Rust LLDB 调试入门指北

谁践踏了优雅 2022-09-02 13:58 481阅读 0赞

本文介绍工具 LLDB 的初步使用

Rust 会使用 DWARF 格式在 binary 中嵌入调试信息，所以可以使用一些通用的调试工具，比如 GDB 和 LLDB。Rust 提供了 `rust-gdb` 和 `rust-lldb` 两个命令用于调试，其相比原生的 `gdb` 和 `lldb` 添加了一些方便调试的脚本

下面来初步的了解 `rust-lldb` 的使用，`rustup` 会安装 `rust-lldb`，但不会安装 `lldb`，需要自行安装

LLDB 的命令结构如下

<noun> <verb> [-options [option-value]] [argument [argument...]]

本文所使用的代码如下:

fn binary_search(nums: Vec<i32>, target: i32) -> i32 {  
        let mut size = nums.len();
    
        if size == 0 {
            return -1;
        }
    
        let mut base = 0usize;
    
        while size > 1 {
            let half = size / 2;
            let mid = base + half;
    
            if nums[mid] <= target {
                base = mid;
            }
            size = size - half;
        }
    
        if nums[base] == target {
            return base as i32;
        }
        return -1;
    }
    
    
    fn main() {  
        assert_eq!(binary_search(vec![1, 4, 7, 10, 16, 19], 10), 3);
    }

`cargo build` 默认会以 debug 模式进行构建，所以含有用于调试的 symbol，需要注意的是 `cargo install` 会以 release 模式构建，需要 `cargo install --debug`

通过 `rust-lldb` 命令加载可执行文件(或者在 REPL 中通过 `file` 载入)，进入 LLDB 的 REPL。比如

`rust-lldb target/debug/<bin-name> first_arg`

启动时会显示

(lldb) settings set -- target.run-args  "first_arg"

字样，参数会存在 `target.run-args` 中，可以在 REPL 中重置命令行参数

(lldb) settings set target.run-args "arg1"
    (lldb) settings show target.run-args
    target.run-args (array of strings) =  
      [0]: "arg1"
    (lldb) settings append target.run-args "arg2"                                                                                          
    (lldb) settings show target.run-args
    target.run-args (array of strings) =  
      [0]: "arg1"
      [1]: "arg2"

`gui` 命令可以进入 GUI，可以借助 [voltron][] 获得更改的体验

通过 `b` 命令来设置断点。`b` 命令是对于 GDB 中 `break` 命令的模拟，并不是 LLDB 中的 `breakpoint` 的 alias。`breakpoint set -n <func name>` 设置支持补全，`breakpoint set -f <文件路径>--line 15` 在指定的行设置断点

(lldb) b binary_search                                                                                                                
    Breakpoint 1: where = b`b::binary_search::hcb6e4fa332e0db67 + 13 at main.rs:2, address = 0x00000000000049bd  
    (lldb) b
    Current breakpoints:  
    1: name = 'binary_search', locations = 1  
      1.1: where = b`b::binary_search::hcb6e4fa332e0db67 + 13 at main.rs:2, address = b[0x00000000000049bd], unresolved, hit count = 0    
    
    2: name = 'binary_s', locations = 0 (pending)
    
    3: name = 'binary_search', locations = 1  
      3.1: where = b`b::binary_search::hcb6e4fa332e0db67 + 13 at main.rs:2, address = b[0x00000000000049bd], unresolved, hit count = 0    
    (lldb) breakpoint delete 2        
    1 breakpoints deleted; 0 breakpoint locations disabled.

通过 `r` 命令来运行程序

(lldb) r
    There is a running process, kill it and restart?: [Y/n] y  
    Process 12494 launched: '/root/T/b/target/debug/b' (x86_64)  
    Process 12494 stopped  
    * thread #1, name = 'b', stop reason = breakpoint 1.1
        frame #0: 0x00005555555589bd b`b::binary_search::hcb6e4fa332e0db67(nums=vec![1, 4, 7, 10, 16, 19], target=10) at main.rs:2        
       1    fn binary_search(nums: Vec<i32>, target: i32) -> i32 {
    -> 2        let mut size = nums.len();
       3
       4        if size == 0 {
       5            return -1;
       6        }
       7

通过 `frame variable` 可以看到当前栈中的变量

(lldb) frame variable
    (alloc::vec::Vec<int>) nums = vec![1, 4, 7, 10, 16, 19]
    (int) target = 10

