在 gdb 中为没有调试符号的应用程序调用系统命令

Question

我只是想了解如何动态重定向 linux 进程输入/输出/错误（0/1/2），我在堆栈溢出中找到了答案 How to redirect output of an already running process

如您所见，指南要求我通过 gdb 附加到进程并在文件描述符上调用 close() 并创建新的。不幸的是 cat （测试应用程序）没有使用调试信息构建，gdb 抱怨没有关闭这样的符号。我尝试的其他系统命令也是如此。

我需要做什么才能访问 gdb 中的这些符号 此外，如果您能够提供替代方法来重定向已运行进程的输出，而无需安装外来工具或使用 gdb，那就太好了。谢谢

编辑： gdb 会话

gdb -p 6836 /bin/cat                                                                                                                                                                                                                                              
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Reading symbols from /bin/cat...
(No debugging symbols found in /bin/cat)
Attaching to program: /usr/bin/cat, process 6836
ptrace: No such process.
(gdb) p close(1)
No symbol table is loaded.  Use the "file" command.
(gdb) file /bin/cat
Reading symbols from /bin/cat...
(No debugging symbols found in /bin/cat)
(gdb) p close(1)
No symbol table is loaded.  Use the "file" command.
(gdb) 

Answer 1

你的问题从这里开始：

Attaching to program: /usr/bin/cat, process 6836
ptrace: No such process.

这很可能会失败，因为您输入了错误的进程 ID，或者您的 /usr/bin/cat 因其他原因退出。

尝试这样做：

cat > /tmp/foo &
[1] 1084976   # Note the PID

gdb /usr/bin/cat -p 1084976  # use the PID printed by the shell

这是您应该看到的：

Attaching to program: /bin/cat, process 1084976

Program received signal SIGTTIN, Stopped (tty input).
0x00007f96c36178be in __GI___libc_read (fd=0, buf=0x7f96c36f6000, nbytes=131072) at ../sysdeps/unix/sysv/linux/read.c:26
26      ../sysdeps/unix/sysv/linux/read.c: No such file or directory.

(gdb) info shared
From                To                  Syms Read   Shared Object Library
0x00007f96c354f330  0x00007f96c3696df9  Yes         /lib/x86_64-linux-gnu/libc.so.6
0x00007f96c3718050  0x00007f96c3737cbe  Yes         /lib64/ld-linux-x86-64.so.2

您没有定义close 的原因是它在libc.so.6 中定义，但是您的GDB 没有从libc.so.6 加载符号因为它未能附加进程。

如果没有进程，即使在主可执行文件中定义了 close，您也无法使用 call。