linux-kernel - Re: RFC: allow empty symlink targets

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Message-ID: <20130526093954.GB7225@amd.pavel.ucw.cz>
Date:	Sun, 26 May 2013 11:39:54 +0200
From:	Pavel Machek <pavel@....cz>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	P??draig Brady <P@...igBrady.com>, linux-kernel@...r.kernel.org,
	Eric Blake <eblake@...hat.com>
Subject: Re: RFC: allow empty symlink targets

On Wed 2013-05-15 23:03:35, Al Viro wrote:
> On Wed, May 15, 2013 at 01:38:48PM +0100, P??draig Brady wrote:
> > >> In today's Austin Group meeting, I was tasked to open a new bug that
> > >> would state specifically how the empty symlink is resolved; the intent
> > >> is to allow both Solaris behavior (current directory) and BSD behavior
> > >> (ENOENT).  Meanwhile, everyone was in agreement that the Linux kernel
> > >> has a bug for rejecting the creation of an empty symlink, but once that
> > >> bug is fixed, then Linux can choose either Solaris or BSD behavior for
> > >> how to resolve such a symlink.
> 
> Austin Group Is At It Again, Demands at 11...
> 
> Would you mind explaining who's "everyone" and why would we possibly
> want to honour that agreement of yours?  Functionality in question is
> utterly pointless, seeing that semantics of such symlinks is OS-dependent
> anyway *and* that blanket refusal to traverse such beasts is a legitimate
> option.  What's the point in allowing to create them in the first
> place?

BSD may have created them. Or evil sysadmin (me) might have created
them with hex editor.

AFAICT we seem to have the Solaris behavior (current directory):

pavel@amd:/mnt/foo$ ls -al link2/
total 3
drwxr-xr-x 2 pavel pavel 1024 May 26 11:34 .
drwxr-xr-x 4 pavel pavel 1024 May 26 11:29 ..
-rw-r--r-- 1 pavel pavel    5 May 26 11:29 file1
lrwxrwxrwx 1 pavel pavel    5 May 26 11:29 link1 -> file1
lrwxrwxrwx 1 pavel pavel   21 May 26 11:34 link2 -> 
lrwxrwxrwx 1 pavel pavel    5 May 26 11:29 this_will_be_null_link ->
file1
pavel@amd:/mnt/foo$ cd link2/
pavel@amd:/mnt/foo/link2$ ls -al
total 3
drwxr-xr-x 2 pavel pavel 1024 May 26 11:34 .
drwxr-xr-x 4 pavel pavel 1024 May 26 11:29 ..
-rw-r--r-- 1 pavel pavel    5 May 26 11:29 file1
lrwxrwxrwx 1 pavel pavel    5 May 26 11:29 link1 -> file1
lrwxrwxrwx 1 pavel pavel   21 May 26 11:34 link2 -> 
lrwxrwxrwx 1 pavel pavel    5 May 26 11:29 this_will_be_null_link ->
file1
pavel@amd:/mnt/foo/link2$ cd link2/
pavel@amd:/mnt/foo/link2/link2$ ls 
file1  link1  link2  this_will_be_null_link
pavel@amd:/mnt/foo/link2/link2$ cd ..

They are _not_ detected as broken during runtime (ext2 fs is not marked as
containing errors) but fsck seems to handle them correctly.

pavel@amd:~/misc$ /sbin/fsck.ext2 -f delme2.fs 
e2fsck 1.41.12 (17-May-2010)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Symlink /foo/link2 (inode #7717) is invalid.
Clear<y>? yes

Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information

delme2.fs: ***** FILE SYSTEM WAS MODIFIED *****
delme2.fs: 15/25064 files (0.0% non-contiguous), 4724/100000 blocks
pavel@amd:~/misc$ 

I was looking for nice kernel crash but alas, nothing :-).
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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