linux-kernel - Re: New system call wanted: fdreopen

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Message-ID: <20121209162746.GA23890@thunk.org>
Date:	Sun, 9 Dec 2012 11:27:46 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Tristan Wibberley <tristan.wibberley@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: New system call wanted: fdreopen

On Sun, Dec 09, 2012 at 03:03:30PM +0000, Tristan Wibberley wrote:
> 
> - /proc/self/fd/* does not solve this problem because the file might no
>   longer be available at the same place in the filesystem. In some
>   otherwise simple message passing or ReSTful IPC a different file will
>   be available at that path.

Actually, /proc/self/fd/* _will_ work.  When you do a ls -l, it looks
like a symlink, but the files in /proc/self/fd (and /proc/<pid>/fd
more generally) are magic.  If you open files in /proc/<pid>/fd/*, it
will do what you want.

See for yourself:

% cat > /tmp/foo.test
foo
bar
^Z
% jobs -l			# (and note the pid, hereafter <pid>)
% ls -l /proc/<pid>/fd 
% mv /tmp/foo.test /tmp/foo2.test
% ls -l /proc/<pid>/fd	# note that the symlink now points at /tmp/foo2.test
% cat /proc/<pid>/fd/1	# note that it works!
% rm /tmp/foo2.test
% ls -l /proc/<pid>/fd	# note that the symlink now has "(deleted)" at the end
% cat /proc/<pid>/fd/1	# note that it works!

This is of course horribly Linux-specific, but so would be a new
system call like your proposed fdrepon.  Better yet, using
/proc/self/fd/* will work *now*.  You don't have to wait for a new
system call in a future verison of the kernel to start shipping in new
distributions.

Regards,

					- Ted
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