From: "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
To: Nick Piggin <npiggin@suse.de>
Cc: hch@infradead.org, viro@zeniv.linux.org.uk, adilger@sun.com,
       corbet@lwn.net, serue@us.ibm.com, neilb@suse.de, hooanon05@yahoo.co.jp,
       bfields@fieldses.org, linux-fsdevel@vger.kernel.org, sfrench@us.ibm.com,
       philippe.deniel@CEA.FR, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -V14 04/11] vfs: Allow handle based open on symlinks
In-Reply-To: <20100707165719.GC9263@laptop>
References: <1276621981-2774-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <1276621981-2774-5-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20100707152303.GS11732@laptop> <87d3uzfeej.fsf@linux.vnet.ibm.com> <20100707165719.GC9263@laptop>
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Date: Wed, 07 Jul 2010 23:23:31 +0530
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On Thu, 8 Jul 2010 02:57:19 +1000, Nick Piggin <npiggin@suse.de> wrote:
> On Wed, Jul 07, 2010 at 09:54:52PM +0530, Aneesh Kumar K. V wrote:
> > On Thu, 8 Jul 2010 01:23:03 +1000, Nick Piggin <npiggin@suse.de> wrote:
> > > On Tue, Jun 15, 2010 at 10:42:54PM +0530, Aneesh Kumar K.V wrote:
> > > > The patch update may_open to allow handle based open on symlinks.
> > > > The file handle based API use file descritor returned from open_by_handle_at
> > > > to do different file system operations. To find the link target name we
> > > > need to get a file descriptor on symlinks.
> > > 
> > > This is a pretty big change, isn't it?
> > 
> > Yes
> > 
> > > Have you looked through vfs to
> > > ensure this is actually OK? I was just looking at remount,ro code, for
> > > example, and it seems to assume only writable open files on ISREG
> > > files.
> > 
> > I verified that write and read returns an error on that descriptor. I
> > will look at rest of code to ensure the it doesn't break assumptions in
> > the rest of VFS.
> > 
> > > 
> > > Is this restricted to RDONLY? Really, it should be O_NONE...
> > > 
> > > 
> > 
> > we need the interface so that we can do fstat(lstat) and readlink using the
> > handle. If we are against the idea of allowing an fd on symlink, how about
> > adding stat_by_handle and readlink_by_handle syscall ? That way we can
> > drop "vfs: Support null pathname in readlink" patch 
> 
> It would be nice to avoid introducing open files to symlinks, but
> I don't know about really the *right* thing to do though. I think
> avoiding adding any by-handle syscalls except for open by handle
> may be a good idea... OTOH if this is really a special case for
> handles...
> 
> I haven't followed this thread closely, so can you just go back
> and explain to me why you need this? Ie. that you can't achieve with
> a path-to-handle follow/nofollow option.


We need to be able to read the link target using file handle. The exact
usecase i have is with respect to implementing READLINK operation on a
userspace NFS server. The request contain the file handle and the
response include target name.

Similarly we need to able to get file attribute based on file handle.
I was using readlinkat and fstat to obtain both the above information
using the descriptor returned by open_by_handle on a file handle
representing symlink

-aneesh
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