[ruby-core:112315] [Ruby master Feature#19347] Add Dir.fchdir

Issue #19347 has been updated by kjtsanaktsidis (KJ Tsanaktsidis).


>  I'm not sure if it is an issue, but Dir.for_fd(fd).path returns nil, because there is no path associated with such a directory. Hopefully that is acceptable. I think it is possible to generate a path by walking upwards in the directory tree to the root of the file system, scanning each directory visited to find the name pointing to the previous directory, and using that to build a path. However, I'm not sure that is worth doing.

On Linux you could implement it as `File.readlink("/proc/self/fd/#{self.fileno}")`, although I don't know if it's better to have `#path` working on only some platforms vs just not working at all anywhere, consistently.

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Feature #19347: Add Dir.fchdir
https://bugs.ruby-lang.org/issues/19347#change-101756

* 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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