[英]How do I debug jonesforth with GDB?
jonesforth is typically started as follows: jonesforth通常如下所示:
cat jonesforth.f - | ./jonesforth
What's a good way to debug jonesforth
? 调试jonesforth
的好方法是jonesforth
?
If you're on Ubuntu, allow gdb
to attach to running processes: 如果您使用的是Ubuntu,请允许gdb
附加到正在运行的进程:
echo 0 > /proc/sys/kernel/yama/ptrace_scope
If you'd like that setting to remain across reboots: 如果您希望该设置保持重新启动:
vim /etc/sysctl.d/10-ptrace.conf
Add the g
flag to your jonesforth
Makefile
recipe: 将g
标志添加到jonesforth
Makefile
配方中:
jonesforth: jonesforth.S
gcc -g -m32 -nostdlib -static $(BUILD_ID_NONE) -o $@ $<
Then, start up jonesforth as usual in a terminal: 然后,像往常一样在终端中启动jonesforth:
cat jonesforth.f - | ./jonesforth
In another terminal, start gdb
and attach it to the running jonesforth: 在另一个终端中,启动gdb
并将其附加到正在运行的jonesforth:
gdb --quiet --pid=`pgrep jonesforth` ./jonesforth
Here's what I see when I start gdb
: 这是我在启动gdb
时看到的内容:
$ gdb --quiet --pid=`pgrep jonesforth` ./jonesforth
Reading symbols from ./jonesforth...done.
Attaching to program: /home/dharmatech/Dropbox/Documents/jonesforth-annexia/jonesforth, process 3406
_KEY () at jonesforth.S:1290
1290 test %eax,%eax // If %eax <= 0, then exit.
(gdb)
Jonesforth is waiting for us to enter something. 琼斯福斯正在等我们输入一些东西。 It's in the _KEY
assembly routine. 它在_KEY
汇编程序中。 This is indicated by gdb
above. 这由上面的gdb
表示。 It also shows that line 1290 is the next one to execute. 它还显示第1290行是下一个要执行的行。 Here's the _KEY
routine: 这是_KEY
例程:
_KEY:
mov (currkey),%ebx
cmp (bufftop),%ebx
jge 1f // exhausted the input buffer?
xor %eax,%eax
mov (%ebx),%al // get next key from input buffer
inc %ebx
mov %ebx,(currkey) // increment currkey
ret
1: // Out of input; use read(2) to fetch more input from stdin.
xor %ebx,%ebx // 1st param: stdin
mov $buffer,%ecx // 2nd param: buffer
mov %ecx,currkey
mov $BUFFER_SIZE,%edx // 3rd param: max length
mov $__NR_read,%eax // syscall: read
int $0x80
test %eax,%eax // If %eax <= 0, then exit.
jbe 2f
addl %eax,%ecx // buffer+%eax = bufftop
mov %ecx,bufftop
jmp _KEY
2: // Error or end of input: exit the program.
xor %ebx,%ebx
mov $__NR_exit,%eax // syscall: exit
int $0x80
_KEY
uses some variables in memory: buffer
, currkey
, and bufftop
. _KEY
在内存中使用了一些变量: buffer
, currkey
和bufftop
。 It also uses a couple of registers. 它还使用了几个寄存器。 Let's use gdb
's Auto Display
feature to display these: 让我们使用gdb
的Auto Display
功能来显示这些:
display/8cb &buffer
display/1xw &currkey
display/1xw &bufftop
display/x $eax
display/x $ebx
Now if we type display
in gdb
, we'll see all of those at once: 现在如果我们在gdb
输入display
,我们会立刻看到所有这些:
(gdb) display
1: x/8cb &buffer
0x804c000: 97 'a' 98 'b' 108 'l' 121 'y' 46 '.' 32 ' ' 32 ' ' 84 'T'
2: x/xw &currkey 0x8049d54: 0x0804c000
3: x/xw &bufftop 0x8049d58: 0x0804c7e3
4: /x $eax = 0xfffffe00
5: /x $ebx = 0x0
This might also be a good time to enable gdb
's TUI: 这也许是启用gdb
的TUI的好时机:
tui enable
gdb should now look like this: gdb现在应该如下所示:
OK, jonesforth is still waiting for input. 好的,jonesforth还在等待输入。 So let's give it something: 所以让我们给它一些东西:
JONESFORTH VERSION 47
14499 CELLS REMAINING
OK 123
Alright, back in gdb, we can finally ask it to step: 好吧,回到gdb,我们终于可以要求它继续:
(gdb) s
1: x/8cb &buffer
0x804c000: 49 '1' 50 '2' 51 '3' 10 '\n' 46 '.' 32 ' ' 32 ' ' 84 'T'
2: x/xw &currkey 0x8049d54: 0x0804c000
3: x/xw &bufftop 0x8049d58: 0x0804c7e3
4: /x $eax = 0x4
5: /x $ebx = 0x0
Hey, look at that! 嘿,看那个! The first 3 characters in buffer
are 1
, 2
, and 3
. 在第一3个字符buffer
是1
, 2
,和3
。
If %eax <= 0
the next step will jump to the 2f
label. 如果%eax <= 0
则下一步将跳转到2f
标签。 But as we can see above, %eax
is 4
. 但正如我们上面所看到的, %eax
是4
。 So it should just continue on. 所以它应该继续下去。
If we step through the next three lines, the bufftop
will be set to the address of buffer
incremented by 4 (three characters of '123' plus a newline character). 如果我们单步执行接下来的三行,则bufftop
将设置为buffer
的地址递增4(三个字符'123'加上一个换行符)。 The value in relation to the address of buffer
checks out: 与buffer
地址相关的值检出:
3: x/xw &bufftop 0x8049d58: 0x0804c004
Now that data has been read into the input buffer, _KEY
will do its job and return back to the caller. 现在数据已被读入输入缓冲区, _KEY
将完成其工作并返回给调用者。 Here's the next few instructions before the return: 这是返回前的下几条指令:
As you step through those, the auto display feature will show the variables and registers updating accordingly. 当您逐步执行这些操作时,自动显示功能将相应地显示变量和寄存器更新。
我(非常有限!)的经验是,当修改汇编语言程序时,LLVM调试器lldb更加友好。
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