] = i * i
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11: }
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12: fmt.Println(nums)
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(dlv)
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```
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输入next命令单步执行进入main函数内部：
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```
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(dlv) next
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> main.main() ./main.go:8 (PC: 0x10ae9cf)
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3: import (
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4: "fmt"
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5: )
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6:
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7: func main() {
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=> 8: nums := make([]int, 5)
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9: for i := 0; i < len(nums); i++ {
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10: nums[i] = i * i
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11: }
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12: fmt.Println(nums)
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13: }
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(dlv)
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```
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进入函数之后可以通过args和locals命令查看函数的参数和局部变量：
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```
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(dlv) args
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(no args)
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(dlv) locals
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nums = []int len: 842350763880, cap: 17491881, nil
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```
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因为main函数没有参数，因此args命令没有任何输出。而locals命令则输出了局部变量nums切片的值：此时切片还未完成初始化，切片的底层指针为nil，长度和容量都是一个随机数值。
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再次输入next命令单步执行后就可以查看到nums切片初始化之后的结果了：
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```
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(dlv) next
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> main.main() ./main.go:9 (PC: 0x10aea12)
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4: "fmt"
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5: )
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6:
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7: func main() {
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8: nums := make([]int, 5)
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=> 9: for i := 0; i < len(nums); i++ {
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10: nums[i] = i * i
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11: }
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12: fmt.Println(nums)
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13: }
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(dlv) locals
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nums = []int len: 5, cap: 5, [...]
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i = 17601536
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(dlv)
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```
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此时因为调试器已经到了for语句行，因此局部变量出现了还未初始化的循环迭代变量i。
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下面我们通过组合使用break和condition命令，在循环内部设置一个条件断点，当循环变量i等于3时断点生效：
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```
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(dlv) break main.go:10
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Breakpoint 2 set at 0x10aea33 for main.main() ./main.go:10
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(dlv) condition 2 i==3
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(dlv)
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```
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然后通过continue执行到刚设置的条件断点，并且输出局部变量：
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```
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(dlv) continue
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> main.main() ./main.go:10 (hits goroutine(1):1 total:1) (PC: 0x10aea33)
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5: )
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6:
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7: func main() {
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8: nums := make([]int, 5)
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9: for i := 0; i < len(nums); i++ {
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=> 10: nums[i] = i * i
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11: }
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12: fmt.Println(nums)
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13: }
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(dlv) locals
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nums = []int len: 5, cap: 5, [...]
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i = 3
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(dlv) print nums
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[]int len: 5, cap: 5, [0,1,4,0,0]
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(dlv)
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```
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我们发现当循环变量i等于3时，nums切片的前3个元素已经正确初始化。
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我们还可以通过stack查看当前执行函数的栈帧信息：
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```
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(dlv) stack
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0 0x00000000010aea33 in main.main
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at ./main.go:10
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1 0x000000000102bd60 in runtime.main
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at /usr/local/go/src/runtime/proc.go:198
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2 0x0000000001053bd1 in runtime.goexit
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at /usr/local/go/src/runtime/asm_amd64.s:2361
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(dlv)
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```
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或者通过goroutine和goroutines命令查看当前Goroutine相关的信息：
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```
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(dlv) goroutine
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Thread 101686 at ./main.go:10
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Goroutine 1:
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Runtime: ./main.go:10 main.main (0x10aea33)
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User: ./main.go:10 main.main (0x10aea33)
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Go: /usr/local/go/src/runtime/asm_amd64.s:258 runtime.rt0_go (0x1051643)
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Start: /usr/local/go/src/runtime/proc.go:109 runtime.main (0x102bb90)
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(dlv) goroutines
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[4 goroutines]
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* Goroutine 1 - User: ./main.go:10 main.main (0x10aea33) (thread 101686)
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Goroutine 2 - User: /usr/local/go/src/runtime/proc.go:292 runtime.gopark (0x102c189)
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Goroutine 3 - User: /usr/local/go/src/runtime/proc.go:292 runtime.gopark (0x102c189)
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Goroutine 4 - User: /usr/local/go/src/runtime/proc.go:292 runtime.gopark (0x102c189)
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(dlv)
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```
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最后完成调试工作后输入quit命令退出调试器。至此我们已经掌握了Delve调试器器的简单用法。
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## 调试汇编程序
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用Delve调试Go汇编程序的过程比调试Go语言程序更加简单。调试汇编程序时，我们需要时刻关注寄存器的状态，如果涉及函数调用或局部变量或参数还需要重点关注栈寄存器SP的状态。
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为了编译演示，我们重新实现一个更简单的main函数：
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```go
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package main
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func main() { asmSayHello() }
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func asmSayHello()
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```
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在main函数中调用汇编语言实现的asmSayHello函数输出一个字符串。
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asmSayHello函数在main_amd64.s文件中实现：
