linux-kernel - Re: [PATCH -V16 04/12] vfs: Allow handle based open on symlinks

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Message-ID: <87d3utyr2o.fsf@linux.vnet.ibm.com>
Date:	Mon, 12 Jul 2010 15:12:07 +0530
From:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	hch@...radead.org, viro@...iv.linux.org.uk, adilger@....com,
	corbet@....net, neilb@...e.de, npiggin@...e.de,
	hooanon05@...oo.co.jp, bfields@...ldses.org,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V16 04/12] vfs: Allow handle based open on symlinks

On Mon, 12 Jul 2010 10:23:21 +0200, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Mon, 12 Jul 2010, 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.
> > 
> > We should be able to read the link target using file handle. The exact
> > usecase is with respect to implementing READLINK operation on a
> > userspace NFS server. The request contain the file handle and the
> > response include target name.
> > 
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> > ---
> >  fs/namei.c         |   10 +++++++++-
> >  fs/open.c          |    9 ++++++++-
> >  include/linux/fs.h |    1 +
> >  3 files changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 4d590a3..a6a8093 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -1456,7 +1456,15 @@ int may_open(struct path *path, int acc_mode, int flag)
> >  
> >  	switch (inode->i_mode & S_IFMT) {
> >  	case S_IFLNK:
> > -		return -ELOOP;
> > +		/*
> > +		 * Allow only if acc_mode contain
> > +		 * open link request and read request.
> > +		 */
> > +		if (acc_mode != (MAY_OPEN_LINK | MAY_READ))
> 
> Why require MAY_READ? 

a value of 0x0 for flag in open(2) implies a read ?

> 
> Actually, open_by_handle() should be a good place to start supporting
> O_NOACCESS from the start.  I.e. neigher read, nor write access is
> permitted on the file.

Yes that would be ideal. But that will include much larger change. I was
hoping we could get something in this merge window with the change
suggested above ?

> 
> 
> > +			return -ELOOP;
> > +		if (flag != O_RDONLY)
> > +			return -ELOOP;
> > +		break;
> >  	case S_IFDIR:
> >  		if (acc_mode & MAY_WRITE)
> >  			return -EISDIR;

-aneesh
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