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如何从写入进程外部读取打开的文件描述符

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如何从写入进程外部读取打开的文件描述符

当从进程写入文件描述符时，如何打开文件描述符并将其回显到终端？

我有一个备份程序 Duplicity，它将其日志写入参数指定的文件描述符--log-fd=16。

果然，如果我运行lsof -p <duplicity PID>我会看到：

python2 9224 myuser    0r      CHR                1,3      0t0         6 /dev/null
python2 9224 myuser    1w      CHR                1,3      0t0         6 /dev/null
python2 9224 myuser    2w      CHR                1,3      0t0         6 /dev/null
python2 9224 myuser    3u  a_inode               0,11        0      7005 [eventfd]
python2 9224 myuser    4u     unix 0x0000000000000000      0t0    158199 type=STREAM
python2 9224 myuser    5u  a_inode               0,11        0      7005 [eventfd]
python2 9224 myuser    6u  a_inode               0,11        0      7005 [eventfd]
python2 9224 myuser    7r      DIR                8,3     4096  22414346 <some random file being accessed during the backup>
python2 9224 myuser    8r      CHR                1,9      0t0        11 /dev/urandom
python2 9224 myuser   15r     FIFO               0,10      0t0    157054 pipe
python2 9224 myuser   16w     FIFO               0,10      0t0    157054 pipe

但是，如果我尝试在 Python 中打开文件描述符，则会收到错误：

>>> import os
>>> os.fdopen(16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 9] Bad file descriptor

为什么是这样？如何读取文件描述符？

答案1

使用strace（跟踪系统调用和信号）。

用法：

sudo strace -p <PID of writing process> -s 9999 -e write=<corresponding FD>

从手册页：

       -p pid      Attach to the process with the process ID pid and begin tracing.  The trace may be terminated
                   at any time by a keyboard interrupt signal (CTRL-C).  strace will respond by detaching itself
                   from  the  traced process(es) leaving it (them) to continue running.  Multiple -p options can
                   be used to attach to many processes in addition to command (which is optional if at least one
                   -p option is given).  -p "`pidof PROG`" syntax is supported.
    
       -s strsize  Specify the maximum string size to print (the default is 32).  Note that  filenames  are  not
                   considered strings and are always printed in full.
    
       -e read=set
              Perform a full hexadecimal and ASCII dump of all the data read from file descriptors listed in the
              specified set.  For example,  to  see  all  input  activity  on  file  descriptors  3  and  5  use
              -e read=3,5.   Note  that  this  is independent from the normal tracing of the read(2) system call
              which is controlled by the option -e trace=read.

       -e write=set
              Perform a full hexadecimal and ASCII dump of all the data written to file  descriptors  listed  in
              the  specified  set.   For  example,  to  see  all output activity on file descriptors 3 and 5 use
              -e write=3,5.  Note that this is independent from the normal tracing of the write(2)  system  call
              which is controlled by the option -e trace=write.

参考：https://man7.org/linux/man-pages/man1/strace.1.html

答案2

我相信口是心非的--log=fd选项适用于您想要与日志分离的复杂stderr管道stdout。

这个答案对这个问题举个例子。这是一个简单的例子：

#!/bin/sh
# Generate output on three different fds
echo hello >&3
echo world >&2
echo today >&1

当像这样执行时，

./foo 2> 2.log 3> 3.log 1> 1.log

结果是

$ cat 1.log 2.log 3.log
today
world
hello

答案3

Linux 最近获得了针对此类事情的系统调用：

  1. 使用pidfd_open从 PID 获取“PID FD”。

  2. 使用pidfd_getfd通过另一个进程的 PID FD 获取文件描述符。

从 Python 3.9 开始，pidfd_open可用作os.pidfd_open。

pidfd_getfd尚未通过Python标准库公开，但幸运的是ctypes让我们打电话syscall，Linux系统调用号永远不会改变，Linux系统调用API和ABI只是以向后兼容的方式改变。

所以！

from ctypes import CDLL, c_int, c_long, c_uint, get_errno
from functools import partial
from os import strerror


_syscall = CDLL(None, use_errno=True).syscall

# Non-variadic system call number argument:
_syscall.argtypes = [c_long]


def pidfd_getfd(pidfd, targetfd):
    fd = _syscall(
             438,  # system call number of pidfd_getfd
             c_int(pidfd),
             c_int(targetfd),
             c_uint(0),  # unused "flags" argument
         )
    if fd == -1:
        errno = get_errno()
        raise OSError(errno, strerror(errno))
    return fd

因此，在您的示例中，感兴趣的 PID 为 9224，os.fdopen(16)您无需调用 ，而是执行os.fdopen(pidfd_getfd(os.pidfd_open(9224), 16))。

请注意，只有当您拥有访问目标进程所需的权限时，这才有效，因此您可能需要使用提升的权限（例如sudo）运行此代码，具体取决于进程的启动方式以及系统的配置方式。

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