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```
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#include "textflag.h"
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#include "funcdata.h"
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// "Hello World!\n"
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DATA text<>+0(SB)/8,$"Hello Wo"
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DATA text<>+8(SB)/8,$"rld!\n"
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GLOBL text<>(SB),NOPTR,$16
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// func asmSayHello()
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TEXT ·asmSayHello(SB), $16-0
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NO_LOCAL_POINTERS
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MOVQ $text<>+0(SB), AX
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MOVQ AX, (SP)
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MOVQ $16, 8(SP)
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CALL runtime·printstring(SB)
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RET
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```
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参考前面的调试流程，在执行到main函数断点时，可以disassemble反汇编命令查看main函数对应的汇编代码：
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```
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(dlv) break main.main
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Breakpoint 1 set at 0x105011f for main.main() ./main.go:3
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(dlv) continue
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> main.main() ./main.go:3 (hits goroutine(1):1 total:1) (PC: 0x105011f)
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1: package main
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2:
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=> 3: func main() { asmSayHello() }
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4:
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5: func asmSayHello()
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(dlv) disassemble
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TEXT main.main(SB) /Users/chai/go/src/github.com/chai2010/advanced-go-programming-book/vendor/gobook.examples/ch3-08-debug/hello-asm/main.go
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main.go:3 0x1050110 65488b0c25a0080000 mov rcx, qword ptr gs:[0x8a0]
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main.go:3 0x1050119 483b6110 cmp rsp, qword ptr [rcx+0x10]
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main.go:3 0x105011d 761a jbe 0x1050139
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=> main.go:3 0x105011f* 4883ec08 sub rsp, 0x8
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main.go:3 0x1050123 48892c24 mov qword ptr [rsp], rbp
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main.go:3 0x1050127 488d2c24 lea rbp, ptr [rsp]
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main.go:3 0x105012b e880000000 call $main.asmSayHello
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main.go:3 0x1050130 488b2c24 mov rbp, qword ptr [rsp]
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main.go:3 0x1050134 4883c408 add rsp, 0x8
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main.go:3 0x1050138 c3 ret
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main.go:3 0x1050139 e87288ffff call $runtime.morestack_noctxt
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main.go:3 0x105013e ebd0 jmp $main.main
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(dlv)
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```
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虽然main函数内部只有一行函数调用语句，但是却生成了很多汇编指令。在函数的开头通过比较rsp寄存器判断栈空间是否不足，如果不足则跳转到0x1050139地址调用runtime.morestack函数进行栈扩容，然后跳回到main函数开始位置重新进行栈空间测试。而在asmSayHello函数调用之前，先扩展rsp空间用于临时存储rbp寄存器的状态，在函数返回后通过栈恢复rbp的值并回收临时栈空间。通过对比Go语言代码和对应的汇编代码，我们可以加深对Go汇编语言的理解。
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从汇编语言角度深刻Go语言各种特性的工作机制对调试工作也是一个很大的帮助。如果希望在汇编指令层面调试Go代码，Delve还提供了一个step-instruction单步执行汇编指令的命令。
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现在我们依然用break命令在asmSayHello函数设置断点，并且输入continue命令让调试器执行到断点位置停下：
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```
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(dlv) break main.asmSayHello
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Breakpoint 2 set at 0x10501bf for main.asmSayHello() ./main_amd64.s:10
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(dlv) continue
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> main.asmSayHello() ./main_amd64.s:10 (hits goroutine(1):1 total:1) (PC: 0x10501bf)
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5: DATA text<>+0(SB)/8,$"Hello Wo"
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6: DATA text<>+8(SB)/8,$"rld!\n"
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7: GLOBL text<>(SB),NOPTR,$16
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8:
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9: // func asmSayHello()
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=> 10: TEXT ·asmSayHello(SB), $16-0
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11: NO_LOCAL_POINTERS
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12: MOVQ $text<>+0(SB), AX
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13: MOVQ AX, (SP)
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14: MOVQ $16, 8(SP)
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15: CALL runtime·printstring(SB)
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(dlv)
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```
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此时我们可以通过regs查看全部的寄存器状态：
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```
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(dlv) regs
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rax = 0x0000000001050110
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rbx = 0x0000000000000000
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rcx = 0x000000c420000300
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rdx = 0x0000000001070be0
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rdi = 0x000000c42007c020
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rsi = 0x0000000000000001
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rbp = 0x000000c420049f78
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rsp = 0x000000c420049f70
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r8 = 0x7fffffffffffffff
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r9 = 0xffffffffffffffff
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r10 = 0x0000000000000100
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r11 = 0x0000000000000286
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r12 = 0x000000c41fffff7c
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r13 = 0x0000000000000000
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r14 = 0x0000000000000178
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r15 = 0x0000000000000004
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rip = 0x00000000010501bf
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rflags = 0x0000000000000206
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...
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(dlv)
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```
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因为AMD64的各种寄存器非常多，项目的信息中刻意省略了非通用的寄存器。如果再单步执行到13行时，可以发现AX寄存器值的变化。
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```
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(dlv) regs
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rax = 0x00000000010a4060
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rbx = 0x0000000000000000
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rcx = 0x000000c420000300
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...
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(dlv)
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```
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因此我们可以推断汇编程序内部定义的`text<>`数据的地址为0x00000000010a4060。我们可以用过print命令来查看该内存内的数据：
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```
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(dlv) print *(*[5]byte)(uintptr(0x00000000010a4060))
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[5]uint8 [72,101,108,108,111]
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(dlv)
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```
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我们可以发现输出的`[5]uint8 [72,101,108,108,111]`刚好是对应“Hello”字符串。通过类似的方法，我们可以通过查看SP对应的栈指针位置，然后查看栈中局部变量的值。
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至此我们就掌握了Go汇编程序的简单调试技术。
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