[ruby-core:111927] [Ruby master Feature#19347] Add Dir.fchdir

Issue #19347 has been updated by ko1 (Koichi Sasada).


At the dev meeting there are 4 ideas:

* 4 ideas
  * `Dir.fchdir(int)`
    * Proposed
    * Consistent to the POSIX API
    * Can be extended for `Dir.fchdir(dir)` and `Dir.fchdir(io)`
  * `Dir.chdir(int)`
    * Do not need to add new method
    * `Dir.chdir` partially implemented on Windows because fchdir is not supported by Windows. `respond_to?` is not usable to test the availability.
    * Can be extended for `Dir.fchdir(dir)` and `Dir.fchdir(io)`
  * `dir = Dir.for_fd(int); dir.chdir`
    * More object oriented?
    * 2 more methods (`Dir.for_fd(int)` and `Dir#chdir`) are needed
    * `Dir.for_fd(io.fileno).chdir` needs two objects (IO and Dir) references one FD.  It would have a problem with GC. `closedir` frees DIR structure and closes FD. Thus, we cannot implement `autoclose: false` for `Dir.for_fd`.
    * Dir.for_fd(io.fileno, autoclose: false)
        * use dup(io.fileno) and close that, leave the original one alone?
  * `IO#fchdir`

but not concluded.

More discussions are welcome.

----------------------------------------
Feature #19347: Add Dir.fchdir
https://bugs.ruby-lang.org/issues/19347#change-101352

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
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Recently, I was working on a program that passes file descriptors over UNIX sockets (using `send_io`/`recv_io`).  For file/socket/device descriptors, this works fine using `File#reopen`.  However, I found that while Ruby supports `Dir#fileno` to return a directory file descriptor, it cannot use that file descriptor when changing directories.

I worked around this in my program by writing the directory path over the UNIX socket, but this results in a TOCTOU vulnerability in certain cases.  Passing the directory file descriptor would be simpler and avoid the vulnerability.

I've submitted a pull request to implement this method: https://github.com/ruby/ruby/pull/7135



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