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Date: 4 Dec 2011 16:38:16 -0500
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From: "George Spelvin" <linux@horizon.com>
To: linux@horizon.com, viro@ZenIV.linux.org.uk
Subject: Re: Is there a reason hard links from /proc/$PID/fd/$NUM are disallowed?
Cc: linux-kernel@vger.kernel.org
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> You do realize that link(2) does *NOT* follow links, do you?  linkat(2)
> does, if you explicitly ask for that:
> 
> linkat(AT_FDCWD, "/proc/42/fd/1", AT_FDCWD, "/tmp/foo", AT_SYMLINK_FOLLOW)
> 
> will do it just fine.

Thank you very much!  What's embarrassing is that I actually *knew* that once.

That seems to change the error message.

cd /run/shm	# A tmpfs directory
echo foo > foo
sleep 10000 < foo &
pid=$!
rm foo
strace ln -L /proc/$pid/fd/0 bar

produces:
stat64("/run/shm/bar", 0xbfb7b46c)      = -1 ENOENT (No such file or directory)
stat64("/proc/25098/fd/0", {st_mode=S_IFREG|0644, st_size=4, ...}) = 0
linkat(AT_FDCWD, "/proc/25098/fd/0", AT_FDCWD, "/run/shm/bar", 1024) = -1 ENOENT
 (No such file or directory)

(1024 is indeed AT_SYMLINK_FOLLOW)

It seems to work, however, if the file is *not* deleted first.