☰

如何從寫入進程外部讀取開啟的檔案描述符

tag-icon • tag-icon files python
如何從寫入進程外部讀取開啟的檔案描述符

當從進程寫入檔案描述符時，如何開啟檔案描述符並將其回顯到終端？

我有備份程式 Duplicity，它將其日誌寫入參數指定的檔案描述符--log-fd=16。

果然，如果我運行lsof -p <duplicity PID>我會看到：

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，如果我嘗試在 Python 中開啟檔案描述符，則會收到錯誤：

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

為什麼是這樣？如何讀取檔案描述符？

答案1

使用strace（追蹤系統調用和信號）。

用法：

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

從手冊頁：

       -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.

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

答案2

我相信口是心非的--log=fd選項適用於您想要與日誌分離的複雜stderr管道stdout。

這個答案對這個問題舉個例子。這是一個簡單的例子：

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

像這樣執行時，

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

結果是

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

答案3

Linux 最近獲得了針對此類事情的系統呼叫：

  1. 使用pidfd_open從 PID 取得“PID FD”。

  2. 使用pidfd_getfd透過另一個程序的 PID FD 取得檔案描述符。

從 Python 3.9 開始，pidfd_open可用作os.pidfd_open。

pidfd_getfd尚未透過Python標準庫公開，但幸運的是ctypes讓我們打電話syscall，Linux系統呼叫號碼永遠不會改變，Linux系統呼叫API和ABI只是以向後相容的方式改變。

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，在您的範例中，感興趣的 PID 為 9224，os.fdopen(16)您無需呼叫 ，而是執行os.fdopen(pidfd_getfd(os.pidfd_open(9224), 16))。

請注意，只有當您擁有存取目標進程所需的權限時，這才有效，因此您可能需要使用提升的權限（例如sudo）執行此程式碼，具體取決於進程的啟動方式以及系統的配置方式。

相關內容