Can I create a link to an inode?

[Joe Smith]

Doug Wyatt wrote:
> 
> Jose Celestino wrote:
>> ...
>> 1. locate the open (deleted) file you want from the opening pid on 
>> /proc/$PID/fd/*
>> 2. cp /proc/$PID/fd/$FD somewhere
>>
> 
> I think I read, somewhere, that in doing that I could end up
> with garbage bytes at the end of the last block in the copy,
> and would need to use the size from the original inode to
> trim the copied file.  Don't know if that's fact or not.
> 
> Also, by re-linking you preserve the mtime w/o hassling with
> touch.

Interesting topic--I was wondering about this just the other day.

A quick test shows that Jose's approach does not suffer from either of 
these problems. I'm using F9/ext3.

I can't say /how/ it works, but it works perfectly:

$ cp -p ~/Music/lib/Various\ Artists/Unknown/afm_St9RvdtvLeE.ogg .
$ ls -li afm_St9RvdtvLeE.ogg
233608 -rw-rw-r-- 1 joe joe 2246026 2008-07-11 21:15 afm_St9RvdtvLeE.ogg
$ audacious afm_St9RvdtvLeE.ogg &
[1] 16404
$ file /proc/16404/fd/10
/proc/16404/fd/10: symbolic link to `/home/joe/tmp/afm_St9RvdtvLeE.ogg'
$ ls -l /proc/16404/fd/10
lr-x------ 1 joe joe 64 2008-08-15 22:15 /proc/16404/fd/10 -> 
/home/joe/tmp/afm_St9RvdtvLeE.ogg
$ rm afm_St9RvdtvLeE.ogg
$ ls -l /proc/16404/fd/10
lr-x------ 1 joe joe 64 2008-08-15 22:15 /proc/16404/fd/10 -> 
/home/joe/tmp/afm_St9RvdtvLeE.ogg (deleted)
$ file /proc/16404/fd/10
/proc/16404/fd/10: broken symbolic link to 
`/home/joe/tmp/afm_St9RvdtvLeE.ogg (deleted)'
$ cp -p /proc/16404/fd/10 afm_back_cp.ogg
$ ls -li afm_back_cp.ogg
233609 -rw-rw-r-- 1 joe joe 2246026 2008-07-11 21:15 afm_back_cp.ogg
$
[1]+  Done                    audacious afm_St9RvdtvLeE.ogg
$ cmp afm_back_cp.ogg ~/Music/lib/Various\ 
Artists/Unknown/afm_St9RvdtvLeE.ogg
$

Note that the copy has the exact same size, mtime and contents as the 
original file--there's no need at all to use debugfs for this.

Even though the /proc/$PID/fd/$FD appears as a symlink in the 
filesystem, it doesn't behave like a normal symlink. A plain symlink 
definitely won't support this sort of copy after the target file is deleted.

<Joe