通过 `n` 进行单步调试

(lldb) n
    Process 20123 stopped  
    * thread #1, name = 'b', stop reason = step over
        frame #0: 0x00005555555589df b`b::binary_search::hcb6e4fa332e0db67(nums=vec![1, 4, 7, 10, 16, 19], target=10) at main.rs:4        
       1    fn binary_search(nums: Vec<i32>, target: i32) -> i32 {
       2        let mut size = nums.len();
       3
    -> 4        if size == 0 {
       5            return -1;
       6        }
       7
    (lldb) frame variable
    (alloc::vec::Vec<int>) nums = vec![1, 4, 7, 10, 16, 19]
    (int) target = 10
    (unsigned long) size = 6

这都是模拟的 GDB 的命令族。LLDB 原生的则是

(lldb) thead select id
    (lldb) thread step-in    // The same as gdb's "step" or "s"
    (lldb) thread step-over  // The same as gdb's "next" or "n"
    (lldb) thread step-out   // The same as gdb's "finish" or "f"

在断点处可以设置命令，比如直接打印堆栈

(lldb) breakpoint command add 1
    Enter your debugger command(s). Type 'DONE' to end.  
    > bt
    > DONE

也可以直接通过 `--command` 参数设置

一个小例子，在 `half == 3` 条件成立的时候打印堆栈信息

(lldb) list main.rs:10
       10
       11       while size > 1 {
       12           let half = size / 2;
       13           let mid = base + half;
       14
       15           if nums[mid] <= target {
       16               base = mid;
       17           }
       18           size = size - half;
       19       }
       20
    (lldb) breakpoint set -c "half == 3" -f main.rs -l 13 -C bt
    Breakpoint 1: where = b`b::binary_search::hfce4ad2766a2fe8f + 166 at main.rs:13, address = 0x0000000000004a56  
    (lldb) r
    Process 31253 launched: '/root/T/b/target/debug/b' (x86_64)  
    (lldb)  bt
    * thread #1, name = 'b', stop reason = breakpoint 1.1
      * frame #0: 0x0000555555558a56 b`b::binary_search::hfce4ad2766a2fe8f(nums=vec![1, 4, 7, 10, 16, 19], target=10) at main.rs:13                                                              
        frame #1: 0x0000555555558b90 b`b::main::he42792ea9d7cc77a at main.rs:29
        frame #2: 0x00005555555582c0 b`std::rt::lang_start::_$u7b$$u7b$closure$u7d$$u7d$::h36f6beb6917ae1e7 at rt.rs:74                                                                          
        frame #3: 0x0000555555561743 b`std::panicking::try::do_call::h4f8262c35e4e88a2 [inlined] std::rt::lang_start_internal::_$u7b$$u7b$closure$u7d$$u7d$::h10f59d0290367560 at rt.rs:59       
        frame #4: 0x0000555555561737 b`std::panicking::try::do_call::h4f8262c35e4e88a2 at panicking.rs:307                                                                                       
        frame #5: 0x000055555556ebfa b`__rust_maybe_catch_panic at lib.rs:102
        frame #6: 0x0000555555562246 b`std::rt::lang_start_internal::ha81f57a8465b4dcb [inlined] std::panicking::try::h8abdfbcaa376c6de at panicking.rs:286                                      
        frame #7: 0x000055555556220b b`std::rt::lang_start_internal::ha81f57a8465b4dcb [inlined] std::panic::catch_unwind::hcc1db352380c01f1 at panic.rs:398                                     
        frame #8: 0x000055555556220b b`std::rt::lang_start_internal::ha81f57a8465b4dcb at rt.rs:58
        frame #9: 0x0000555555558299 b`std::rt::lang_start::h1bce8d1d740917a0(main=&0x555555558b30, argc=1, argv=&0x7fffffffe3c8) at rt.rs:74                                                    
        frame #10: 0x0000555555558f1a b`main + 42
        frame #11: 0x00007ffff7dc9223 libc.so.6`__libc_start_main + 243
        frame #12: 0x000055555555817e b`_start + 46
    
    Process 31253 stopped  
    * thread #1, name = 'b', stop reason = breakpoint 1.1
        frame #0: 0x0000555555558a56 b`b::binary_search::hfce4ad2766a2fe8f(nums=vec![1, 4, 7, 10, 16, 19], target=10) at main.rs:13                                                              
       10
       11       while size > 1 {
       12           let half = size / 2;
    -> 13           let mid = base + half;
       14
       15           if nums[mid] <= target {
       16               base = mid;

可以通过 `watch` 来监控变量的变化

`expr` 命令可以对表达式求值

lldb) frame variable  
    (unsigned long) size = 24
    (unsigned long) align = 4
    (lldb) expr size == 24
    (bool) $0 = true

### Reference ###

[LLDB Tutorial][]

[voltron]: https://github.com/snare/voltron
[LLDB Tutorial]: https://lldb.llvm.org/tutorial.html