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114 lines
4.0 KiB
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114 lines
4.0 KiB
Markdown
Translating by Torival Three steps to learning GDB
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============================================================
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Debugging C programs used to scare me a lot. Then I was writing my [operating system][2] and I had so many bugs to debug! I was extremely fortunate to be using the emulator qemu, which lets me attach a debugger to my operating system. The debugger is called `gdb`.
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I’m going to explain a couple of small things you can do with `gdb`, because I found it really confusing to get started. We’re going to set a breakpoint and examine some memory in a tiny program.
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### 1\. Set breakpoints
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If you’ve ever used a debugger before, you’ve probably set a breakpoint.
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Here’s the program that we’re going to be “debugging” (though there aren’t any bugs):
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```
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#include <stdio.h>
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void do_thing() {
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printf("Hi!\n");
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}
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int main() {
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do_thing();
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}
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```
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Save this as `hello.c`. We can debug it with gdb like this:
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```
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bork@kiwi ~> gcc -g hello.c -o hello
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bork@kiwi ~> cat
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bork@kiwi ~> gdb ./hello
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```
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This compiles `hello.c` with debugging symbols (so that gdb can do better work), and gives us kind of scary prompt that just says
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`(gdb)`
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We can then set a breakpoint using the `break` command, and then `run` the program.
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```
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(gdb) break do_thing
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Breakpoint 1 at 0x4004f8
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(gdb) run
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Starting program: /home/bork/hello
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Breakpoint 1, 0x00000000004004f8 in do_thing ()
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```
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This stops the program at the beginning of `do_thing`.
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We can find out where we are in the call stack with `where`: (thanks to [@mgedmin][3] for the tip)
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```
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(gdb) where
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#0 do_thing () at hello.c:3
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#1 0x08050cdb in main () at hello.c:6
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(gdb)
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```
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### 2\. Look at some assembly code
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We can look at the assembly code for our function using the `disassemble`command! This is cool. This is x86 assembly. I don’t understand it very well, but the line that says `callq` is what does the `printf` function call.
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```
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(gdb) disassemble do_thing
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Dump of assembler code for function do_thing:
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0x00000000004004f4 <+0>: push %rbp
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0x00000000004004f5 <+1>: mov %rsp,%rbp
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=> 0x00000000004004f8 <+4>: mov $0x40060c,%edi
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0x00000000004004fd <+9>: callq 0x4003f0
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0x0000000000400502 <+14>: pop %rbp
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0x0000000000400503 <+15>: retq
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```
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You can also shorten `disassemble` to `disas`
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### 3\. Examine some memory!
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The main thing I used `gdb` for when I was debugging my kernel was to examine regions of memory to make sure they were what I thought they were. The command for examining memory is `examine`, or `x` for short. We’re going to use `x`.
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From looking at that assembly above, it seems like `0x40060c` might be the address of the string we’re printing. Let’s check!
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```
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(gdb) x/s 0x40060c
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0x40060c: "Hi!"
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```
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It is! Neat! Look at that. The `/s` part of `x/s` means “show it to me like it’s a string”. I could also have said “show me 10 characters” like this:
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```
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(gdb) x/10c 0x40060c
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0x40060c: 72 'H' 105 'i' 33 '!' 0 '\000' 1 '\001' 27 '\033' 3 '\003' 59 ';'
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0x400614: 52 '4' 0 '\000'
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```
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You can see that the first four characters are ‘H’, ‘i’, and ‘!’, and ‘\0’ and then after that there’s more unrelated stuff.
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I know that gdb does lots of other stuff, but I still don’t know it very well and `x`and `break` got me pretty far. You can read the [documentation for examining memory][4].
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--------------------------------------------------------------------------------
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via: https://jvns.ca/blog/2014/02/10/three-steps-to-learning-gdb/
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作者：[Julia Evans ][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://jvns.ca
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[1]:https://jvns.ca/categories/spytools
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[2]:http://jvns.ca/blog/categories/kernel
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[3]:https://twitter.com/mgedmin
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[4]:https://ftp.gnu.org/old-gnu/Manuals/gdb-5.1.1/html_chapter/gdb_9.html#SEC56
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