gdb 在 malloc 中冻结

Question

假设我有一些这样的 C 程序：

#include <stdlib.h>
#include <stdbool.h>

int main()
{
    while (true) {
        void *p = malloc(1000);
        free(p);
    }
    return 0;
}

我用gdb 附加到它，就像gdb a.out PID 这样。 gdb 成功附加到它，但我尝试做类似call printf("bla bla bla") gdb 冻结，如果我按Ctrl^C 我得到这个：

(gdb) call printf("bla bla bla")
^C
Program received signal SIGINT, Interrupt.
__lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95  ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: No such file or directory.
The program being debugged was signaled while in a function called from GDB.
GDB remains in the frame where the signal was received.
To change this behavior use "set unwindonsignal on".
Evaluation of the expression containing the function
(malloc) will be abandoned.
When the function is done executing, GDB will silently stop.

我想这是因为我的a.out 正在创建一个对象并在malloc.c 中获得了一个锁，此时我与gdb 连接并尝试使用malloc 创建字符串“bla bla bla”。

我的问题是如何检测到我在malloc.c 中并让我的程序完成此执行？我不需要在命令行中执行此操作，而是使用某种 gdb 脚本（我只能在 gdb 中使用 -ex 选项执行命令）。

Answer 1

您被冻结的原因可能是您的程序持有的锁，printf 也需要该锁。当您尝试获取它两次时 - 您失败了。

一种可能的 WA 是在将程序中断为 call printf 时，就在您进行调用之前，键入 finish - 这将导致当前函数完成并返回到主框架。这将确保在您调用 printf 之前锁是空闲的。

Answer 2

如果“完成”解决方案对您不起作用。这是另一个想法。

您可以在中断程序时检查您是否在 malloc 中。根据布尔输入/输出，您跳过调用打印命令。这是一个工作示例。

# gdb script: pygdb-logg.gdb
# easier interface for pygdb-logg.py stuff
# from within gdb: (gdb) source -v pygdb-logg.gdb
# from cdmline: gdb -x pygdb-logg.gdb -se test.exe

# first, "include" the python file:
source -v pygdb-logg.py

# define shorthand for inMalloc():
define inMalloc
  python inMalloc()
end

相关的python文件：

# gdb will 'recognize' this as python
#  upon 'source pygdb-logg.py'
# however, from gdb functions still have
#  to be called like:
#  (gdb) python print logExecCapture("bt")

import sys
import gdb
import os

def logExecCapture(instr):
  # /dev/shm - save file in RAM
  ltxname="/dev/shm/c.log"

  gdb.execute("set logging file "+ltxname) # lpfname
  gdb.execute("set logging redirect on")
  gdb.execute("set logging overwrite on")
  gdb.execute("set logging on")
  gdb.execute("bt")
  gdb.execute("set logging off")

  replyContents = open(ltxname, 'r').read() # read entire file
  return replyContents

# in malloc?
def inMalloc():
  isInMalloc = -1;
  # as long as we don't find "Breakpoint" in report:
  while isInMalloc == -1:
    REP=logExecCapture("n")
#Look for calls that have '_malloc' in them 
    isInMalloc = REP.find("_malloc")
    if(isInMalloc != -1):
#       print ("Malloc:: ", isInMalloc, "\n", REP)
       gdb.execute("set $inMalloc=1")
       return True
    else:
#       print ("No Malloc:: ", isInMalloc, "\n", REP)
       gdb.execute("set $inMalloc=0")
       return False

gdb -x pygdb-logg.gdb -se test.exe

从命令行或脚本，

(gdb) inMalloc
(gdb) print $inMalloc

来自实际测试程序：

Program received signal SIGINT, Interrupt.
0x00007ffff7a94dba in _int_malloc (av=<optimized out>, bytes=1) at malloc.c:3806
3806    malloc.c: No such file or directory.
(gdb) inMalloc
(gdb) if $inMalloc
 >print $inMalloc
 >end
$1 = 1

我相信您的脚本可以使用类似的“if”结构来执行/不执行 printf

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