linux-kernel - Re: [PATCH -V18 04/13] vfs: Allow handle based open on symlinks

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Message-ID: <m339u7l80o.fsf@linux.vnet.ibm.com>
Date:	Sun, 22 Aug 2010 07:32:15 +0530
From:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...nel.dk>, Neil Brown <neilb@...e.de>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>, adilger@....com,
	corbet@....net, npiggin@...nel.dk, hooanon05@...oo.co.jp,
	bfields@...ldses.org, miklos@...redi.hu,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V18 04/13] vfs: Allow handle based open on symlinks

On Sat, 21 Aug 2010 15:12:15 +0530, "Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:
> On Sat, 21 Aug 2010 18:30:24 +1000, Nick Piggin <npiggin@...nel.dk> wrote:
> > Thanks, I had both of the same concerns as Christoph with API
> > change and exposing symlink fds last time I looked at the patces,
> > actually.
> > 
> > But they can probably be worked around or avoided. I think the more
> > important thing is whether it is worth supporting. This is
> > all restricted to root (or CAP_DAC_READ_SEARCH) only, right, and
> > what exact semantics they want. I would like to see more discussion
> > of what this enables and some results.
> > 
> > For the case of avoiding expensive network revalidations in path name
> > lookup, do we even need to open symlinks? Could the security issues be
> > avoided by always having handle attached to an open fd?
> > 
> 
> For implementing a userspace file server that use handle for
> representing files (like NFS) we would require to have the ability to do
> different file system operations that can operate on symlink to work on
> handle too. 
> 

linkat change to take NULL path name is needed for regular files
also. The file server will get a request to create a link with handle
of oldpath and directory handle of new path and new name.

The changes would also help in the implementation of 9P server in Qemu
for implementing a fast virtualization passthrough file system (aka
VirtFS)

http://thread.gmane.org/gmane.comp.emulators.qemu/68992

-aneesh
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