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Date: Sun, 4 Jul 2010 13:17:10 -0400
From: Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
To: cygwin <cygwin AT cygwin DOT com>
Subject: Re: tar: symlinks unpacked to empty files (tar security problem?)
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On Sun, Jul 04, 2010 at 04:50:41AM -0500, Yaakov (Cygwin/X) wrote:
>With tar-1.23-1 and recent snapshot:
>
>echo foo > foo
>ln -s $PWD/foo bar
>tar cf test.tar bar foo
>rm -f bar foo
>tar xf test.tar
>ls -l bar foo
>
>You will see that 'bar' is a 0-byte file with 0000 permissions instead
>of a symlink.  The symlink reference need not be absolute; it also
>happens with relative links in different directories, but does not
>happen if I just "ln -s foo bar".

That's because of the way that tar handles symlinks.  If you have a
reference to an absolute path, tar makes a zero-length regular file
placeholder.  Then when it is done extracting, tar is supposed to remove
this file and create the real symlink.  However, the test to make sure
that it is ok to do this was broken by a recent DLL change.  The inode
returned the first time that the file was created was != the inode when
the file is checked later.  So tar thought that the zero-length file was
modified and silently decided not to create the symlink.

I've fixed the cygwin problem - it should be in the next snapshot.  I
think this silent behavior of tar is not too user friendly though.  It
seems like there is a pathological situation there where you'd end
thinking that you'd extracted a symlink without getting the symlink.  In
fact, I think this is actually a security problem.

Eric, am I missing something about tar's behavior here?

cgf

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